json/develop/json.hpp

/*
    __ _____ _____ _____
 __|  |   __|     |   | |  JSON for Modern C++
|  |  |__   |  |  | | | |  version 3.0.1
|_____|_____|_____|_|___|  https://github.com/nlohmann/json

Licensed under the MIT License <http://opensource.org/licenses/MIT>.
Copyright (c) 2013-2017 Niels Lohmann <http://nlohmann.me>.

Permission is hereby  granted, free of charge, to any  person obtaining a copy
of this software and associated  documentation files (the "Software"), to deal
in the Software  without restriction, including without  limitation the rights
to  use, copy,  modify, merge,  publish, distribute,  sublicense, and/or  sell
copies  of  the Software,  and  to  permit persons  to  whom  the Software  is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE  IS PROVIDED "AS  IS", WITHOUT WARRANTY  OF ANY KIND,  EXPRESS OR
IMPLIED,  INCLUDING BUT  NOT  LIMITED TO  THE  WARRANTIES OF  MERCHANTABILITY,
FITNESS FOR  A PARTICULAR PURPOSE AND  NONINFRINGEMENT. IN NO EVENT  SHALL THE
AUTHORS  OR COPYRIGHT  HOLDERS  BE  LIABLE FOR  ANY  CLAIM,  DAMAGES OR  OTHER
LIABILITY, WHETHER IN AN ACTION OF  CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

#ifndef NLOHMANN_JSON_HPP
#define NLOHMANN_JSON_HPP

#include <algorithm> // all_of, find, for_each
#include <cassert> // assert
#include <ciso646> // and, not, or
#include <cstddef> // nullptr_t, ptrdiff_t, size_t
#include <functional> // hash, less
#include <initializer_list> // initializer_list
#include <iosfwd> // istream, ostream
#include <iterator> // iterator_traits, random_access_iterator_tag
#include <numeric> // accumulate
#include <string> // string, stoi, to_string
#include <utility> // declval, forward, move, pair, swap

#include "json_fwd.hpp"
#include "detail/macro_scope.hpp"
#include "detail/meta.hpp"
#include "detail/exceptions.hpp"
#include "detail/value_t.hpp"
#include "detail/conversions/from_json.hpp"
#include "detail/conversions/to_json.hpp"
#include "detail/parsing/input_adapters.hpp"
#include "detail/parsing/lexer.hpp"
#include "detail/parsing/parser.hpp"
#include "detail/iterators/primitive_iterator.hpp"
#include "detail/iterators/internal_iterator.hpp"
#include "detail/iterators/iter_impl.hpp"
#include "detail/iterators/iteration_proxy.hpp"
#include "detail/iterators/json_reverse_iterator.hpp"
#include "detail/parsing/output_adapters.hpp"
#include "detail/parsing/binary_reader.hpp"
#include "detail/parsing/binary_writer.hpp"
#include "detail/serializer.hpp"
#include "detail/json_ref.hpp"
#include "adl_serializer.hpp"

/*!
@brief namespace for Niels Lohmann
@see https://github.com/nlohmann
@since version 1.0.0
*/
namespace nlohmann
{
class json_pointer
{
    /// allow basic_json to access private members
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    friend class basic_json;

  public:
    /*!
    @brief create JSON pointer

    Create a JSON pointer according to the syntax described in
    [Section 3 of RFC6901](https://tools.ietf.org/html/rfc6901#section-3).

    @param[in] s  string representing the JSON pointer; if omitted, the empty
                  string is assumed which references the whole JSON value

    @throw parse_error.107 if the given JSON pointer @a s is nonempty and
    does not begin with a slash (`/`); see example below

    @throw parse_error.108 if a tilde (`~`) in the given JSON pointer @a s
    is not followed by `0` (representing `~`) or `1` (representing `/`);
    see example below

    @liveexample{The example shows the construction several valid JSON
    pointers as well as the exceptional behavior.,json_pointer}

    @since version 2.0.0
    */
    explicit json_pointer(const std::string& s = "") : reference_tokens(split(s)) {}

    /*!
    @brief return a string representation of the JSON pointer

    @invariant For each JSON pointer `ptr`, it holds:
    @code {.cpp}
    ptr == json_pointer(ptr.to_string());
    @endcode

    @return a string representation of the JSON pointer

    @liveexample{The example shows the result of `to_string`.,
    json_pointer__to_string}

    @since version 2.0.0
    */
    std::string to_string() const noexcept
    {
        return std::accumulate(reference_tokens.begin(), reference_tokens.end(),
                               std::string{},
                               [](const std::string & a, const std::string & b)
        {
            return a + "/" + escape(b);
        });
    }

    /// @copydoc to_string()
    operator std::string() const
    {
        return to_string();
    }

    /*!
    @param[in] s  reference token to be converted into an array index

    @return integer representation of @a s

    @throw out_of_range.404 if string @a s could not be converted to an integer
    */
    static int array_index(const std::string& s)
    {
        size_t processed_chars = 0;
        const int res = std::stoi(s, &processed_chars);

        // check if the string was completely read
        if (JSON_UNLIKELY(processed_chars != s.size()))
        {
            JSON_THROW(detail::out_of_range::create(404, "unresolved reference token '" + s + "'"));
        }

        return res;
    }

  private:
    /*!
    @brief remove and return last reference pointer
    @throw out_of_range.405 if JSON pointer has no parent
    */
    std::string pop_back()
    {
        if (JSON_UNLIKELY(is_root()))
        {
            JSON_THROW(detail::out_of_range::create(405, "JSON pointer has no parent"));
        }

        auto last = reference_tokens.back();
        reference_tokens.pop_back();
        return last;
    }

    /// return whether pointer points to the root document
    bool is_root() const
    {
        return reference_tokens.empty();
    }

    json_pointer top() const
    {
        if (JSON_UNLIKELY(is_root()))
        {
            JSON_THROW(detail::out_of_range::create(405, "JSON pointer has no parent"));
        }

        json_pointer result = *this;
        result.reference_tokens = {reference_tokens[0]};
        return result;
    }

    /*!
    @brief create and return a reference to the pointed to value

    @complexity Linear in the number of reference tokens.

    @throw parse_error.109 if array index is not a number
    @throw type_error.313 if value cannot be unflattened
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    NLOHMANN_BASIC_JSON_TPL& get_and_create(NLOHMANN_BASIC_JSON_TPL& j) const;

    /*!
    @brief return a reference to the pointed to value

    @note This version does not throw if a value is not present, but tries to
          create nested values instead. For instance, calling this function
          with pointer `"/this/that"` on a null value is equivalent to calling
          `operator[]("this").operator[]("that")` on that value, effectively
          changing the null value to an object.

    @param[in] ptr  a JSON value

    @return reference to the JSON value pointed to by the JSON pointer

    @complexity Linear in the length of the JSON pointer.

    @throw parse_error.106   if an array index begins with '0'
    @throw parse_error.109   if an array index was not a number
    @throw out_of_range.404  if the JSON pointer can not be resolved
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    NLOHMANN_BASIC_JSON_TPL& get_unchecked(NLOHMANN_BASIC_JSON_TPL* ptr) const;

    /*!
    @throw parse_error.106   if an array index begins with '0'
    @throw parse_error.109   if an array index was not a number
    @throw out_of_range.402  if the array index '-' is used
    @throw out_of_range.404  if the JSON pointer can not be resolved
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    NLOHMANN_BASIC_JSON_TPL& get_checked(NLOHMANN_BASIC_JSON_TPL* ptr) const;

    /*!
    @brief return a const reference to the pointed to value

    @param[in] ptr  a JSON value

    @return const reference to the JSON value pointed to by the JSON
    pointer

    @throw parse_error.106   if an array index begins with '0'
    @throw parse_error.109   if an array index was not a number
    @throw out_of_range.402  if the array index '-' is used
    @throw out_of_range.404  if the JSON pointer can not be resolved
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    const NLOHMANN_BASIC_JSON_TPL& get_unchecked(const NLOHMANN_BASIC_JSON_TPL* ptr) const;

    /*!
    @throw parse_error.106   if an array index begins with '0'
    @throw parse_error.109   if an array index was not a number
    @throw out_of_range.402  if the array index '-' is used
    @throw out_of_range.404  if the JSON pointer can not be resolved
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    const NLOHMANN_BASIC_JSON_TPL& get_checked(const NLOHMANN_BASIC_JSON_TPL* ptr) const;

    /*!
    @brief split the string input to reference tokens

    @note This function is only called by the json_pointer constructor.
          All exceptions below are documented there.

    @throw parse_error.107  if the pointer is not empty or begins with '/'
    @throw parse_error.108  if character '~' is not followed by '0' or '1'
    */
    static std::vector<std::string> split(const std::string& reference_string)
    {
        std::vector<std::string> result;

        // special case: empty reference string -> no reference tokens
        if (reference_string.empty())
        {
            return result;
        }

        // check if nonempty reference string begins with slash
        if (JSON_UNLIKELY(reference_string[0] != '/'))
        {
            JSON_THROW(detail::parse_error::create(107, 1,
                                                   "JSON pointer must be empty or begin with '/' - was: '" +
                                                   reference_string + "'"));
        }

        // extract the reference tokens:
        // - slash: position of the last read slash (or end of string)
        // - start: position after the previous slash
        for (
            // search for the first slash after the first character
            std::size_t slash = reference_string.find_first_of('/', 1),
            // set the beginning of the first reference token
            start = 1;
            // we can stop if start == string::npos+1 = 0
            start != 0;
            // set the beginning of the next reference token
            // (will eventually be 0 if slash == std::string::npos)
            start = slash + 1,
            // find next slash
            slash = reference_string.find_first_of('/', start))
        {
            // use the text between the beginning of the reference token
            // (start) and the last slash (slash).
            auto reference_token = reference_string.substr(start, slash - start);

            // check reference tokens are properly escaped
            for (std::size_t pos = reference_token.find_first_of('~');
                    pos != std::string::npos;
                    pos = reference_token.find_first_of('~', pos + 1))
            {
                assert(reference_token[pos] == '~');

                // ~ must be followed by 0 or 1
                if (JSON_UNLIKELY(pos == reference_token.size() - 1 or
                                  (reference_token[pos + 1] != '0' and
                                   reference_token[pos + 1] != '1')))
                {
                    JSON_THROW(detail::parse_error::create(108, 0, "escape character '~' must be followed with '0' or '1'"));
                }
            }

            // finally, store the reference token
            unescape(reference_token);
            result.push_back(reference_token);
        }

        return result;
    }

    /*!
    @brief replace all occurrences of a substring by another string

    @param[in,out] s  the string to manipulate; changed so that all
                   occurrences of @a f are replaced with @a t
    @param[in]     f  the substring to replace with @a t
    @param[in]     t  the string to replace @a f

    @pre The search string @a f must not be empty. **This precondition is
    enforced with an assertion.**

    @since version 2.0.0
    */
    static void replace_substring(std::string& s, const std::string& f,
                                  const std::string& t)
    {
        assert(not f.empty());
        for (auto pos = s.find(f);                // find first occurrence of f
                pos != std::string::npos;         // make sure f was found
                s.replace(pos, f.size(), t),      // replace with t, and
                pos = s.find(f, pos + t.size()))  // find next occurrence of f
        {}
    }

    /// escape "~"" to "~0" and "/" to "~1"
    static std::string escape(std::string s)
    {
        replace_substring(s, "~", "~0");
        replace_substring(s, "/", "~1");
        return s;
    }

    /// unescape "~1" to tilde and "~0" to slash (order is important!)
    static void unescape(std::string& s)
    {
        replace_substring(s, "~1", "/");
        replace_substring(s, "~0", "~");
    }

    /*!
    @param[in] reference_string  the reference string to the current value
    @param[in] value             the value to consider
    @param[in,out] result        the result object to insert values to

    @note Empty objects or arrays are flattened to `null`.
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    static void flatten(const std::string& reference_string,
                        const NLOHMANN_BASIC_JSON_TPL& value,
                        NLOHMANN_BASIC_JSON_TPL& result);

    /*!
    @param[in] value  flattened JSON

    @return unflattened JSON

    @throw parse_error.109 if array index is not a number
    @throw type_error.314  if value is not an object
    @throw type_error.315  if object values are not primitive
    @throw type_error.313  if value cannot be unflattened
    */
    NLOHMANN_BASIC_JSON_TPL_DECLARATION
    static NLOHMANN_BASIC_JSON_TPL
    unflatten(const NLOHMANN_BASIC_JSON_TPL& value);

    friend bool operator==(json_pointer const& lhs,
                           json_pointer const& rhs) noexcept;

    friend bool operator!=(json_pointer const& lhs,
                           json_pointer const& rhs) noexcept;

    /// the reference tokens
    std::vector<std::string> reference_tokens;
};

/*!
@brief a class to store JSON values

@tparam ObjectType type for JSON objects (`std::map` by default; will be used
in @ref object_t)
@tparam ArrayType type for JSON arrays (`std::vector` by default; will be used
in @ref array_t)
@tparam StringType type for JSON strings and object keys (`std::string` by
default; will be used in @ref string_t)
@tparam BooleanType type for JSON booleans (`bool` by default; will be used
in @ref boolean_t)
@tparam NumberIntegerType type for JSON integer numbers (`int64_t` by
default; will be used in @ref number_integer_t)
@tparam NumberUnsignedType type for JSON unsigned integer numbers (@c
`uint64_t` by default; will be used in @ref number_unsigned_t)
@tparam NumberFloatType type for JSON floating-point numbers (`double` by
default; will be used in @ref number_float_t)
@tparam AllocatorType type of the allocator to use (`std::allocator` by
default)
@tparam JSONSerializer the serializer to resolve internal calls to `to_json()`
and `from_json()` (@ref adl_serializer by default)

@requirement The class satisfies the following concept requirements:
- Basic
 - [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible):
   JSON values can be default constructed. The result will be a JSON null
   value.
 - [MoveConstructible](http://en.cppreference.com/w/cpp/concept/MoveConstructible):
   A JSON value can be constructed from an rvalue argument.
 - [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible):
   A JSON value can be copy-constructed from an lvalue expression.
 - [MoveAssignable](http://en.cppreference.com/w/cpp/concept/MoveAssignable):
   A JSON value van be assigned from an rvalue argument.
 - [CopyAssignable](http://en.cppreference.com/w/cpp/concept/CopyAssignable):
   A JSON value can be copy-assigned from an lvalue expression.
 - [Destructible](http://en.cppreference.com/w/cpp/concept/Destructible):
   JSON values can be destructed.
- Layout
 - [StandardLayoutType](http://en.cppreference.com/w/cpp/concept/StandardLayoutType):
   JSON values have
   [standard layout](http://en.cppreference.com/w/cpp/language/data_members#Standard_layout):
   All non-static data members are private and standard layout types, the
   class has no virtual functions or (virtual) base classes.
- Library-wide
 - [EqualityComparable](http://en.cppreference.com/w/cpp/concept/EqualityComparable):
   JSON values can be compared with `==`, see @ref
   operator==(const_reference,const_reference).
 - [LessThanComparable](http://en.cppreference.com/w/cpp/concept/LessThanComparable):
   JSON values can be compared with `<`, see @ref
   operator<(const_reference,const_reference).
 - [Swappable](http://en.cppreference.com/w/cpp/concept/Swappable):
   Any JSON lvalue or rvalue of can be swapped with any lvalue or rvalue of
   other compatible types, using unqualified function call @ref swap().
 - [NullablePointer](http://en.cppreference.com/w/cpp/concept/NullablePointer):
   JSON values can be compared against `std::nullptr_t` objects which are used
   to model the `null` value.
- Container
 - [Container](http://en.cppreference.com/w/cpp/concept/Container):
   JSON values can be used like STL containers and provide iterator access.
 - [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer);
   JSON values can be used like STL containers and provide reverse iterator
   access.

@invariant The member variables @a m_value and @a m_type have the following
relationship:
- If `m_type == value_t::object`, then `m_value.object != nullptr`.
- If `m_type == value_t::array`, then `m_value.array != nullptr`.
- If `m_type == value_t::string`, then `m_value.string != nullptr`.
The invariants are checked by member function assert_invariant().

@internal
@note ObjectType trick from http://stackoverflow.com/a/9860911
@endinternal

@see [RFC 7159: The JavaScript Object Notation (JSON) Data Interchange
Format](http://rfc7159.net/rfc7159)

@since version 1.0.0

@nosubgrouping
*/
NLOHMANN_BASIC_JSON_TPL_DECLARATION
class basic_json
{
  private:
    template<detail::value_t> friend struct detail::external_constructor;
    friend ::nlohmann::json_pointer;
    friend ::nlohmann::detail::parser<basic_json>;
    friend ::nlohmann::detail::serializer<basic_json>;
    template<typename BasicJsonType>
    friend class ::nlohmann::detail::iter_impl;
    template<typename BasicJsonType, typename CharType>
    friend class ::nlohmann::detail::binary_writer;
    template<typename BasicJsonType>
    friend class ::nlohmann::detail::binary_reader;

    /// workaround type for MSVC
    using basic_json_t = NLOHMANN_BASIC_JSON_TPL;

    // convenience aliases for types residing in namespace detail;
    using lexer = ::nlohmann::detail::lexer<basic_json>;
    using parser = ::nlohmann::detail::parser<basic_json>;

    using primitive_iterator_t = ::nlohmann::detail::primitive_iterator_t;
    template<typename BasicJsonType>
    using internal_iterator = ::nlohmann::detail::internal_iterator<BasicJsonType>;
    template<typename BasicJsonType>
    using iter_impl = ::nlohmann::detail::iter_impl<BasicJsonType>;
    template<typename Iterator>
    using iteration_proxy = ::nlohmann::detail::iteration_proxy<Iterator>;
    template<typename Base> using json_reverse_iterator = ::nlohmann::detail::json_reverse_iterator<Base>;

    template<typename CharType>
    using output_adapter_t = ::nlohmann::detail::output_adapter_t<CharType>;

    using binary_reader = ::nlohmann::detail::binary_reader<basic_json>;
    template<typename CharType> using binary_writer = ::nlohmann::detail::binary_writer<basic_json, CharType>;

    using serializer = ::nlohmann::detail::serializer<basic_json>;

  public:
    using value_t = detail::value_t;
    /// @copydoc nlohmann::json_pointer
    using json_pointer = ::nlohmann::json_pointer;
    template<typename T, typename SFINAE>
    using json_serializer = JSONSerializer<T, SFINAE>;
    /// helper type for initializer lists of basic_json values
    using initializer_list_t = std::initializer_list<detail::json_ref<basic_json>>;

    ////////////////
    // exceptions //
    ////////////////

    /// @name exceptions
    /// Classes to implement user-defined exceptions.
    /// @{

    /// @copydoc detail::exception
    using exception = detail::exception;
    /// @copydoc detail::parse_error
    using parse_error = detail::parse_error;
    /// @copydoc detail::invalid_iterator
    using invalid_iterator = detail::invalid_iterator;
    /// @copydoc detail::type_error
    using type_error = detail::type_error;
    /// @copydoc detail::out_of_range
    using out_of_range = detail::out_of_range;
    /// @copydoc detail::other_error
    using other_error = detail::other_error;

    /// @}


    /////////////////////
    // container types //
    /////////////////////

    /// @name container types
    /// The canonic container types to use @ref basic_json like any other STL
    /// container.
    /// @{

    /// the type of elements in a basic_json container
    using value_type = basic_json;

    /// the type of an element reference
    using reference = value_type&;
    /// the type of an element const reference
    using const_reference = const value_type&;

    /// a type to represent differences between iterators
    using difference_type = std::ptrdiff_t;
    /// a type to represent container sizes
    using size_type = std::size_t;

    /// the allocator type
    using allocator_type = AllocatorType<basic_json>;

    /// the type of an element pointer
    using pointer = typename std::allocator_traits<allocator_type>::pointer;
    /// the type of an element const pointer
    using const_pointer = typename std::allocator_traits<allocator_type>::const_pointer;

    /// an iterator for a basic_json container
    using iterator = iter_impl<basic_json>;
    /// a const iterator for a basic_json container
    using const_iterator = iter_impl<const basic_json>;
    /// a reverse iterator for a basic_json container
    using reverse_iterator = json_reverse_iterator<typename basic_json::iterator>;
    /// a const reverse iterator for a basic_json container
    using const_reverse_iterator = json_reverse_iterator<typename basic_json::const_iterator>;

    /// @}


    /*!
    @brief returns the allocator associated with the container
    */
    static allocator_type get_allocator()
    {
        return allocator_type();
    }

    /*!
    @brief returns version information on the library

    This function returns a JSON object with information about the library,
    including the version number and information on the platform and compiler.

    @return JSON object holding version information
    key         | description
    ----------- | ---------------
    `compiler`  | Information on the used compiler. It is an object with the following keys: `c++` (the used C++ standard), `family` (the compiler family; possible values are `clang`, `icc`, `gcc`, `ilecpp`, `msvc`, `pgcpp`, `sunpro`, and `unknown`), and `version` (the compiler version).
    `copyright` | The copyright line for the library as string.
    `name`      | The name of the library as string.
    `platform`  | The used platform as string. Possible values are `win32`, `linux`, `apple`, `unix`, and `unknown`.
    `url`       | The URL of the project as string.
    `version`   | The version of the library. It is an object with the following keys: `major`, `minor`, and `patch` as defined by [Semantic Versioning](http://semver.org), and `string` (the version string).

    @liveexample{The following code shows an example output of the `meta()`
    function.,meta}

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @complexity Constant.

    @since 2.1.0
    */
    static basic_json meta()
    {
        basic_json result;

        result["copyright"] = "(C) 2013-2017 Niels Lohmann";
        result["name"] = "JSON for Modern C++";
        result["url"] = "https://github.com/nlohmann/json";
        result["version"] =
        {
            {"string", "3.0.1"}, {"major", 3}, {"minor", 0}, {"patch", 1}
        };

#ifdef _WIN32
        result["platform"] = "win32";
#elif defined __linux__
        result["platform"] = "linux";
#elif defined __APPLE__
        result["platform"] = "apple";
#elif defined __unix__
        result["platform"] = "unix";
#else
        result["platform"] = "unknown";
#endif

#if defined(__ICC) || defined(__INTEL_COMPILER)
        result["compiler"] = {{"family", "icc"}, {"version", __INTEL_COMPILER}};
#elif defined(__clang__)
        result["compiler"] = {{"family", "clang"}, {"version", __clang_version__}};
#elif defined(__GNUC__) || defined(__GNUG__)
        result["compiler"] = {{"family", "gcc"}, {"version", std::to_string(__GNUC__) + "." + std::to_string(__GNUC_MINOR__) + "." + std::to_string(__GNUC_PATCHLEVEL__)}};
#elif defined(__HP_cc) || defined(__HP_aCC)
        result["compiler"] = "hp"
#elif defined(__IBMCPP__)
        result["compiler"] = {{"family", "ilecpp"}, {"version", __IBMCPP__}};
#elif defined(_MSC_VER)
        result["compiler"] = {{"family", "msvc"}, {"version", _MSC_VER}};
#elif defined(__PGI)
        result["compiler"] = {{"family", "pgcpp"}, {"version", __PGI}};
#elif defined(__SUNPRO_CC)
        result["compiler"] = {{"family", "sunpro"}, {"version", __SUNPRO_CC}};
#else
        result["compiler"] = {{"family", "unknown"}, {"version", "unknown"}};
#endif

#ifdef __cplusplus
        result["compiler"]["c++"] = std::to_string(__cplusplus);
#else
        result["compiler"]["c++"] = "unknown";
#endif
        return result;
    }


    ///////////////////////////
    // JSON value data types //
    ///////////////////////////

    /// @name JSON value data types
    /// The data types to store a JSON value. These types are derived from
    /// the template arguments passed to class @ref basic_json.
    /// @{

#if defined(JSON_HAS_CPP_14)
    // Use transparent comparator if possible, combined with perfect forwarding
    // on find() and count() calls prevents unnecessary string construction.
    using object_comparator_t = std::less<>;
#else
    using object_comparator_t = std::less<StringType>;
#endif

    /*!
    @brief a type for an object

    [RFC 7159](http://rfc7159.net/rfc7159) describes JSON objects as follows:
    > An object is an unordered collection of zero or more name/value pairs,
    > where a name is a string and a value is a string, number, boolean, null,
    > object, or array.

    To store objects in C++, a type is defined by the template parameters
    described below.

    @tparam ObjectType  the container to store objects (e.g., `std::map` or
    `std::unordered_map`)
    @tparam StringType the type of the keys or names (e.g., `std::string`).
    The comparison function `std::less<StringType>` is used to order elements
    inside the container.
    @tparam AllocatorType the allocator to use for objects (e.g.,
    `std::allocator`)

    #### Default type

    With the default values for @a ObjectType (`std::map`), @a StringType
    (`std::string`), and @a AllocatorType (`std::allocator`), the default
    value for @a object_t is:

    @code {.cpp}
    std::map<
      std::string, // key_type
      basic_json, // value_type
      std::less<std::string>, // key_compare
      std::allocator<std::pair<const std::string, basic_json>> // allocator_type
    >
    @endcode

    #### Behavior

    The choice of @a object_t influences the behavior of the JSON class. With
    the default type, objects have the following behavior:

    - When all names are unique, objects will be interoperable in the sense
      that all software implementations receiving that object will agree on
      the name-value mappings.
    - When the names within an object are not unique, later stored name/value
      pairs overwrite previously stored name/value pairs, leaving the used
      names unique. For instance, `{"key": 1}` and `{"key": 2, "key": 1}` will
      be treated as equal and both stored as `{"key": 1}`.
    - Internally, name/value pairs are stored in lexicographical order of the
      names. Objects will also be serialized (see @ref dump) in this order.
      For instance, `{"b": 1, "a": 2}` and `{"a": 2, "b": 1}` will be stored
      and serialized as `{"a": 2, "b": 1}`.
    - When comparing objects, the order of the name/value pairs is irrelevant.
      This makes objects interoperable in the sense that they will not be
      affected by these differences. For instance, `{"b": 1, "a": 2}` and
      `{"a": 2, "b": 1}` will be treated as equal.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the maximum depth of nesting.

    In this class, the object's limit of nesting is not explicitly constrained.
    However, a maximum depth of nesting may be introduced by the compiler or
    runtime environment. A theoretical limit can be queried by calling the
    @ref max_size function of a JSON object.

    #### Storage

    Objects are stored as pointers in a @ref basic_json type. That is, for any
    access to object values, a pointer of type `object_t*` must be
    dereferenced.

    @sa @ref array_t -- type for an array value

    @since version 1.0.0

    @note The order name/value pairs are added to the object is *not*
    preserved by the library. Therefore, iterating an object may return
    name/value pairs in a different order than they were originally stored. In
    fact, keys will be traversed in alphabetical order as `std::map` with
    `std::less` is used by default. Please note this behavior conforms to [RFC
    7159](http://rfc7159.net/rfc7159), because any order implements the
    specified "unordered" nature of JSON objects.
    */
    using object_t = ObjectType<StringType,
          basic_json,
          object_comparator_t,
          AllocatorType<std::pair<const StringType,
          basic_json>>>;

    /*!
    @brief a type for an array

    [RFC 7159](http://rfc7159.net/rfc7159) describes JSON arrays as follows:
    > An array is an ordered sequence of zero or more values.

    To store objects in C++, a type is defined by the template parameters
    explained below.

    @tparam ArrayType  container type to store arrays (e.g., `std::vector` or
    `std::list`)
    @tparam AllocatorType allocator to use for arrays (e.g., `std::allocator`)

    #### Default type

    With the default values for @a ArrayType (`std::vector`) and @a
    AllocatorType (`std::allocator`), the default value for @a array_t is:

    @code {.cpp}
    std::vector<
      basic_json, // value_type
      std::allocator<basic_json> // allocator_type
    >
    @endcode

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the maximum depth of nesting.

    In this class, the array's limit of nesting is not explicitly constrained.
    However, a maximum depth of nesting may be introduced by the compiler or
    runtime environment. A theoretical limit can be queried by calling the
    @ref max_size function of a JSON array.

    #### Storage

    Arrays are stored as pointers in a @ref basic_json type. That is, for any
    access to array values, a pointer of type `array_t*` must be dereferenced.

    @sa @ref object_t -- type for an object value

    @since version 1.0.0
    */
    using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;

    /*!
    @brief a type for a string

    [RFC 7159](http://rfc7159.net/rfc7159) describes JSON strings as follows:
    > A string is a sequence of zero or more Unicode characters.

    To store objects in C++, a type is defined by the template parameter
    described below. Unicode values are split by the JSON class into
    byte-sized characters during deserialization.

    @tparam StringType  the container to store strings (e.g., `std::string`).
    Note this container is used for keys/names in objects, see @ref object_t.

    #### Default type

    With the default values for @a StringType (`std::string`), the default
    value for @a string_t is:

    @code {.cpp}
    std::string
    @endcode

    #### Encoding

    Strings are stored in UTF-8 encoding. Therefore, functions like
    `std::string::size()` or `std::string::length()` return the number of
    bytes in the string rather than the number of characters or glyphs.

    #### String comparison

    [RFC 7159](http://rfc7159.net/rfc7159) states:
    > Software implementations are typically required to test names of object
    > members for equality. Implementations that transform the textual
    > representation into sequences of Unicode code units and then perform the
    > comparison numerically, code unit by code unit, are interoperable in the
    > sense that implementations will agree in all cases on equality or
    > inequality of two strings. For example, implementations that compare
    > strings with escaped characters unconverted may incorrectly find that
    > `"a\\b"` and `"a\u005Cb"` are not equal.

    This implementation is interoperable as it does compare strings code unit
    by code unit.

    #### Storage

    String values are stored as pointers in a @ref basic_json type. That is,
    for any access to string values, a pointer of type `string_t*` must be
    dereferenced.

    @since version 1.0.0
    */
    using string_t = StringType;

    /*!
    @brief a type for a boolean

    [RFC 7159](http://rfc7159.net/rfc7159) implicitly describes a boolean as a
    type which differentiates the two literals `true` and `false`.

    To store objects in C++, a type is defined by the template parameter @a
    BooleanType which chooses the type to use.

    #### Default type

    With the default values for @a BooleanType (`bool`), the default value for
    @a boolean_t is:

    @code {.cpp}
    bool
    @endcode

    #### Storage

    Boolean values are stored directly inside a @ref basic_json type.

    @since version 1.0.0
    */
    using boolean_t = BooleanType;

    /*!
    @brief a type for a number (integer)

    [RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
    > The representation of numbers is similar to that used in most
    > programming languages. A number is represented in base 10 using decimal
    > digits. It contains an integer component that may be prefixed with an
    > optional minus sign, which may be followed by a fraction part and/or an
    > exponent part. Leading zeros are not allowed. (...) Numeric values that
    > cannot be represented in the grammar below (such as Infinity and NaN)
    > are not permitted.

    This description includes both integer and floating-point numbers.
    However, C++ allows more precise storage if it is known whether the number
    is a signed integer, an unsigned integer or a floating-point number.
    Therefore, three different types, @ref number_integer_t, @ref
    number_unsigned_t and @ref number_float_t are used.

    To store integer numbers in C++, a type is defined by the template
    parameter @a NumberIntegerType which chooses the type to use.

    #### Default type

    With the default values for @a NumberIntegerType (`int64_t`), the default
    value for @a number_integer_t is:

    @code {.cpp}
    int64_t
    @endcode

    #### Default behavior

    - The restrictions about leading zeros is not enforced in C++. Instead,
      leading zeros in integer literals lead to an interpretation as octal
      number. Internally, the value will be stored as decimal number. For
      instance, the C++ integer literal `010` will be serialized to `8`.
      During deserialization, leading zeros yield an error.
    - Not-a-number (NaN) values will be serialized to `null`.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be
    stored is `9223372036854775807` (INT64_MAX) and the minimal integer number
    that can be stored is `-9223372036854775808` (INT64_MIN). Integer numbers
    that are out of range will yield over/underflow when used in a
    constructor. During deserialization, too large or small integer numbers
    will be automatically be stored as @ref number_unsigned_t or @ref
    number_float_t.

    [RFC 7159](http://rfc7159.net/rfc7159) further states:
    > Note that when such software is used, numbers that are integers and are
    > in the range \f$[-2^{53}+1, 2^{53}-1]\f$ are interoperable in the sense
    > that implementations will agree exactly on their numeric values.

    As this range is a subrange of the exactly supported range [INT64_MIN,
    INT64_MAX], this class's integer type is interoperable.

    #### Storage

    Integer number values are stored directly inside a @ref basic_json type.

    @sa @ref number_float_t -- type for number values (floating-point)

    @sa @ref number_unsigned_t -- type for number values (unsigned integer)

    @since version 1.0.0
    */
    using number_integer_t = NumberIntegerType;

    /*!
    @brief a type for a number (unsigned)

    [RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
    > The representation of numbers is similar to that used in most
    > programming languages. A number is represented in base 10 using decimal
    > digits. It contains an integer component that may be prefixed with an
    > optional minus sign, which may be followed by a fraction part and/or an
    > exponent part. Leading zeros are not allowed. (...) Numeric values that
    > cannot be represented in the grammar below (such as Infinity and NaN)
    > are not permitted.

    This description includes both integer and floating-point numbers.
    However, C++ allows more precise storage if it is known whether the number
    is a signed integer, an unsigned integer or a floating-point number.
    Therefore, three different types, @ref number_integer_t, @ref
    number_unsigned_t and @ref number_float_t are used.

    To store unsigned integer numbers in C++, a type is defined by the
    template parameter @a NumberUnsignedType which chooses the type to use.

    #### Default type

    With the default values for @a NumberUnsignedType (`uint64_t`), the
    default value for @a number_unsigned_t is:

    @code {.cpp}
    uint64_t
    @endcode

    #### Default behavior

    - The restrictions about leading zeros is not enforced in C++. Instead,
      leading zeros in integer literals lead to an interpretation as octal
      number. Internally, the value will be stored as decimal number. For
      instance, the C++ integer literal `010` will be serialized to `8`.
      During deserialization, leading zeros yield an error.
    - Not-a-number (NaN) values will be serialized to `null`.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be
    stored is `18446744073709551615` (UINT64_MAX) and the minimal integer
    number that can be stored is `0`. Integer numbers that are out of range
    will yield over/underflow when used in a constructor. During
    deserialization, too large or small integer numbers will be automatically
    be stored as @ref number_integer_t or @ref number_float_t.

    [RFC 7159](http://rfc7159.net/rfc7159) further states:
    > Note that when such software is used, numbers that are integers and are
    > in the range \f$[-2^{53}+1, 2^{53}-1]\f$ are interoperable in the sense
    > that implementations will agree exactly on their numeric values.

    As this range is a subrange (when considered in conjunction with the
    number_integer_t type) of the exactly supported range [0, UINT64_MAX],
    this class's integer type is interoperable.

    #### Storage

    Integer number values are stored directly inside a @ref basic_json type.

    @sa @ref number_float_t -- type for number values (floating-point)
    @sa @ref number_integer_t -- type for number values (integer)

    @since version 2.0.0
    */
    using number_unsigned_t = NumberUnsignedType;

    /*!
    @brief a type for a number (floating-point)

    [RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
    > The representation of numbers is similar to that used in most
    > programming languages. A number is represented in base 10 using decimal
    > digits. It contains an integer component that may be prefixed with an
    > optional minus sign, which may be followed by a fraction part and/or an
    > exponent part. Leading zeros are not allowed. (...) Numeric values that
    > cannot be represented in the grammar below (such as Infinity and NaN)
    > are not permitted.

    This description includes both integer and floating-point numbers.
    However, C++ allows more precise storage if it is known whether the number
    is a signed integer, an unsigned integer or a floating-point number.
    Therefore, three different types, @ref number_integer_t, @ref
    number_unsigned_t and @ref number_float_t are used.

    To store floating-point numbers in C++, a type is defined by the template
    parameter @a NumberFloatType which chooses the type to use.

    #### Default type

    With the default values for @a NumberFloatType (`double`), the default
    value for @a number_float_t is:

    @code {.cpp}
    double
    @endcode

    #### Default behavior

    - The restrictions about leading zeros is not enforced in C++. Instead,
      leading zeros in floating-point literals will be ignored. Internally,
      the value will be stored as decimal number. For instance, the C++
      floating-point literal `01.2` will be serialized to `1.2`. During
      deserialization, leading zeros yield an error.
    - Not-a-number (NaN) values will be serialized to `null`.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) states:
    > This specification allows implementations to set limits on the range and
    > precision of numbers accepted. Since software that implements IEEE
    > 754-2008 binary64 (double precision) numbers is generally available and
    > widely used, good interoperability can be achieved by implementations
    > that expect no more precision or range than these provide, in the sense
    > that implementations will approximate JSON numbers within the expected
    > precision.

    This implementation does exactly follow this approach, as it uses double
    precision floating-point numbers. Note values smaller than
    `-1.79769313486232e+308` and values greater than `1.79769313486232e+308`
    will be stored as NaN internally and be serialized to `null`.

    #### Storage

    Floating-point number values are stored directly inside a @ref basic_json
    type.

    @sa @ref number_integer_t -- type for number values (integer)

    @sa @ref number_unsigned_t -- type for number values (unsigned integer)

    @since version 1.0.0
    */
    using number_float_t = NumberFloatType;

    /// @}

  private:

    /// helper for exception-safe object creation
    template<typename T, typename... Args>
    static T* create(Args&& ... args)
    {
        AllocatorType<T> alloc;
        using AllocatorTraits = std::allocator_traits<AllocatorType<T>>;

        auto deleter = [&](T * object)
        {
            AllocatorTraits::deallocate(alloc, object, 1);
        };
        std::unique_ptr<T, decltype(deleter)> object(AllocatorTraits::allocate(alloc, 1), deleter);
        AllocatorTraits::construct(alloc, object.get(), std::forward<Args>(args)...);
        assert(object != nullptr);
        return object.release();
    }

    ////////////////////////
    // JSON value storage //
    ////////////////////////

    /*!
    @brief a JSON value

    The actual storage for a JSON value of the @ref basic_json class. This
    union combines the different storage types for the JSON value types
    defined in @ref value_t.

    JSON type | value_t type    | used type
    --------- | --------------- | ------------------------
    object    | object          | pointer to @ref object_t
    array     | array           | pointer to @ref array_t
    string    | string          | pointer to @ref string_t
    boolean   | boolean         | @ref boolean_t
    number    | number_integer  | @ref number_integer_t
    number    | number_unsigned | @ref number_unsigned_t
    number    | number_float    | @ref number_float_t
    null      | null            | *no value is stored*

    @note Variable-length types (objects, arrays, and strings) are stored as
    pointers. The size of the union should not exceed 64 bits if the default
    value types are used.

    @since version 1.0.0
    */
    union json_value
    {
        /// object (stored with pointer to save storage)
        object_t* object;
        /// array (stored with pointer to save storage)
        array_t* array;
        /// string (stored with pointer to save storage)
        string_t* string;
        /// boolean
        boolean_t boolean;
        /// number (integer)
        number_integer_t number_integer;
        /// number (unsigned integer)
        number_unsigned_t number_unsigned;
        /// number (floating-point)
        number_float_t number_float;

        /// default constructor (for null values)
        json_value() = default;
        /// constructor for booleans
        json_value(boolean_t v) noexcept : boolean(v) {}
        /// constructor for numbers (integer)
        json_value(number_integer_t v) noexcept : number_integer(v) {}
        /// constructor for numbers (unsigned)
        json_value(number_unsigned_t v) noexcept : number_unsigned(v) {}
        /// constructor for numbers (floating-point)
        json_value(number_float_t v) noexcept : number_float(v) {}
        /// constructor for empty values of a given type
        json_value(value_t t)
        {
            switch (t)
            {
                case value_t::object:
                {
                    object = create<object_t>();
                    break;
                }

                case value_t::array:
                {
                    array = create<array_t>();
                    break;
                }

                case value_t::string:
                {
                    string = create<string_t>("");
                    break;
                }

                case value_t::boolean:
                {
                    boolean = boolean_t(false);
                    break;
                }

                case value_t::number_integer:
                {
                    number_integer = number_integer_t(0);
                    break;
                }

                case value_t::number_unsigned:
                {
                    number_unsigned = number_unsigned_t(0);
                    break;
                }

                case value_t::number_float:
                {
                    number_float = number_float_t(0.0);
                    break;
                }

                case value_t::null:
                {
                    object = nullptr;  // silence warning, see #821
                    break;
                }

                default:
                {
                    object = nullptr;  // silence warning, see #821
                    if (JSON_UNLIKELY(t == value_t::null))
                    {
                        JSON_THROW(other_error::create(500, "961c151d2e87f2686a955a9be24d316f1362bf21 3.0.1")); // LCOV_EXCL_LINE
                    }
                    break;
                }
            }
        }

        /// constructor for strings
        json_value(const string_t& value)
        {
            string = create<string_t>(value);
        }

        /// constructor for rvalue strings
        json_value(string_t&& value)
        {
            string = create<string_t>(std::move(value));
        }

        /// constructor for objects
        json_value(const object_t& value)
        {
            object = create<object_t>(value);
        }

        /// constructor for rvalue objects
        json_value(object_t&& value)
        {
            object = create<object_t>(std::move(value));
        }

        /// constructor for arrays
        json_value(const array_t& value)
        {
            array = create<array_t>(value);
        }

        /// constructor for rvalue arrays
        json_value(array_t&& value)
        {
            array = create<array_t>(std::move(value));
        }

        void destroy(value_t t) noexcept
        {
            switch (t)
            {
                case value_t::object:
                {
                    AllocatorType<object_t> alloc;
                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
                    break;
                }

                case value_t::array:
                {
                    AllocatorType<array_t> alloc;
                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
                    break;
                }

                case value_t::string:
                {
                    AllocatorType<string_t> alloc;
                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
                    break;
                }

                default:
                {
                    break;
                }
            }
        }
    };

    /*!
    @brief checks the class invariants

    This function asserts the class invariants. It needs to be called at the
    end of every constructor to make sure that created objects respect the
    invariant. Furthermore, it has to be called each time the type of a JSON
    value is changed, because the invariant expresses a relationship between
    @a m_type and @a m_value.
    */
    void assert_invariant() const noexcept
    {
        assert(m_type != value_t::object or m_value.object != nullptr);
        assert(m_type != value_t::array or m_value.array != nullptr);
        assert(m_type != value_t::string or m_value.string != nullptr);
    }

  public:
    //////////////////////////
    // JSON parser callback //
    //////////////////////////

    /*!
    @brief parser event types

    The parser callback distinguishes the following events:
    - `object_start`: the parser read `{` and started to process a JSON object
    - `key`: the parser read a key of a value in an object
    - `object_end`: the parser read `}` and finished processing a JSON object
    - `array_start`: the parser read `[` and started to process a JSON array
    - `array_end`: the parser read `]` and finished processing a JSON array
    - `value`: the parser finished reading a JSON value

    @image html callback_events.png "Example when certain parse events are triggered"

    @sa @ref parser_callback_t for more information and examples
    */
    using parse_event_t = typename parser::parse_event_t;

    /*!
    @brief per-element parser callback type

    With a parser callback function, the result of parsing a JSON text can be
    influenced. When passed to @ref parse, it is called on certain events
    (passed as @ref parse_event_t via parameter @a event) with a set recursion
    depth @a depth and context JSON value @a parsed. The return value of the
    callback function is a boolean indicating whether the element that emitted
    the callback shall be kept or not.

    We distinguish six scenarios (determined by the event type) in which the
    callback function can be called. The following table describes the values
    of the parameters @a depth, @a event, and @a parsed.

    parameter @a event | description | parameter @a depth | parameter @a parsed
    ------------------ | ----------- | ------------------ | -------------------
    parse_event_t::object_start | the parser read `{` and started to process a JSON object | depth of the parent of the JSON object | a JSON value with type discarded
    parse_event_t::key | the parser read a key of a value in an object | depth of the currently parsed JSON object | a JSON string containing the key
    parse_event_t::object_end | the parser read `}` and finished processing a JSON object | depth of the parent of the JSON object | the parsed JSON object
    parse_event_t::array_start | the parser read `[` and started to process a JSON array | depth of the parent of the JSON array | a JSON value with type discarded
    parse_event_t::array_end | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array
    parse_event_t::value | the parser finished reading a JSON value | depth of the value | the parsed JSON value

    @image html callback_events.png "Example when certain parse events are triggered"

    Discarding a value (i.e., returning `false`) has different effects
    depending on the context in which function was called:

    - Discarded values in structured types are skipped. That is, the parser
      will behave as if the discarded value was never read.
    - In case a value outside a structured type is skipped, it is replaced
      with `null`. This case happens if the top-level element is skipped.

    @param[in] depth  the depth of the recursion during parsing

    @param[in] event  an event of type parse_event_t indicating the context in
    the callback function has been called

    @param[in,out] parsed  the current intermediate parse result; note that
    writing to this value has no effect for parse_event_t::key events

    @return Whether the JSON value which called the function during parsing
    should be kept (`true`) or not (`false`). In the latter case, it is either
    skipped completely or replaced by an empty discarded object.

    @sa @ref parse for examples

    @since version 1.0.0
    */
    using parser_callback_t = typename parser::parser_callback_t;


    //////////////////
    // constructors //
    //////////////////

    /// @name constructors and destructors
    /// Constructors of class @ref basic_json, copy/move constructor, copy
    /// assignment, static functions creating objects, and the destructor.
    /// @{

    /*!
    @brief create an empty value with a given type

    Create an empty JSON value with a given type. The value will be default
    initialized with an empty value which depends on the type:

    Value type  | initial value
    ----------- | -------------
    null        | `null`
    boolean     | `false`
    string      | `""`
    number      | `0`
    object      | `{}`
    array       | `[]`

    @param[in] v  the type of the value to create

    @complexity Constant.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @liveexample{The following code shows the constructor for different @ref
    value_t values,basic_json__value_t}

    @sa @ref clear() -- restores the postcondition of this constructor

    @since version 1.0.0
    */
    basic_json(const value_t v)
        : m_type(v), m_value(v)
    {
        assert_invariant();
    }

    /*!
    @brief create a null object

    Create a `null` JSON value. It either takes a null pointer as parameter
    (explicitly creating `null`) or no parameter (implicitly creating `null`).
    The passed null pointer itself is not read -- it is only used to choose
    the right constructor.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this constructor never throws
    exceptions.

    @liveexample{The following code shows the constructor with and without a
    null pointer parameter.,basic_json__nullptr_t}

    @since version 1.0.0
    */
    basic_json(std::nullptr_t = nullptr) noexcept
        : basic_json(value_t::null)
    {
        assert_invariant();
    }

    /*!
    @brief create a JSON value

    This is a "catch all" constructor for all compatible JSON types; that is,
    types for which a `to_json()` method exists. The constructor forwards the
    parameter @a val to that method (to `json_serializer<U>::to_json` method
    with `U = uncvref_t<CompatibleType>`, to be exact).

    Template type @a CompatibleType includes, but is not limited to, the
    following types:
    - **arrays**: @ref array_t and all kinds of compatible containers such as
      `std::vector`, `std::deque`, `std::list`, `std::forward_list`,
      `std::array`, `std::valarray`, `std::set`, `std::unordered_set`,
      `std::multiset`, and `std::unordered_multiset` with a `value_type` from
      which a @ref basic_json value can be constructed.
    - **objects**: @ref object_t and all kinds of compatible associative
      containers such as `std::map`, `std::unordered_map`, `std::multimap`,
      and `std::unordered_multimap` with a `key_type` compatible to
      @ref string_t and a `value_type` from which a @ref basic_json value can
      be constructed.
    - **strings**: @ref string_t, string literals, and all compatible string
      containers can be used.
    - **numbers**: @ref number_integer_t, @ref number_unsigned_t,
      @ref number_float_t, and all convertible number types such as `int`,
      `size_t`, `int64_t`, `float` or `double` can be used.
    - **boolean**: @ref boolean_t / `bool` can be used.

    See the examples below.

    @tparam CompatibleType a type such that:
    - @a CompatibleType is not derived from `std::istream`,
    - @a CompatibleType is not @ref basic_json (to avoid hijacking copy/move
         constructors),
    - @a CompatibleType is not a @ref basic_json nested type (e.g.,
         @ref json_pointer, @ref iterator, etc ...)
    - @ref @ref json_serializer<U> has a
         `to_json(basic_json_t&, CompatibleType&&)` method

    @tparam U = `uncvref_t<CompatibleType>`

    @param[in] val the value to be forwarded to the respective constructor

    @complexity Usually linear in the size of the passed @a val, also
                depending on the implementation of the called `to_json()`
                method.

    @exceptionsafety Depends on the called constructor. For types directly
    supported by the library (i.e., all types for which no `to_json()` function
    was provided), strong guarantee holds: if an exception is thrown, there are
    no changes to any JSON value.

    @liveexample{The following code shows the constructor with several
    compatible types.,basic_json__CompatibleType}

    @since version 2.1.0
    */
    template <typename CompatibleType,
              typename U = detail::uncvref_t<CompatibleType>,
              detail::enable_if_t<
                  detail::is_compatible_type<basic_json_t, U>::value, int> = 0>
    basic_json(CompatibleType && val) noexcept(noexcept(
                JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
                                           std::forward<CompatibleType>(val))))
    {
        JSONSerializer<U>::to_json(*this, std::forward<CompatibleType>(val));
        assert_invariant();
    }

    /*!
    @brief create a container (array or object) from an initializer list

    Creates a JSON value of type array or object from the passed initializer
    list @a init. In case @a type_deduction is `true` (default), the type of
    the JSON value to be created is deducted from the initializer list @a init
    according to the following rules:

    1. If the list is empty, an empty JSON object value `{}` is created.
    2. If the list consists of pairs whose first element is a string, a JSON
       object value is created where the first elements of the pairs are
       treated as keys and the second elements are as values.
    3. In all other cases, an array is created.

    The rules aim to create the best fit between a C++ initializer list and
    JSON values. The rationale is as follows:

    1. The empty initializer list is written as `{}` which is exactly an empty
       JSON object.
    2. C++ has no way of describing mapped types other than to list a list of
       pairs. As JSON requires that keys must be of type string, rule 2 is the
       weakest constraint one can pose on initializer lists to interpret them
       as an object.
    3. In all other cases, the initializer list could not be interpreted as
       JSON object type, so interpreting it as JSON array type is safe.

    With the rules described above, the following JSON values cannot be
    expressed by an initializer list:

    - the empty array (`[]`): use @ref array(initializer_list_t)
      with an empty initializer list in this case
    - arrays whose elements satisfy rule 2: use @ref
      array(initializer_list_t) with the same initializer list
      in this case

    @note When used without parentheses around an empty initializer list, @ref
    basic_json() is called instead of this function, yielding the JSON null
    value.

    @param[in] init  initializer list with JSON values

    @param[in] type_deduction internal parameter; when set to `true`, the type
    of the JSON value is deducted from the initializer list @a init; when set
    to `false`, the type provided via @a manual_type is forced. This mode is
    used by the functions @ref array(initializer_list_t) and
    @ref object(initializer_list_t).

    @param[in] manual_type internal parameter; when @a type_deduction is set
    to `false`, the created JSON value will use the provided type (only @ref
    value_t::array and @ref value_t::object are valid); when @a type_deduction
    is set to `true`, this parameter has no effect

    @throw type_error.301 if @a type_deduction is `false`, @a manual_type is
    `value_t::object`, but @a init contains an element which is not a pair
    whose first element is a string. In this case, the constructor could not
    create an object. If @a type_deduction would have be `true`, an array
    would have been created. See @ref object(initializer_list_t)
    for an example.

    @complexity Linear in the size of the initializer list @a init.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @liveexample{The example below shows how JSON values are created from
    initializer lists.,basic_json__list_init_t}

    @sa @ref array(initializer_list_t) -- create a JSON array
    value from an initializer list
    @sa @ref object(initializer_list_t) -- create a JSON object
    value from an initializer list

    @since version 1.0.0
    */
    basic_json(initializer_list_t init,
               bool type_deduction = true,
               value_t manual_type = value_t::array)
    {
        // check if each element is an array with two elements whose first
        // element is a string
        bool is_an_object = std::all_of(init.begin(), init.end(),
                                        [](const detail::json_ref<basic_json>& element_ref)
        {
            return (element_ref->is_array() and element_ref->size() == 2 and (*element_ref)[0].is_string());
        });

        // adjust type if type deduction is not wanted
        if (not type_deduction)
        {
            // if array is wanted, do not create an object though possible
            if (manual_type == value_t::array)
            {
                is_an_object = false;
            }

            // if object is wanted but impossible, throw an exception
            if (JSON_UNLIKELY(manual_type == value_t::object and not is_an_object))
            {
                JSON_THROW(type_error::create(301, "cannot create object from initializer list"));
            }
        }

        if (is_an_object)
        {
            // the initializer list is a list of pairs -> create object
            m_type = value_t::object;
            m_value = value_t::object;

            std::for_each(init.begin(), init.end(), [this](const detail::json_ref<basic_json>& element_ref)
            {
                auto element = element_ref.moved_or_copied();
                m_value.object->emplace(
                    std::move(*((*element.m_value.array)[0].m_value.string)),
                    std::move((*element.m_value.array)[1]));
            });
        }
        else
        {
            // the initializer list describes an array -> create array
            m_type = value_t::array;
            m_value.array = create<array_t>(init.begin(), init.end());
        }

        assert_invariant();
    }

    /*!
    @brief explicitly create an array from an initializer list

    Creates a JSON array value from a given initializer list. That is, given a
    list of values `a, b, c`, creates the JSON value `[a, b, c]`. If the
    initializer list is empty, the empty array `[]` is created.

    @note This function is only needed to express two edge cases that cannot
    be realized with the initializer list constructor (@ref
    basic_json(initializer_list_t, bool, value_t)). These cases
    are:
    1. creating an array whose elements are all pairs whose first element is a
    string -- in this case, the initializer list constructor would create an
    object, taking the first elements as keys
    2. creating an empty array -- passing the empty initializer list to the
    initializer list constructor yields an empty object

    @param[in] init  initializer list with JSON values to create an array from
    (optional)

    @return JSON array value

    @complexity Linear in the size of @a init.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @liveexample{The following code shows an example for the `array`
    function.,array}

    @sa @ref basic_json(initializer_list_t, bool, value_t) --
    create a JSON value from an initializer list
    @sa @ref object(initializer_list_t) -- create a JSON object
    value from an initializer list

    @since version 1.0.0
    */
    static basic_json array(initializer_list_t init = {})
    {
        return basic_json(init, false, value_t::array);
    }

    /*!
    @brief explicitly create an object from an initializer list

    Creates a JSON object value from a given initializer list. The initializer
    lists elements must be pairs, and their first elements must be strings. If
    the initializer list is empty, the empty object `{}` is created.

    @note This function is only added for symmetry reasons. In contrast to the
    related function @ref array(initializer_list_t), there are
    no cases which can only be expressed by this function. That is, any
    initializer list @a init can also be passed to the initializer list
    constructor @ref basic_json(initializer_list_t, bool, value_t).

    @param[in] init  initializer list to create an object from (optional)

    @return JSON object value

    @throw type_error.301 if @a init is not a list of pairs whose first
    elements are strings. In this case, no object can be created. When such a
    value is passed to @ref basic_json(initializer_list_t, bool, value_t),
    an array would have been created from the passed initializer list @a init.
    See example below.

    @complexity Linear in the size of @a init.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @liveexample{The following code shows an example for the `object`
    function.,object}

    @sa @ref basic_json(initializer_list_t, bool, value_t) --
    create a JSON value from an initializer list
    @sa @ref array(initializer_list_t) -- create a JSON array
    value from an initializer list

    @since version 1.0.0
    */
    static basic_json object(initializer_list_t init = {})
    {
        return basic_json(init, false, value_t::object);
    }

    /*!
    @brief construct an array with count copies of given value

    Constructs a JSON array value by creating @a cnt copies of a passed value.
    In case @a cnt is `0`, an empty array is created.

    @param[in] cnt  the number of JSON copies of @a val to create
    @param[in] val  the JSON value to copy

    @post `std::distance(begin(),end()) == cnt` holds.

    @complexity Linear in @a cnt.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @liveexample{The following code shows examples for the @ref
    basic_json(size_type\, const basic_json&)
    constructor.,basic_json__size_type_basic_json}

    @since version 1.0.0
    */
    basic_json(size_type cnt, const basic_json& val)
        : m_type(value_t::array)
    {
        m_value.array = create<array_t>(cnt, val);
        assert_invariant();
    }

    /*!
    @brief construct a JSON container given an iterator range

    Constructs the JSON value with the contents of the range `[first, last)`.
    The semantics depends on the different types a JSON value can have:
    - In case of a null type, invalid_iterator.206 is thrown.
    - In case of other primitive types (number, boolean, or string), @a first
      must be `begin()` and @a last must be `end()`. In this case, the value is
      copied. Otherwise, invalid_iterator.204 is thrown.
    - In case of structured types (array, object), the constructor behaves as
      similar versions for `std::vector` or `std::map`; that is, a JSON array
      or object is constructed from the values in the range.

    @tparam InputIT an input iterator type (@ref iterator or @ref
    const_iterator)

    @param[in] first begin of the range to copy from (included)
    @param[in] last end of the range to copy from (excluded)

    @pre Iterators @a first and @a last must be initialized. **This
         precondition is enforced with an assertion (see warning).** If
         assertions are switched off, a violation of this precondition yields
         undefined behavior.

    @pre Range `[first, last)` is valid. Usually, this precondition cannot be
         checked efficiently. Only certain edge cases are detected; see the
         description of the exceptions below. A violation of this precondition
         yields undefined behavior.

    @warning A precondition is enforced with a runtime assertion that will
             result in calling `std::abort` if this precondition is not met.
             Assertions can be disabled by defining `NDEBUG` at compile time.
             See http://en.cppreference.com/w/cpp/error/assert for more
             information.

    @throw invalid_iterator.201 if iterators @a first and @a last are not
    compatible (i.e., do not belong to the same JSON value). In this case,
    the range `[first, last)` is undefined.
    @throw invalid_iterator.204 if iterators @a first and @a last belong to a
    primitive type (number, boolean, or string), but @a first does not point
    to the first element any more. In this case, the range `[first, last)` is
    undefined. See example code below.
    @throw invalid_iterator.206 if iterators @a first and @a last belong to a
    null value. In this case, the range `[first, last)` is undefined.

    @complexity Linear in distance between @a first and @a last.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @liveexample{The example below shows several ways to create JSON values by
    specifying a subrange with iterators.,basic_json__InputIt_InputIt}

    @since version 1.0.0
    */
    template<class InputIT, typename std::enable_if<
                 std::is_same<InputIT, typename basic_json_t::iterator>::value or
                 std::is_same<InputIT, typename basic_json_t::const_iterator>::value, int>::type = 0>
    basic_json(InputIT first, InputIT last)
    {
        assert(first.m_object != nullptr);
        assert(last.m_object != nullptr);

        // make sure iterator fits the current value
        if (JSON_UNLIKELY(first.m_object != last.m_object))
        {
            JSON_THROW(invalid_iterator::create(201, "iterators are not compatible"));
        }

        // copy type from first iterator
        m_type = first.m_object->m_type;

        // check if iterator range is complete for primitive values
        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (JSON_UNLIKELY(not first.m_it.primitive_iterator.is_begin()
                                  or not last.m_it.primitive_iterator.is_end()))
                {
                    JSON_THROW(invalid_iterator::create(204, "iterators out of range"));
                }
                break;
            }

            default:
                break;
        }

        switch (m_type)
        {
            case value_t::number_integer:
            {
                m_value.number_integer = first.m_object->m_value.number_integer;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value.number_unsigned = first.m_object->m_value.number_unsigned;
                break;
            }

            case value_t::number_float:
            {
                m_value.number_float = first.m_object->m_value.number_float;
                break;
            }

            case value_t::boolean:
            {
                m_value.boolean = first.m_object->m_value.boolean;
                break;
            }

            case value_t::string:
            {
                m_value = *first.m_object->m_value.string;
                break;
            }

            case value_t::object:
            {
                m_value.object = create<object_t>(first.m_it.object_iterator,
                                                  last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                m_value.array = create<array_t>(first.m_it.array_iterator,
                                                last.m_it.array_iterator);
                break;
            }

            default:
                JSON_THROW(invalid_iterator::create(206, "cannot construct with iterators from " +
                                                    std::string(first.m_object->type_name())));
        }

        assert_invariant();
    }


    ///////////////////////////////////////
    // other constructors and destructor //
    ///////////////////////////////////////

    /// @private
    basic_json(const detail::json_ref<basic_json>& ref)
        : basic_json(ref.moved_or_copied())
    {}

    /*!
    @brief copy constructor

    Creates a copy of a given JSON value.

    @param[in] other  the JSON value to copy

    @post `*this == other`

    @complexity Linear in the size of @a other.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes to any JSON value.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is linear.
    - As postcondition, it holds: `other == basic_json(other)`.

    @liveexample{The following code shows an example for the copy
    constructor.,basic_json__basic_json}

    @since version 1.0.0
    */
    basic_json(const basic_json& other)
        : m_type(other.m_type)
    {
        // check of passed value is valid
        other.assert_invariant();

        switch (m_type)
        {
            case value_t::object:
            {
                m_value = *other.m_value.object;
                break;
            }

            case value_t::array:
            {
                m_value = *other.m_value.array;
                break;
            }

            case value_t::string:
            {
                m_value = *other.m_value.string;
                break;
            }

            case value_t::boolean:
            {
                m_value = other.m_value.boolean;
                break;
            }

            case value_t::number_integer:
            {
                m_value = other.m_value.number_integer;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value = other.m_value.number_unsigned;
                break;
            }

            case value_t::number_float:
            {
                m_value = other.m_value.number_float;
                break;
            }

            default:
                break;
        }

        assert_invariant();
    }

    /*!
    @brief move constructor

    Move constructor. Constructs a JSON value with the contents of the given
    value @a other using move semantics. It "steals" the resources from @a
    other and leaves it as JSON null value.

    @param[in,out] other  value to move to this object

    @post `*this` has the same value as @a other before the call.
    @post @a other is a JSON null value.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this constructor never throws
    exceptions.

    @requirement This function helps `basic_json` satisfying the
    [MoveConstructible](http://en.cppreference.com/w/cpp/concept/MoveConstructible)
    requirements.

    @liveexample{The code below shows the move constructor explicitly called
    via std::move.,basic_json__moveconstructor}

    @since version 1.0.0
    */
    basic_json(basic_json&& other) noexcept
        : m_type(std::move(other.m_type)),
          m_value(std::move(other.m_value))
    {
        // check that passed value is valid
        other.assert_invariant();

        // invalidate payload
        other.m_type = value_t::null;
        other.m_value = {};

        assert_invariant();
    }

    /*!
    @brief copy assignment

    Copy assignment operator. Copies a JSON value via the "copy and swap"
    strategy: It is expressed in terms of the copy constructor, destructor,
    and the `swap()` member function.

    @param[in] other  value to copy from

    @complexity Linear.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is linear.

    @liveexample{The code below shows and example for the copy assignment. It
    creates a copy of value `a` which is then swapped with `b`. Finally\, the
    copy of `a` (which is the null value after the swap) is
    destroyed.,basic_json__copyassignment}

    @since version 1.0.0
    */
    reference& operator=(basic_json other) noexcept (
        std::is_nothrow_move_constructible<value_t>::value and
        std::is_nothrow_move_assignable<value_t>::value and
        std::is_nothrow_move_constructible<json_value>::value and
        std::is_nothrow_move_assignable<json_value>::value
    )
    {
        // check that passed value is valid
        other.assert_invariant();

        using std::swap;
        swap(m_type, other.m_type);
        swap(m_value, other.m_value);

        assert_invariant();
        return *this;
    }

    /*!
    @brief destructor

    Destroys the JSON value and frees all allocated memory.

    @complexity Linear.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is linear.
    - All stored elements are destroyed and all memory is freed.

    @since version 1.0.0
    */
    ~basic_json() noexcept
    {
        assert_invariant();
        m_value.destroy(m_type);
    }

    /// @}

  public:
    ///////////////////////
    // object inspection //
    ///////////////////////

    /// @name object inspection
    /// Functions to inspect the type of a JSON value.
    /// @{

    /*!
    @brief serialization

    Serialization function for JSON values. The function tries to mimic
    Python's `json.dumps()` function, and currently supports its @a indent
    and @a ensure_ascii parameters.

    @param[in] indent If indent is nonnegative, then array elements and object
    members will be pretty-printed with that indent level. An indent level of
    `0` will only insert newlines. `-1` (the default) selects the most compact
    representation.
    @param[in] indent_char The character to use for indentation if @a indent is
    greater than `0`. The default is ` ` (space).
    @param[in] ensure_ascii If @a ensure_ascii is true, all non-ASCII characters
    in the output are escaped with `\uXXXX` sequences, and the result consists
    of ASCII characters only.

    @return string containing the serialization of the JSON value

    @throw type_error.316 if a string stored inside the JSON value is not
                          UTF-8 encoded

    @complexity Linear.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @liveexample{The following example shows the effect of different @a indent\,
    @a indent_char\, and @a ensure_ascii parameters to the result of the
    serialization.,dump}

    @see https://docs.python.org/2/library/json.html#json.dump

    @since version 1.0.0; indentation character @a indent_char, option
           @a ensure_ascii and exceptions added in version 3.0.0
    */
    string_t dump(const int indent = -1, const char indent_char = ' ',
                  const bool ensure_ascii = false) const
    {
        string_t result;
        serializer s(detail::output_adapter<char>(result), indent_char);

        if (indent >= 0)
        {
            s.dump(*this, true, ensure_ascii, static_cast<unsigned int>(indent));
        }
        else
        {
            s.dump(*this, false, ensure_ascii, 0);
        }

        return result;
    }

    /*!
    @brief return the type of the JSON value (explicit)

    Return the type of the JSON value as a value from the @ref value_t
    enumeration.

    @return the type of the JSON value
            Value type                | return value
            ------------------------- | -------------------------
            null                      | value_t::null
            boolean                   | value_t::boolean
            string                    | value_t::string
            number (integer)          | value_t::number_integer
            number (unsigned integer) | value_t::number_unsigned
            number (floating-point)   | value_t::number_float
            object                    | value_t::object
            array                     | value_t::array
            discarded                 | value_t::discarded

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `type()` for all JSON
    types.,type}

    @sa @ref operator value_t() -- return the type of the JSON value (implicit)
    @sa @ref type_name() -- return the type as string

    @since version 1.0.0
    */
    constexpr value_t type() const noexcept
    {
        return m_type;
    }

    /*!
    @brief return whether type is primitive

    This function returns true if and only if the JSON type is primitive
    (string, number, boolean, or null).

    @return `true` if type is primitive (string, number, boolean, or null),
    `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_primitive()` for all JSON
    types.,is_primitive}

    @sa @ref is_structured() -- returns whether JSON value is structured
    @sa @ref is_null() -- returns whether JSON value is `null`
    @sa @ref is_string() -- returns whether JSON value is a string
    @sa @ref is_boolean() -- returns whether JSON value is a boolean
    @sa @ref is_number() -- returns whether JSON value is a number

    @since version 1.0.0
    */
    constexpr bool is_primitive() const noexcept
    {
        return is_null() or is_string() or is_boolean() or is_number();
    }

    /*!
    @brief return whether type is structured

    This function returns true if and only if the JSON type is structured
    (array or object).

    @return `true` if type is structured (array or object), `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_structured()` for all JSON
    types.,is_structured}

    @sa @ref is_primitive() -- returns whether value is primitive
    @sa @ref is_array() -- returns whether value is an array
    @sa @ref is_object() -- returns whether value is an object

    @since version 1.0.0
    */
    constexpr bool is_structured() const noexcept
    {
        return is_array() or is_object();
    }

    /*!
    @brief return whether value is null

    This function returns true if and only if the JSON value is null.

    @return `true` if type is null, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_null()` for all JSON
    types.,is_null}

    @since version 1.0.0
    */
    constexpr bool is_null() const noexcept
    {
        return (m_type == value_t::null);
    }

    /*!
    @brief return whether value is a boolean

    This function returns true if and only if the JSON value is a boolean.

    @return `true` if type is boolean, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_boolean()` for all JSON
    types.,is_boolean}

    @since version 1.0.0
    */
    constexpr bool is_boolean() const noexcept
    {
        return (m_type == value_t::boolean);
    }

    /*!
    @brief return whether value is a number

    This function returns true if and only if the JSON value is a number. This
    includes both integer (signed and unsigned) and floating-point values.

    @return `true` if type is number (regardless whether integer, unsigned
    integer or floating-type), `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number()` for all JSON
    types.,is_number}

    @sa @ref is_number_integer() -- check if value is an integer or unsigned
    integer number
    @sa @ref is_number_unsigned() -- check if value is an unsigned integer
    number
    @sa @ref is_number_float() -- check if value is a floating-point number

    @since version 1.0.0
    */
    constexpr bool is_number() const noexcept
    {
        return is_number_integer() or is_number_float();
    }

    /*!
    @brief return whether value is an integer number

    This function returns true if and only if the JSON value is a signed or
    unsigned integer number. This excludes floating-point values.

    @return `true` if type is an integer or unsigned integer number, `false`
    otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number_integer()` for all
    JSON types.,is_number_integer}

    @sa @ref is_number() -- check if value is a number
    @sa @ref is_number_unsigned() -- check if value is an unsigned integer
    number
    @sa @ref is_number_float() -- check if value is a floating-point number

    @since version 1.0.0
    */
    constexpr bool is_number_integer() const noexcept
    {
        return (m_type == value_t::number_integer or m_type == value_t::number_unsigned);
    }

    /*!
    @brief return whether value is an unsigned integer number

    This function returns true if and only if the JSON value is an unsigned
    integer number. This excludes floating-point and signed integer values.

    @return `true` if type is an unsigned integer number, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number_unsigned()` for all
    JSON types.,is_number_unsigned}

    @sa @ref is_number() -- check if value is a number
    @sa @ref is_number_integer() -- check if value is an integer or unsigned
    integer number
    @sa @ref is_number_float() -- check if value is a floating-point number

    @since version 2.0.0
    */
    constexpr bool is_number_unsigned() const noexcept
    {
        return (m_type == value_t::number_unsigned);
    }

    /*!
    @brief return whether value is a floating-point number

    This function returns true if and only if the JSON value is a
    floating-point number. This excludes signed and unsigned integer values.

    @return `true` if type is a floating-point number, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number_float()` for all
    JSON types.,is_number_float}

    @sa @ref is_number() -- check if value is number
    @sa @ref is_number_integer() -- check if value is an integer number
    @sa @ref is_number_unsigned() -- check if value is an unsigned integer
    number

    @since version 1.0.0
    */
    constexpr bool is_number_float() const noexcept
    {
        return (m_type == value_t::number_float);
    }

    /*!
    @brief return whether value is an object

    This function returns true if and only if the JSON value is an object.

    @return `true` if type is object, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_object()` for all JSON
    types.,is_object}

    @since version 1.0.0
    */
    constexpr bool is_object() const noexcept
    {
        return (m_type == value_t::object);
    }

    /*!
    @brief return whether value is an array

    This function returns true if and only if the JSON value is an array.

    @return `true` if type is array, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_array()` for all JSON
    types.,is_array}

    @since version 1.0.0
    */
    constexpr bool is_array() const noexcept
    {
        return (m_type == value_t::array);
    }

    /*!
    @brief return whether value is a string

    This function returns true if and only if the JSON value is a string.

    @return `true` if type is string, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_string()` for all JSON
    types.,is_string}

    @since version 1.0.0
    */
    constexpr bool is_string() const noexcept
    {
        return (m_type == value_t::string);
    }

    /*!
    @brief return whether value is discarded

    This function returns true if and only if the JSON value was discarded
    during parsing with a callback function (see @ref parser_callback_t).

    @note This function will always be `false` for JSON values after parsing.
    That is, discarded values can only occur during parsing, but will be
    removed when inside a structured value or replaced by null in other cases.

    @return `true` if type is discarded, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_discarded()` for all JSON
    types.,is_discarded}

    @since version 1.0.0
    */
    constexpr bool is_discarded() const noexcept
    {
        return (m_type == value_t::discarded);
    }

    /*!
    @brief return the type of the JSON value (implicit)

    Implicitly return the type of the JSON value as a value from the @ref
    value_t enumeration.

    @return the type of the JSON value

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies the @ref value_t operator for
    all JSON types.,operator__value_t}

    @sa @ref type() -- return the type of the JSON value (explicit)
    @sa @ref type_name() -- return the type as string

    @since version 1.0.0
    */
    constexpr operator value_t() const noexcept
    {
        return m_type;
    }

    /// @}

  private:
    //////////////////
    // value access //
    //////////////////

    /// get a boolean (explicit)
    boolean_t get_impl(boolean_t* /*unused*/) const
    {
        if (JSON_LIKELY(is_boolean()))
        {
            return m_value.boolean;
        }

        JSON_THROW(type_error::create(302, "type must be boolean, but is " + std::string(type_name())));
    }

    /// get a pointer to the value (object)
    object_t* get_impl_ptr(object_t* /*unused*/) noexcept
    {
        return is_object() ? m_value.object : nullptr;
    }

    /// get a pointer to the value (object)
    constexpr const object_t* get_impl_ptr(const object_t* /*unused*/) const noexcept
    {
        return is_object() ? m_value.object : nullptr;
    }

    /// get a pointer to the value (array)
    array_t* get_impl_ptr(array_t* /*unused*/) noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

    /// get a pointer to the value (array)
    constexpr const array_t* get_impl_ptr(const array_t* /*unused*/) const noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

    /// get a pointer to the value (string)
    string_t* get_impl_ptr(string_t* /*unused*/) noexcept
    {
        return is_string() ? m_value.string : nullptr;
    }

    /// get a pointer to the value (string)
    constexpr const string_t* get_impl_ptr(const string_t* /*unused*/) const noexcept
    {
        return is_string() ? m_value.string : nullptr;
    }

    /// get a pointer to the value (boolean)
    boolean_t* get_impl_ptr(boolean_t* /*unused*/) noexcept
    {
        return is_boolean() ? &m_value.boolean : nullptr;
    }

    /// get a pointer to the value (boolean)
    constexpr const boolean_t* get_impl_ptr(const boolean_t* /*unused*/) const noexcept
    {
        return is_boolean() ? &m_value.boolean : nullptr;
    }

    /// get a pointer to the value (integer number)
    number_integer_t* get_impl_ptr(number_integer_t* /*unused*/) noexcept
    {
        return is_number_integer() ? &m_value.number_integer : nullptr;
    }

    /// get a pointer to the value (integer number)
    constexpr const number_integer_t* get_impl_ptr(const number_integer_t* /*unused*/) const noexcept
    {
        return is_number_integer() ? &m_value.number_integer : nullptr;
    }

    /// get a pointer to the value (unsigned number)
    number_unsigned_t* get_impl_ptr(number_unsigned_t* /*unused*/) noexcept
    {
        return is_number_unsigned() ? &m_value.number_unsigned : nullptr;
    }

    /// get a pointer to the value (unsigned number)
    constexpr const number_unsigned_t* get_impl_ptr(const number_unsigned_t* /*unused*/) const noexcept
    {
        return is_number_unsigned() ? &m_value.number_unsigned : nullptr;
    }

    /// get a pointer to the value (floating-point number)
    number_float_t* get_impl_ptr(number_float_t* /*unused*/) noexcept
    {
        return is_number_float() ? &m_value.number_float : nullptr;
    }

    /// get a pointer to the value (floating-point number)
    constexpr const number_float_t* get_impl_ptr(const number_float_t* /*unused*/) const noexcept
    {
        return is_number_float() ? &m_value.number_float : nullptr;
    }

    /*!
    @brief helper function to implement get_ref()

    This function helps to implement get_ref() without code duplication for
    const and non-const overloads

    @tparam ThisType will be deduced as `basic_json` or `const basic_json`

    @throw type_error.303 if ReferenceType does not match underlying value
    type of the current JSON
    */
    template<typename ReferenceType, typename ThisType>
    static ReferenceType get_ref_impl(ThisType& obj)
    {
        // delegate the call to get_ptr<>()
        auto ptr = obj.template get_ptr<typename std::add_pointer<ReferenceType>::type>();

        if (JSON_LIKELY(ptr != nullptr))
        {
            return *ptr;
        }

        JSON_THROW(type_error::create(303, "incompatible ReferenceType for get_ref, actual type is " + std::string(obj.type_name())));
    }

  public:
    /// @name value access
    /// Direct access to the stored value of a JSON value.
    /// @{

    /*!
    @brief get special-case overload

    This overloads avoids a lot of template boilerplate, it can be seen as the
    identity method

    @tparam BasicJsonType == @ref basic_json

    @return a copy of *this

    @complexity Constant.

    @since version 2.1.0
    */
    template<typename BasicJsonType, detail::enable_if_t<
                 std::is_same<typename std::remove_const<BasicJsonType>::type, basic_json_t>::value,
                 int> = 0>
    basic_json get() const
    {
        return *this;
    }

    /*!
    @brief get a value (explicit)

    Explicit type conversion between the JSON value and a compatible value
    which is [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible)
    and [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible).
    The value is converted by calling the @ref json_serializer<ValueType>
    `from_json()` method.

    The function is equivalent to executing
    @code {.cpp}
    ValueType ret;
    JSONSerializer<ValueType>::from_json(*this, ret);
    return ret;
    @endcode

    This overloads is chosen if:
    - @a ValueType is not @ref basic_json,
    - @ref json_serializer<ValueType> has a `from_json()` method of the form
      `void from_json(const basic_json&, ValueType&)`, and
    - @ref json_serializer<ValueType> does not have a `from_json()` method of
      the form `ValueType from_json(const basic_json&)`

    @tparam ValueTypeCV the provided value type
    @tparam ValueType the returned value type

    @return copy of the JSON value, converted to @a ValueType

    @throw what @ref json_serializer<ValueType> `from_json()` method throws

    @liveexample{The example below shows several conversions from JSON values
    to other types. There a few things to note: (1) Floating-point numbers can
    be converted to integers\, (2) A JSON array can be converted to a standard
    `std::vector<short>`\, (3) A JSON object can be converted to C++
    associative containers such as `std::unordered_map<std::string\,
    json>`.,get__ValueType_const}

    @since version 2.1.0
    */
    template<typename ValueTypeCV, typename ValueType = detail::uncvref_t<ValueTypeCV>,
             detail::enable_if_t <
                 not std::is_same<basic_json_t, ValueType>::value and
                 detail::has_from_json<basic_json_t, ValueType>::value and
                 not detail::has_non_default_from_json<basic_json_t, ValueType>::value,
                 int> = 0>
    ValueType get() const noexcept(noexcept(
                                       JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
    {
        // we cannot static_assert on ValueTypeCV being non-const, because
        // there is support for get<const basic_json_t>(), which is why we
        // still need the uncvref
        static_assert(not std::is_reference<ValueTypeCV>::value,
                      "get() cannot be used with reference types, you might want to use get_ref()");
        static_assert(std::is_default_constructible<ValueType>::value,
                      "types must be DefaultConstructible when used with get()");

        ValueType ret;
        JSONSerializer<ValueType>::from_json(*this, ret);
        return ret;
    }

    /*!
    @brief get a value (explicit); special case

    Explicit type conversion between the JSON value and a compatible value
    which is **not** [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible)
    and **not** [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible).
    The value is converted by calling the @ref json_serializer<ValueType>
    `from_json()` method.

    The function is equivalent to executing
    @code {.cpp}
    return JSONSerializer<ValueTypeCV>::from_json(*this);
    @endcode

    This overloads is chosen if:
    - @a ValueType is not @ref basic_json and
    - @ref json_serializer<ValueType> has a `from_json()` method of the form
      `ValueType from_json(const basic_json&)`

    @note If @ref json_serializer<ValueType> has both overloads of
    `from_json()`, this one is chosen.

    @tparam ValueTypeCV the provided value type
    @tparam ValueType the returned value type

    @return copy of the JSON value, converted to @a ValueType

    @throw what @ref json_serializer<ValueType> `from_json()` method throws

    @since version 2.1.0
    */
    template<typename ValueTypeCV, typename ValueType = detail::uncvref_t<ValueTypeCV>,
             detail::enable_if_t<not std::is_same<basic_json_t, ValueType>::value and
                                 detail::has_non_default_from_json<basic_json_t, ValueType>::value,
                                 int> = 0>
    ValueType get() const noexcept(noexcept(
                                       JSONSerializer<ValueTypeCV>::from_json(std::declval<const basic_json_t&>())))
    {
        static_assert(not std::is_reference<ValueTypeCV>::value,
                      "get() cannot be used with reference types, you might want to use get_ref()");
        return JSONSerializer<ValueTypeCV>::from_json(*this);
    }

    /*!
    @brief get a pointer value (explicit)

    Explicit pointer access to the internally stored JSON value. No copies are
    made.

    @warning The pointer becomes invalid if the underlying JSON object
    changes.

    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
    @ref number_unsigned_t, or @ref number_float_t.

    @return pointer to the internally stored JSON value if the requested
    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise

    @complexity Constant.

    @liveexample{The example below shows how pointers to internal values of a
    JSON value can be requested. Note that no type conversions are made and a
    `nullptr` is returned if the value and the requested pointer type does not
    match.,get__PointerType}

    @sa @ref get_ptr() for explicit pointer-member access

    @since version 1.0.0
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value, int>::type = 0>
    PointerType get() noexcept
    {
        // delegate the call to get_ptr
        return get_ptr<PointerType>();
    }

    /*!
    @brief get a pointer value (explicit)
    @copydoc get()
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value, int>::type = 0>
    constexpr const PointerType get() const noexcept
    {
        // delegate the call to get_ptr
        return get_ptr<PointerType>();
    }

    /*!
    @brief get a pointer value (implicit)

    Implicit pointer access to the internally stored JSON value. No copies are
    made.

    @warning Writing data to the pointee of the result yields an undefined
    state.

    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
    assertion.

    @return pointer to the internally stored JSON value if the requested
    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise

    @complexity Constant.

    @liveexample{The example below shows how pointers to internal values of a
    JSON value can be requested. Note that no type conversions are made and a
    `nullptr` is returned if the value and the requested pointer type does not
    match.,get_ptr}

    @since version 1.0.0
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value, int>::type = 0>
    PointerType get_ptr() noexcept
    {
        // get the type of the PointerType (remove pointer and const)
        using pointee_t = typename std::remove_const<typename
                          std::remove_pointer<typename
                          std::remove_const<PointerType>::type>::type>::type;
        // make sure the type matches the allowed types
        static_assert(
            std::is_same<object_t, pointee_t>::value
            or std::is_same<array_t, pointee_t>::value
            or std::is_same<string_t, pointee_t>::value
            or std::is_same<boolean_t, pointee_t>::value
            or std::is_same<number_integer_t, pointee_t>::value
            or std::is_same<number_unsigned_t, pointee_t>::value
            or std::is_same<number_float_t, pointee_t>::value
            , "incompatible pointer type");

        // delegate the call to get_impl_ptr<>()
        return get_impl_ptr(static_cast<PointerType>(nullptr));
    }

    /*!
    @brief get a pointer value (implicit)
    @copydoc get_ptr()
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value and
                 std::is_const<typename std::remove_pointer<PointerType>::type>::value, int>::type = 0>
    constexpr const PointerType get_ptr() const noexcept
    {
        // get the type of the PointerType (remove pointer and const)
        using pointee_t = typename std::remove_const<typename
                          std::remove_pointer<typename
                          std::remove_const<PointerType>::type>::type>::type;
        // make sure the type matches the allowed types
        static_assert(
            std::is_same<object_t, pointee_t>::value
            or std::is_same<array_t, pointee_t>::value
            or std::is_same<string_t, pointee_t>::value
            or std::is_same<boolean_t, pointee_t>::value
            or std::is_same<number_integer_t, pointee_t>::value
            or std::is_same<number_unsigned_t, pointee_t>::value
            or std::is_same<number_float_t, pointee_t>::value
            , "incompatible pointer type");

        // delegate the call to get_impl_ptr<>() const
        return get_impl_ptr(static_cast<PointerType>(nullptr));
    }

    /*!
    @brief get a reference value (implicit)

    Implicit reference access to the internally stored JSON value. No copies
    are made.

    @warning Writing data to the referee of the result yields an undefined
    state.

    @tparam ReferenceType reference type; must be a reference to @ref array_t,
    @ref object_t, @ref string_t, @ref boolean_t, @ref number_integer_t, or
    @ref number_float_t. Enforced by static assertion.

    @return reference to the internally stored JSON value if the requested
    reference type @a ReferenceType fits to the JSON value; throws
    type_error.303 otherwise

    @throw type_error.303 in case passed type @a ReferenceType is incompatible
    with the stored JSON value; see example below

    @complexity Constant.

    @liveexample{The example shows several calls to `get_ref()`.,get_ref}

    @since version 1.1.0
    */
    template<typename ReferenceType, typename std::enable_if<
                 std::is_reference<ReferenceType>::value, int>::type = 0>
    ReferenceType get_ref()
    {
        // delegate call to get_ref_impl
        return get_ref_impl<ReferenceType>(*this);
    }

    /*!
    @brief get a reference value (implicit)
    @copydoc get_ref()
    */
    template<typename ReferenceType, typename std::enable_if<
                 std::is_reference<ReferenceType>::value and
                 std::is_const<typename std::remove_reference<ReferenceType>::type>::value, int>::type = 0>
    ReferenceType get_ref() const
    {
        // delegate call to get_ref_impl
        return get_ref_impl<ReferenceType>(*this);
    }

    /*!
    @brief get a value (implicit)

    Implicit type conversion between the JSON value and a compatible value.
    The call is realized by calling @ref get() const.

    @tparam ValueType non-pointer type compatible to the JSON value, for
    instance `int` for JSON integer numbers, `bool` for JSON booleans, or
    `std::vector` types for JSON arrays. The character type of @ref string_t
    as well as an initializer list of this type is excluded to avoid
    ambiguities as these types implicitly convert to `std::string`.

    @return copy of the JSON value, converted to type @a ValueType

    @throw type_error.302 in case passed type @a ValueType is incompatible
    to the JSON value type (e.g., the JSON value is of type boolean, but a
    string is requested); see example below

    @complexity Linear in the size of the JSON value.

    @liveexample{The example below shows several conversions from JSON values
    to other types. There a few things to note: (1) Floating-point numbers can
    be converted to integers\, (2) A JSON array can be converted to a standard
    `std::vector<short>`\, (3) A JSON object can be converted to C++
    associative containers such as `std::unordered_map<std::string\,
    json>`.,operator__ValueType}

    @since version 1.0.0
    */
    template < typename ValueType, typename std::enable_if <
                   not std::is_pointer<ValueType>::value and
                   not std::is_same<ValueType, detail::json_ref<basic_json>>::value and
                   not std::is_same<ValueType, typename string_t::value_type>::value
#ifndef _MSC_VER  // fix for issue #167 operator<< ambiguity under VS2015
                   and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
#endif
#if defined(JSON_HAS_CPP_17)
                   and not std::is_same<ValueType, typename std::string_view>::value
#endif
                   , int >::type = 0 >
    operator ValueType() const
    {
        // delegate the call to get<>() const
        return get<ValueType>();
    }

    /// @}


    ////////////////////
    // element access //
    ////////////////////

    /// @name element access
    /// Access to the JSON value.
    /// @{

    /*!
    @brief access specified array element with bounds checking

    Returns a reference to the element at specified location @a idx, with
    bounds checking.

    @param[in] idx  index of the element to access

    @return reference to the element at index @a idx

    @throw type_error.304 if the JSON value is not an array; in this case,
    calling `at` with an index makes no sense. See example below.
    @throw out_of_range.401 if the index @a idx is out of range of the array;
    that is, `idx >= size()`. See example below.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Constant.

    @since version 1.0.0

    @liveexample{The example below shows how array elements can be read and
    written using `at()`. It also demonstrates the different exceptions that
    can be thrown.,at__size_type}
    */
    reference at(size_type idx)
    {
        // at only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            JSON_TRY
            {
                return m_value.array->at(idx);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(out_of_range::create(401, "array index " + std::to_string(idx) + " is out of range"));
            }
        }
        else
        {
            JSON_THROW(type_error::create(304, "cannot use at() with " + std::string(type_name())));
        }
    }

    /*!
    @brief access specified array element with bounds checking

    Returns a const reference to the element at specified location @a idx,
    with bounds checking.

    @param[in] idx  index of the element to access

    @return const reference to the element at index @a idx

    @throw type_error.304 if the JSON value is not an array; in this case,
    calling `at` with an index makes no sense. See example below.
    @throw out_of_range.401 if the index @a idx is out of range of the array;
    that is, `idx >= size()`. See example below.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Constant.

    @since version 1.0.0

    @liveexample{The example below shows how array elements can be read using
    `at()`. It also demonstrates the different exceptions that can be thrown.,
    at__size_type_const}
    */
    const_reference at(size_type idx) const
    {
        // at only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            JSON_TRY
            {
                return m_value.array->at(idx);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(out_of_range::create(401, "array index " + std::to_string(idx) + " is out of range"));
            }
        }
        else
        {
            JSON_THROW(type_error::create(304, "cannot use at() with " + std::string(type_name())));
        }
    }

    /*!
    @brief access specified object element with bounds checking

    Returns a reference to the element at with specified key @a key, with
    bounds checking.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw type_error.304 if the JSON value is not an object; in this case,
    calling `at` with a key makes no sense. See example below.
    @throw out_of_range.403 if the key @a key is is not stored in the object;
    that is, `find(key) == end()`. See example below.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Logarithmic in the size of the container.

    @sa @ref operator[](const typename object_t::key_type&) for unchecked
    access by reference
    @sa @ref value() for access by value with a default value

    @since version 1.0.0

    @liveexample{The example below shows how object elements can be read and
    written using `at()`. It also demonstrates the different exceptions that
    can be thrown.,at__object_t_key_type}
    */
    reference at(const typename object_t::key_type& key)
    {
        // at only works for objects
        if (JSON_LIKELY(is_object()))
        {
            JSON_TRY
            {
                return m_value.object->at(key);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(out_of_range::create(403, "key '" + key + "' not found"));
            }
        }
        else
        {
            JSON_THROW(type_error::create(304, "cannot use at() with " + std::string(type_name())));
        }
    }

    /*!
    @brief access specified object element with bounds checking

    Returns a const reference to the element at with specified key @a key,
    with bounds checking.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @throw type_error.304 if the JSON value is not an object; in this case,
    calling `at` with a key makes no sense. See example below.
    @throw out_of_range.403 if the key @a key is is not stored in the object;
    that is, `find(key) == end()`. See example below.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Logarithmic in the size of the container.

    @sa @ref operator[](const typename object_t::key_type&) for unchecked
    access by reference
    @sa @ref value() for access by value with a default value

    @since version 1.0.0

    @liveexample{The example below shows how object elements can be read using
    `at()`. It also demonstrates the different exceptions that can be thrown.,
    at__object_t_key_type_const}
    */
    const_reference at(const typename object_t::key_type& key) const
    {
        // at only works for objects
        if (JSON_LIKELY(is_object()))
        {
            JSON_TRY
            {
                return m_value.object->at(key);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(out_of_range::create(403, "key '" + key + "' not found"));
            }
        }
        else
        {
            JSON_THROW(type_error::create(304, "cannot use at() with " + std::string(type_name())));
        }
    }

    /*!
    @brief access specified array element

    Returns a reference to the element at specified location @a idx.

    @note If @a idx is beyond the range of the array (i.e., `idx >= size()`),
    then the array is silently filled up with `null` values to make `idx` a
    valid reference to the last stored element.

    @param[in] idx  index of the element to access

    @return reference to the element at index @a idx

    @throw type_error.305 if the JSON value is not an array or null; in that
    cases, using the [] operator with an index makes no sense.

    @complexity Constant if @a idx is in the range of the array. Otherwise
    linear in `idx - size()`.

    @liveexample{The example below shows how array elements can be read and
    written using `[]` operator. Note the addition of `null`
    values.,operatorarray__size_type}

    @since version 1.0.0
    */
    reference operator[](size_type idx)
    {
        // implicitly convert null value to an empty array
        if (is_null())
        {
            m_type = value_t::array;
            m_value.array = create<array_t>();
            assert_invariant();
        }

        // operator[] only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            // fill up array with null values if given idx is outside range
            if (idx >= m_value.array->size())
            {
                m_value.array->insert(m_value.array->end(),
                                      idx - m_value.array->size() + 1,
                                      basic_json());
            }

            return m_value.array->operator[](idx);
        }

        JSON_THROW(type_error::create(305, "cannot use operator[] with " + std::string(type_name())));
    }

    /*!
    @brief access specified array element

    Returns a const reference to the element at specified location @a idx.

    @param[in] idx  index of the element to access

    @return const reference to the element at index @a idx

    @throw type_error.305 if the JSON value is not an array; in that case,
    using the [] operator with an index makes no sense.

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read using
    the `[]` operator.,operatorarray__size_type_const}

    @since version 1.0.0
    */
    const_reference operator[](size_type idx) const
    {
        // const operator[] only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            return m_value.array->operator[](idx);
        }

        JSON_THROW(type_error::create(305, "cannot use operator[] with " + std::string(type_name())));
    }

    /*!
    @brief access specified object element

    Returns a reference to the element at with specified key @a key.

    @note If @a key is not found in the object, then it is silently added to
    the object and filled with a `null` value to make `key` a valid reference.
    In case the value was `null` before, it is converted to an object.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw type_error.305 if the JSON value is not an object or null; in that
    cases, using the [] operator with a key makes no sense.

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using the `[]` operator.,operatorarray__key_type}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    reference operator[](const typename object_t::key_type& key)
    {
        // implicitly convert null value to an empty object
        if (is_null())
        {
            m_type = value_t::object;
            m_value.object = create<object_t>();
            assert_invariant();
        }

        // operator[] only works for objects
        if (JSON_LIKELY(is_object()))
        {
            return m_value.object->operator[](key);
        }

        JSON_THROW(type_error::create(305, "cannot use operator[] with " + std::string(type_name())));
    }

    /*!
    @brief read-only access specified object element

    Returns a const reference to the element at with specified key @a key. No
    bounds checking is performed.

    @warning If the element with key @a key does not exist, the behavior is
    undefined.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @pre The element with key @a key must exist. **This precondition is
         enforced with an assertion.**

    @throw type_error.305 if the JSON value is not an object; in that case,
    using the [] operator with a key makes no sense.

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    the `[]` operator.,operatorarray__key_type_const}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    const_reference operator[](const typename object_t::key_type& key) const
    {
        // const operator[] only works for objects
        if (JSON_LIKELY(is_object()))
        {
            assert(m_value.object->find(key) != m_value.object->end());
            return m_value.object->find(key)->second;
        }

        JSON_THROW(type_error::create(305, "cannot use operator[] with " + std::string(type_name())));
    }

    /*!
    @brief access specified object element

    Returns a reference to the element at with specified key @a key.

    @note If @a key is not found in the object, then it is silently added to
    the object and filled with a `null` value to make `key` a valid reference.
    In case the value was `null` before, it is converted to an object.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw type_error.305 if the JSON value is not an object or null; in that
    cases, using the [] operator with a key makes no sense.

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using the `[]` operator.,operatorarray__key_type}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.1.0
    */
    template<typename T>
    reference operator[](T* key)
    {
        // implicitly convert null to object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }

        // at only works for objects
        if (JSON_LIKELY(is_object()))
        {
            return m_value.object->operator[](key);
        }

        JSON_THROW(type_error::create(305, "cannot use operator[] with " + std::string(type_name())));
    }

    /*!
    @brief read-only access specified object element

    Returns a const reference to the element at with specified key @a key. No
    bounds checking is performed.

    @warning If the element with key @a key does not exist, the behavior is
    undefined.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @pre The element with key @a key must exist. **This precondition is
         enforced with an assertion.**

    @throw type_error.305 if the JSON value is not an object; in that case,
    using the [] operator with a key makes no sense.

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    the `[]` operator.,operatorarray__key_type_const}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.1.0
    */
    template<typename T>
    const_reference operator[](T* key) const
    {
        // at only works for objects
        if (JSON_LIKELY(is_object()))
        {
            assert(m_value.object->find(key) != m_value.object->end());
            return m_value.object->find(key)->second;
        }

        JSON_THROW(type_error::create(305, "cannot use operator[] with " + std::string(type_name())));
    }

    /*!
    @brief access specified object element with default value

    Returns either a copy of an object's element at the specified key @a key
    or a given default value if no element with key @a key exists.

    The function is basically equivalent to executing
    @code {.cpp}
    try {
        return at(key);
    } catch(out_of_range) {
        return default_value;
    }
    @endcode

    @note Unlike @ref at(const typename object_t::key_type&), this function
    does not throw if the given key @a key was not found.

    @note Unlike @ref operator[](const typename object_t::key_type& key), this
    function does not implicitly add an element to the position defined by @a
    key. This function is furthermore also applicable to const objects.

    @param[in] key  key of the element to access
    @param[in] default_value  the value to return if @a key is not found

    @tparam ValueType type compatible to JSON values, for instance `int` for
    JSON integer numbers, `bool` for JSON booleans, or `std::vector` types for
    JSON arrays. Note the type of the expected value at @a key and the default
    value @a default_value must be compatible.

    @return copy of the element at key @a key or @a default_value if @a key
    is not found

    @throw type_error.306 if the JSON value is not an object; in that case,
    using `value()` with a key makes no sense.

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be queried
    with a default value.,basic_json__value}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref operator[](const typename object_t::key_type&) for unchecked
    access by reference

    @since version 1.0.0
    */
    template<class ValueType, typename std::enable_if<
                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
    ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
    {
        // at only works for objects
        if (JSON_LIKELY(is_object()))
        {
            // if key is found, return value and given default value otherwise
            const auto it = find(key);
            if (it != end())
            {
                return *it;
            }

            return default_value;
        }

        JSON_THROW(type_error::create(306, "cannot use value() with " + std::string(type_name())));
    }

    /*!
    @brief overload for a default value of type const char*
    @copydoc basic_json::value(const typename object_t::key_type&, ValueType) const
    */
    string_t value(const typename object_t::key_type& key, const char* default_value) const
    {
        return value(key, string_t(default_value));
    }

    /*!
    @brief access specified object element via JSON Pointer with default value

    Returns either a copy of an object's element at the specified key @a key
    or a given default value if no element with key @a key exists.

    The function is basically equivalent to executing
    @code {.cpp}
    try {
        return at(ptr);
    } catch(out_of_range) {
        return default_value;
    }
    @endcode

    @note Unlike @ref at(const json_pointer&), this function does not throw
    if the given key @a key was not found.

    @param[in] ptr  a JSON pointer to the element to access
    @param[in] default_value  the value to return if @a ptr found no value

    @tparam ValueType type compatible to JSON values, for instance `int` for
    JSON integer numbers, `bool` for JSON booleans, or `std::vector` types for
    JSON arrays. Note the type of the expected value at @a key and the default
    value @a default_value must be compatible.

    @return copy of the element at key @a key or @a default_value if @a key
    is not found

    @throw type_error.306 if the JSON value is not an objec; in that case,
    using `value()` with a key makes no sense.

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be queried
    with a default value.,basic_json__value_ptr}

    @sa @ref operator[](const json_pointer&) for unchecked access by reference

    @since version 2.0.2
    */
    template<class ValueType, typename std::enable_if<
                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
    ValueType value(const json_pointer& ptr, const ValueType& default_value) const
    {
        // at only works for objects
        if (JSON_LIKELY(is_object()))
        {
            // if pointer resolves a value, return it or use default value
            JSON_TRY
            {
                return ptr.get_checked(this);
            }
            JSON_CATCH (out_of_range&)
            {
                return default_value;
            }
        }

        JSON_THROW(type_error::create(306, "cannot use value() with " + std::string(type_name())));
    }

    /*!
    @brief overload for a default value of type const char*
    @copydoc basic_json::value(const json_pointer&, ValueType) const
    */
    string_t value(const json_pointer& ptr, const char* default_value) const
    {
        return value(ptr, string_t(default_value));
    }

    /*!
    @brief access the first element

    Returns a reference to the first element in the container. For a JSON
    container `c`, the expression `c.front()` is equivalent to `*c.begin()`.

    @return In case of a structured type (array or object), a reference to the
    first element is returned. In case of number, string, or boolean values, a
    reference to the value is returned.

    @complexity Constant.

    @pre The JSON value must not be `null` (would throw `std::out_of_range`)
    or an empty array or object (undefined behavior, **guarded by
    assertions**).
    @post The JSON value remains unchanged.

    @throw invalid_iterator.214 when called on `null` value

    @liveexample{The following code shows an example for `front()`.,front}

    @sa @ref back() -- access the last element

    @since version 1.0.0
    */
    reference front()
    {
        return *begin();
    }

    /*!
    @copydoc basic_json::front()
    */
    const_reference front() const
    {
        return *cbegin();
    }

    /*!
    @brief access the last element

    Returns a reference to the last element in the container. For a JSON
    container `c`, the expression `c.back()` is equivalent to
    @code {.cpp}
    auto tmp = c.end();
    --tmp;
    return *tmp;
    @endcode

    @return In case of a structured type (array or object), a reference to the
    last element is returned. In case of number, string, or boolean values, a
    reference to the value is returned.

    @complexity Constant.

    @pre The JSON value must not be `null` (would throw `std::out_of_range`)
    or an empty array or object (undefined behavior, **guarded by
    assertions**).
    @post The JSON value remains unchanged.

    @throw invalid_iterator.214 when called on a `null` value. See example
    below.

    @liveexample{The following code shows an example for `back()`.,back}

    @sa @ref front() -- access the first element

    @since version 1.0.0
    */
    reference back()
    {
        auto tmp = end();
        --tmp;
        return *tmp;
    }

    /*!
    @copydoc basic_json::back()
    */
    const_reference back() const
    {
        auto tmp = cend();
        --tmp;
        return *tmp;
    }

    /*!
    @brief remove element given an iterator

    Removes the element specified by iterator @a pos. The iterator @a pos must
    be valid and dereferenceable. Thus the `end()` iterator (which is valid,
    but is not dereferenceable) cannot be used as a value for @a pos.

    If called on a primitive type other than `null`, the resulting JSON value
    will be `null`.

    @param[in] pos iterator to the element to remove
    @return Iterator following the last removed element. If the iterator @a
    pos refers to the last element, the `end()` iterator is returned.

    @tparam IteratorType an @ref iterator or @ref const_iterator

    @post Invalidates iterators and references at or after the point of the
    erase, including the `end()` iterator.

    @throw type_error.307 if called on a `null` value; example: `"cannot use
    erase() with null"`
    @throw invalid_iterator.202 if called on an iterator which does not belong
    to the current JSON value; example: `"iterator does not fit current
    value"`
    @throw invalid_iterator.205 if called on a primitive type with invalid
    iterator (i.e., any iterator which is not `begin()`); example: `"iterator
    out of range"`

    @complexity The complexity depends on the type:
    - objects: amortized constant
    - arrays: linear in distance between @a pos and the end of the container
    - strings: linear in the length of the string
    - other types: constant

    @liveexample{The example shows the result of `erase()` for different JSON
    types.,erase__IteratorType}

    @sa @ref erase(IteratorType, IteratorType) -- removes the elements in
    the given range
    @sa @ref erase(const typename object_t::key_type&) -- removes the element
    from an object at the given key
    @sa @ref erase(const size_type) -- removes the element from an array at
    the given index

    @since version 1.0.0
    */
    template<class IteratorType, typename std::enable_if<
                 std::is_same<IteratorType, typename basic_json_t::iterator>::value or
                 std::is_same<IteratorType, typename basic_json_t::const_iterator>::value, int>::type
             = 0>
    IteratorType erase(IteratorType pos)
    {
        // make sure iterator fits the current value
        if (JSON_UNLIKELY(this != pos.m_object))
        {
            JSON_THROW(invalid_iterator::create(202, "iterator does not fit current value"));
        }

        IteratorType result = end();

        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (JSON_UNLIKELY(not pos.m_it.primitive_iterator.is_begin()))
                {
                    JSON_THROW(invalid_iterator::create(205, "iterator out of range"));
                }

                if (is_string())
                {
                    AllocatorType<string_t> alloc;
                    std::allocator_traits<decltype(alloc)>::destroy(alloc, m_value.string);
                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, m_value.string, 1);
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                assert_invariant();
                break;
            }

            case value_t::object:
            {
                result.m_it.object_iterator = m_value.object->erase(pos.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                result.m_it.array_iterator = m_value.array->erase(pos.m_it.array_iterator);
                break;
            }

            default:
                JSON_THROW(type_error::create(307, "cannot use erase() with " + std::string(type_name())));
        }

        return result;
    }

    /*!
    @brief remove elements given an iterator range

    Removes the element specified by the range `[first; last)`. The iterator
    @a first does not need to be dereferenceable if `first == last`: erasing
    an empty range is a no-op.

    If called on a primitive type other than `null`, the resulting JSON value
    will be `null`.

    @param[in] first iterator to the beginning of the range to remove
    @param[in] last iterator past the end of the range to remove
    @return Iterator following the last removed element. If the iterator @a
    second refers to the last element, the `end()` iterator is returned.

    @tparam IteratorType an @ref iterator or @ref const_iterator

    @post Invalidates iterators and references at or after the point of the
    erase, including the `end()` iterator.

    @throw type_error.307 if called on a `null` value; example: `"cannot use
    erase() with null"`
    @throw invalid_iterator.203 if called on iterators which does not belong
    to the current JSON value; example: `"iterators do not fit current value"`
    @throw invalid_iterator.204 if called on a primitive type with invalid
    iterators (i.e., if `first != begin()` and `last != end()`); example:
    `"iterators out of range"`

    @complexity The complexity depends on the type:
    - objects: `log(size()) + std::distance(first, last)`
    - arrays: linear in the distance between @a first and @a last, plus linear
      in the distance between @a last and end of the container
    - strings: linear in the length of the string
    - other types: constant

    @liveexample{The example shows the result of `erase()` for different JSON
    types.,erase__IteratorType_IteratorType}

    @sa @ref erase(IteratorType) -- removes the element at a given position
    @sa @ref erase(const typename object_t::key_type&) -- removes the element
    from an object at the given key
    @sa @ref erase(const size_type) -- removes the element from an array at
    the given index

    @since version 1.0.0
    */
    template<class IteratorType, typename std::enable_if<
                 std::is_same<IteratorType, typename basic_json_t::iterator>::value or
                 std::is_same<IteratorType, typename basic_json_t::const_iterator>::value, int>::type
             = 0>
    IteratorType erase(IteratorType first, IteratorType last)
    {
        // make sure iterator fits the current value
        if (JSON_UNLIKELY(this != first.m_object or this != last.m_object))
        {
            JSON_THROW(invalid_iterator::create(203, "iterators do not fit current value"));
        }

        IteratorType result = end();

        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (JSON_LIKELY(not first.m_it.primitive_iterator.is_begin()
                                or not last.m_it.primitive_iterator.is_end()))
                {
                    JSON_THROW(invalid_iterator::create(204, "iterators out of range"));
                }

                if (is_string())
                {
                    AllocatorType<string_t> alloc;
                    std::allocator_traits<decltype(alloc)>::destroy(alloc, m_value.string);
                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, m_value.string, 1);
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                assert_invariant();
                break;
            }

            case value_t::object:
            {
                result.m_it.object_iterator = m_value.object->erase(first.m_it.object_iterator,
                                              last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                result.m_it.array_iterator = m_value.array->erase(first.m_it.array_iterator,
                                             last.m_it.array_iterator);
                break;
            }

            default:
                JSON_THROW(type_error::create(307, "cannot use erase() with " + std::string(type_name())));
        }

        return result;
    }

    /*!
    @brief remove element from a JSON object given a key

    Removes elements from a JSON object with the key value @a key.

    @param[in] key value of the elements to remove

    @return Number of elements removed. If @a ObjectType is the default
    `std::map` type, the return value will always be `0` (@a key was not
    found) or `1` (@a key was found).

    @post References and iterators to the erased elements are invalidated.
    Other references and iterators are not affected.

    @throw type_error.307 when called on a type other than JSON object;
    example: `"cannot use erase() with null"`

    @complexity `log(size()) + count(key)`

    @liveexample{The example shows the effect of `erase()`.,erase__key_type}

    @sa @ref erase(IteratorType) -- removes the element at a given position
    @sa @ref erase(IteratorType, IteratorType) -- removes the elements in
    the given range
    @sa @ref erase(const size_type) -- removes the element from an array at
    the given index

    @since version 1.0.0
    */
    size_type erase(const typename object_t::key_type& key)
    {
        // this erase only works for objects
        if (JSON_LIKELY(is_object()))
        {
            return m_value.object->erase(key);
        }

        JSON_THROW(type_error::create(307, "cannot use erase() with " + std::string(type_name())));
    }

    /*!
    @brief remove element from a JSON array given an index

    Removes element from a JSON array at the index @a idx.

    @param[in] idx index of the element to remove

    @throw type_error.307 when called on a type other than JSON object;
    example: `"cannot use erase() with null"`
    @throw out_of_range.401 when `idx >= size()`; example: `"array index 17
    is out of range"`

    @complexity Linear in distance between @a idx and the end of the container.

    @liveexample{The example shows the effect of `erase()`.,erase__size_type}

    @sa @ref erase(IteratorType) -- removes the element at a given position
    @sa @ref erase(IteratorType, IteratorType) -- removes the elements in
    the given range
    @sa @ref erase(const typename object_t::key_type&) -- removes the element
    from an object at the given key

    @since version 1.0.0
    */
    void erase(const size_type idx)
    {
        // this erase only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            if (JSON_UNLIKELY(idx >= size()))
            {
                JSON_THROW(out_of_range::create(401, "array index " + std::to_string(idx) + " is out of range"));
            }

            m_value.array->erase(m_value.array->begin() + static_cast<difference_type>(idx));
        }
        else
        {
            JSON_THROW(type_error::create(307, "cannot use erase() with " + std::string(type_name())));
        }
    }

    /// @}


    ////////////
    // lookup //
    ////////////

    /// @name lookup
    /// @{

    /*!
    @brief find an element in a JSON object

    Finds an element in a JSON object with key equivalent to @a key. If the
    element is not found or the JSON value is not an object, end() is
    returned.

    @note This method always returns @ref end() when executed on a JSON type
          that is not an object.

    @param[in] key key value of the element to search for.

    @return Iterator to an element with key equivalent to @a key. If no such
    element is found or the JSON value is not an object, past-the-end (see
    @ref end()) iterator is returned.

    @complexity Logarithmic in the size of the JSON object.

    @liveexample{The example shows how `find()` is used.,find__key_type}

    @since version 1.0.0
    */
    template<typename KeyT>
    iterator find(KeyT&& key)
    {
        auto result = end();

        if (is_object())
        {
            result.m_it.object_iterator = m_value.object->find(std::forward<KeyT>(key));
        }

        return result;
    }

    /*!
    @brief find an element in a JSON object
    @copydoc find(KeyT&&)
    */
    template<typename KeyT>
    const_iterator find(KeyT&& key) const
    {
        auto result = cend();

        if (is_object())
        {
            result.m_it.object_iterator = m_value.object->find(std::forward<KeyT>(key));
        }

        return result;
    }

    /*!
    @brief returns the number of occurrences of a key in a JSON object

    Returns the number of elements with key @a key. If ObjectType is the
    default `std::map` type, the return value will always be `0` (@a key was
    not found) or `1` (@a key was found).

    @note This method always returns `0` when executed on a JSON type that is
          not an object.

    @param[in] key key value of the element to count

    @return Number of elements with key @a key. If the JSON value is not an
    object, the return value will be `0`.

    @complexity Logarithmic in the size of the JSON object.

    @liveexample{The example shows how `count()` is used.,count}

    @since version 1.0.0
    */
    template<typename KeyT>
    size_type count(KeyT&& key) const
    {
        // return 0 for all nonobject types
        return is_object() ? m_value.object->count(std::forward<KeyT>(key)) : 0;
    }

    /// @}


    ///////////////
    // iterators //
    ///////////////

    /// @name iterators
    /// @{

    /*!
    @brief returns an iterator to the first element

    Returns an iterator to the first element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return iterator to the first element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.

    @liveexample{The following code shows an example for `begin()`.,begin}

    @sa @ref cbegin() -- returns a const iterator to the beginning
    @sa @ref end() -- returns an iterator to the end
    @sa @ref cend() -- returns a const iterator to the end

    @since version 1.0.0
    */
    iterator begin() noexcept
    {
        iterator result(this);
        result.set_begin();
        return result;
    }

    /*!
    @copydoc basic_json::cbegin()
    */
    const_iterator begin() const noexcept
    {
        return cbegin();
    }

    /*!
    @brief returns a const iterator to the first element

    Returns a const iterator to the first element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return const iterator to the first element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).begin()`.

    @liveexample{The following code shows an example for `cbegin()`.,cbegin}

    @sa @ref begin() -- returns an iterator to the beginning
    @sa @ref end() -- returns an iterator to the end
    @sa @ref cend() -- returns a const iterator to the end

    @since version 1.0.0
    */
    const_iterator cbegin() const noexcept
    {
        const_iterator result(this);
        result.set_begin();
        return result;
    }

    /*!
    @brief returns an iterator to one past the last element

    Returns an iterator to one past the last element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return iterator one past the last element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.

    @liveexample{The following code shows an example for `end()`.,end}

    @sa @ref cend() -- returns a const iterator to the end
    @sa @ref begin() -- returns an iterator to the beginning
    @sa @ref cbegin() -- returns a const iterator to the beginning

    @since version 1.0.0
    */
    iterator end() noexcept
    {
        iterator result(this);
        result.set_end();
        return result;
    }

    /*!
    @copydoc basic_json::cend()
    */
    const_iterator end() const noexcept
    {
        return cend();
    }

    /*!
    @brief returns a const iterator to one past the last element

    Returns a const iterator to one past the last element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return const iterator one past the last element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).end()`.

    @liveexample{The following code shows an example for `cend()`.,cend}

    @sa @ref end() -- returns an iterator to the end
    @sa @ref begin() -- returns an iterator to the beginning
    @sa @ref cbegin() -- returns a const iterator to the beginning

    @since version 1.0.0
    */
    const_iterator cend() const noexcept
    {
        const_iterator result(this);
        result.set_end();
        return result;
    }

    /*!
    @brief returns an iterator to the reverse-beginning

    Returns an iterator to the reverse-beginning; that is, the last element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `reverse_iterator(end())`.

    @liveexample{The following code shows an example for `rbegin()`.,rbegin}

    @sa @ref crbegin() -- returns a const reverse iterator to the beginning
    @sa @ref rend() -- returns a reverse iterator to the end
    @sa @ref crend() -- returns a const reverse iterator to the end

    @since version 1.0.0
    */
    reverse_iterator rbegin() noexcept
    {
        return reverse_iterator(end());
    }

    /*!
    @copydoc basic_json::crbegin()
    */
    const_reverse_iterator rbegin() const noexcept
    {
        return crbegin();
    }

    /*!
    @brief returns an iterator to the reverse-end

    Returns an iterator to the reverse-end; that is, one before the first
    element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `reverse_iterator(begin())`.

    @liveexample{The following code shows an example for `rend()`.,rend}

    @sa @ref crend() -- returns a const reverse iterator to the end
    @sa @ref rbegin() -- returns a reverse iterator to the beginning
    @sa @ref crbegin() -- returns a const reverse iterator to the beginning

    @since version 1.0.0
    */
    reverse_iterator rend() noexcept
    {
        return reverse_iterator(begin());
    }

    /*!
    @copydoc basic_json::crend()
    */
    const_reverse_iterator rend() const noexcept
    {
        return crend();
    }

    /*!
    @brief returns a const reverse iterator to the last element

    Returns a const iterator to the reverse-beginning; that is, the last
    element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).rbegin()`.

    @liveexample{The following code shows an example for `crbegin()`.,crbegin}

    @sa @ref rbegin() -- returns a reverse iterator to the beginning
    @sa @ref rend() -- returns a reverse iterator to the end
    @sa @ref crend() -- returns a const reverse iterator to the end

    @since version 1.0.0
    */
    const_reverse_iterator crbegin() const noexcept
    {
        return const_reverse_iterator(cend());
    }

    /*!
    @brief returns a const reverse iterator to one before the first

    Returns a const reverse iterator to the reverse-end; that is, one before
    the first element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).rend()`.

    @liveexample{The following code shows an example for `crend()`.,crend}

    @sa @ref rend() -- returns a reverse iterator to the end
    @sa @ref rbegin() -- returns a reverse iterator to the beginning
    @sa @ref crbegin() -- returns a const reverse iterator to the beginning

    @since version 1.0.0
    */
    const_reverse_iterator crend() const noexcept
    {
        return const_reverse_iterator(cbegin());
    }

  public:
    /*!
    @brief wrapper to access iterator member functions in range-based for

    This function allows to access @ref iterator::key() and @ref
    iterator::value() during range-based for loops. In these loops, a
    reference to the JSON values is returned, so there is no access to the
    underlying iterator.

    For loop without iterator_wrapper:

    @code{cpp}
    for (auto it = j_object.begin(); it != j_object.end(); ++it)
    {
        std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
    }
    @endcode

    Range-based for loop without iterator proxy:

    @code{cpp}
    for (auto it : j_object)
    {
        // "it" is of type json::reference and has no key() member
        std::cout << "value: " << it << '\n';
    }
    @endcode

    Range-based for loop with iterator proxy:

    @code{cpp}
    for (auto it : json::iterator_wrapper(j_object))
    {
        std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
    }
    @endcode

    @note When iterating over an array, `key()` will return the index of the
          element as string (see example).

    @param[in] ref  reference to a JSON value
    @return iteration proxy object wrapping @a ref with an interface to use in
            range-based for loops

    @liveexample{The following code shows how the wrapper is used,iterator_wrapper}

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Constant.

    @note The name of this function is not yet final and may change in the
    future.
    */
    static iteration_proxy<iterator> iterator_wrapper(reference ref) noexcept
    {
        return iteration_proxy<iterator>(ref);
    }

    /*!
    @copydoc iterator_wrapper(reference)
    */
    static iteration_proxy<const_iterator> iterator_wrapper(const_reference ref) noexcept
    {
        return iteration_proxy<const_iterator>(ref);
    }

    /*!
    @brief helper to access iterator member functions in range-based for

    This function allows to access @ref iterator::key() and @ref
    iterator::value() during range-based for loops. In these loops, a
    reference to the JSON values is returned, so there is no access to the
    underlying iterator.

    For loop without `items()` function:

    @code{cpp}
    for (auto it = j_object.begin(); it != j_object.end(); ++it)
    {
        std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
    }
    @endcode

    Range-based for loop without `items()` function:

    @code{cpp}
    for (auto it : j_object)
    {
        // "it" is of type json::reference and has no key() member
        std::cout << "value: " << it << '\n';
    }
    @endcode

    Range-based for loop with `items()` function:

    @code{cpp}
    for (auto it : j_object.items())
    {
        std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
    }
    @endcode

    @note When iterating over an array, `key()` will return the index of the
          element as string (see example). For primitive types (e.g., numbers),
          `key()` returns an empty string.

    @return iteration proxy object wrapping @a ref with an interface to use in
            range-based for loops

    @liveexample{The following code shows how the function is used.,items}

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Constant.

    @since version 3.x.x.
    */
    iteration_proxy<iterator> items() noexcept
    {
        return iteration_proxy<iterator>(*this);
    }

    /*!
    @copydoc items()
    */
    iteration_proxy<const_iterator> items() const noexcept
    {
        return iteration_proxy<const_iterator>(*this);
    }

    /// @}


    //////////////
    // capacity //
    //////////////

    /// @name capacity
    /// @{

    /*!
    @brief checks whether the container is empty.

    Checks if a JSON value has no elements (i.e. whether its @ref size is `0`).

    @return The return value depends on the different types and is
            defined as follows:
            Value type  | return value
            ----------- | -------------
            null        | `true`
            boolean     | `false`
            string      | `false`
            number      | `false`
            object      | result of function `object_t::empty()`
            array       | result of function `array_t::empty()`

    @liveexample{The following code uses `empty()` to check if a JSON
    object contains any elements.,empty}

    @complexity Constant, as long as @ref array_t and @ref object_t satisfy
    the Container concept; that is, their `empty()` functions have constant
    complexity.

    @iterators No changes.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @note This function does not return whether a string stored as JSON value
    is empty - it returns whether the JSON container itself is empty which is
    false in the case of a string.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `begin() == end()`.

    @sa @ref size() -- returns the number of elements

    @since version 1.0.0
    */
    bool empty() const noexcept
    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return true;
            }

            case value_t::array:
            {
                // delegate call to array_t::empty()
                return m_value.array->empty();
            }

            case value_t::object:
            {
                // delegate call to object_t::empty()
                return m_value.object->empty();
            }

            default:
            {
                // all other types are nonempty
                return false;
            }
        }
    }

    /*!
    @brief returns the number of elements

    Returns the number of elements in a JSON value.

    @return The return value depends on the different types and is
            defined as follows:
            Value type  | return value
            ----------- | -------------
            null        | `0`
            boolean     | `1`
            string      | `1`
            number      | `1`
            object      | result of function object_t::size()
            array       | result of function array_t::size()

    @liveexample{The following code calls `size()` on the different value
    types.,size}

    @complexity Constant, as long as @ref array_t and @ref object_t satisfy
    the Container concept; that is, their size() functions have constant
    complexity.

    @iterators No changes.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @note This function does not return the length of a string stored as JSON
    value - it returns the number of elements in the JSON value which is 1 in
    the case of a string.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `std::distance(begin(), end())`.

    @sa @ref empty() -- checks whether the container is empty
    @sa @ref max_size() -- returns the maximal number of elements

    @since version 1.0.0
    */
    size_type size() const noexcept
    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return 0;
            }

            case value_t::array:
            {
                // delegate call to array_t::size()
                return m_value.array->size();
            }

            case value_t::object:
            {
                // delegate call to object_t::size()
                return m_value.object->size();
            }

            default:
            {
                // all other types have size 1
                return 1;
            }
        }
    }

    /*!
    @brief returns the maximum possible number of elements

    Returns the maximum number of elements a JSON value is able to hold due to
    system or library implementation limitations, i.e. `std::distance(begin(),
    end())` for the JSON value.

    @return The return value depends on the different types and is
            defined as follows:
            Value type  | return value
            ----------- | -------------
            null        | `0` (same as `size()`)
            boolean     | `1` (same as `size()`)
            string      | `1` (same as `size()`)
            number      | `1` (same as `size()`)
            object      | result of function `object_t::max_size()`
            array       | result of function `array_t::max_size()`

    @liveexample{The following code calls `max_size()` on the different value
    types. Note the output is implementation specific.,max_size}

    @complexity Constant, as long as @ref array_t and @ref object_t satisfy
    the Container concept; that is, their `max_size()` functions have constant
    complexity.

    @iterators No changes.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of returning `b.size()` where `b` is the largest
      possible JSON value.

    @sa @ref size() -- returns the number of elements

    @since version 1.0.0
    */
    size_type max_size() const noexcept
    {
        switch (m_type)
        {
            case value_t::array:
            {
                // delegate call to array_t::max_size()
                return m_value.array->max_size();
            }

            case value_t::object:
            {
                // delegate call to object_t::max_size()
                return m_value.object->max_size();
            }

            default:
            {
                // all other types have max_size() == size()
                return size();
            }
        }
    }

    /// @}


    ///////////////
    // modifiers //
    ///////////////

    /// @name modifiers
    /// @{

    /*!
    @brief clears the contents

    Clears the content of a JSON value and resets it to the default value as
    if @ref basic_json(value_t) would have been called with the current value
    type from @ref type():

    Value type  | initial value
    ----------- | -------------
    null        | `null`
    boolean     | `false`
    string      | `""`
    number      | `0`
    object      | `{}`
    array       | `[]`

    @post Has the same effect as calling
    @code {.cpp}
    *this = basic_json(type());
    @endcode

    @liveexample{The example below shows the effect of `clear()` to different
    JSON types.,clear}

    @complexity Linear in the size of the JSON value.

    @iterators All iterators, pointers and references related to this container
               are invalidated.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @sa @ref basic_json(value_t) -- constructor that creates an object with the
        same value than calling `clear()`

    @since version 1.0.0
    */
    void clear() noexcept
    {
        switch (m_type)
        {
            case value_t::number_integer:
            {
                m_value.number_integer = 0;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value.number_unsigned = 0;
                break;
            }

            case value_t::number_float:
            {
                m_value.number_float = 0.0;
                break;
            }

            case value_t::boolean:
            {
                m_value.boolean = false;
                break;
            }

            case value_t::string:
            {
                m_value.string->clear();
                break;
            }

            case value_t::array:
            {
                m_value.array->clear();
                break;
            }

            case value_t::object:
            {
                m_value.object->clear();
                break;
            }

            default:
                break;
        }
    }

    /*!
    @brief add an object to an array

    Appends the given element @a val to the end of the JSON value. If the
    function is called on a JSON null value, an empty array is created before
    appending @a val.

    @param[in] val the value to add to the JSON array

    @throw type_error.308 when called on a type other than JSON array or
    null; example: `"cannot use push_back() with number"`

    @complexity Amortized constant.

    @liveexample{The example shows how `push_back()` and `+=` can be used to
    add elements to a JSON array. Note how the `null` value was silently
    converted to a JSON array.,push_back}

    @since version 1.0.0
    */
    void push_back(basic_json&& val)
    {
        // push_back only works for null objects or arrays
        if (JSON_UNLIKELY(not(is_null() or is_array())))
        {
            JSON_THROW(type_error::create(308, "cannot use push_back() with " + std::string(type_name())));
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
            assert_invariant();
        }

        // add element to array (move semantics)
        m_value.array->push_back(std::move(val));
        // invalidate object
        val.m_type = value_t::null;
    }

    /*!
    @brief add an object to an array
    @copydoc push_back(basic_json&&)
    */
    reference operator+=(basic_json&& val)
    {
        push_back(std::move(val));
        return *this;
    }

    /*!
    @brief add an object to an array
    @copydoc push_back(basic_json&&)
    */
    void push_back(const basic_json& val)
    {
        // push_back only works for null objects or arrays
        if (JSON_UNLIKELY(not(is_null() or is_array())))
        {
            JSON_THROW(type_error::create(308, "cannot use push_back() with " + std::string(type_name())));
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
            assert_invariant();
        }

        // add element to array
        m_value.array->push_back(val);
    }

    /*!
    @brief add an object to an array
    @copydoc push_back(basic_json&&)
    */
    reference operator+=(const basic_json& val)
    {
        push_back(val);
        return *this;
    }

    /*!
    @brief add an object to an object

    Inserts the given element @a val to the JSON object. If the function is
    called on a JSON null value, an empty object is created before inserting
    @a val.

    @param[in] val the value to add to the JSON object

    @throw type_error.308 when called on a type other than JSON object or
    null; example: `"cannot use push_back() with number"`

    @complexity Logarithmic in the size of the container, O(log(`size()`)).

    @liveexample{The example shows how `push_back()` and `+=` can be used to
    add elements to a JSON object. Note how the `null` value was silently
    converted to a JSON object.,push_back__object_t__value}

    @since version 1.0.0
    */
    void push_back(const typename object_t::value_type& val)
    {
        // push_back only works for null objects or objects
        if (JSON_UNLIKELY(not(is_null() or is_object())))
        {
            JSON_THROW(type_error::create(308, "cannot use push_back() with " + std::string(type_name())));
        }

        // transform null object into an object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }

        // add element to array
        m_value.object->insert(val);
    }

    /*!
    @brief add an object to an object
    @copydoc push_back(const typename object_t::value_type&)
    */
    reference operator+=(const typename object_t::value_type& val)
    {
        push_back(val);
        return *this;
    }

    /*!
    @brief add an object to an object

    This function allows to use `push_back` with an initializer list. In case

    1. the current value is an object,
    2. the initializer list @a init contains only two elements, and
    3. the first element of @a init is a string,

    @a init is converted into an object element and added using
    @ref push_back(const typename object_t::value_type&). Otherwise, @a init
    is converted to a JSON value and added using @ref push_back(basic_json&&).

    @param[in] init  an initializer list

    @complexity Linear in the size of the initializer list @a init.

    @note This function is required to resolve an ambiguous overload error,
          because pairs like `{"key", "value"}` can be both interpreted as
          `object_t::value_type` or `std::initializer_list<basic_json>`, see
          https://github.com/nlohmann/json/issues/235 for more information.

    @liveexample{The example shows how initializer lists are treated as
    objects when possible.,push_back__initializer_list}
    */
    void push_back(initializer_list_t init)
    {
        if (is_object() and init.size() == 2 and (*init.begin())->is_string())
        {
            basic_json&& key = init.begin()->moved_or_copied();
            push_back(typename object_t::value_type(
                          std::move(key.get_ref<string_t&>()), (init.begin() + 1)->moved_or_copied()));
        }
        else
        {
            push_back(basic_json(init));
        }
    }

    /*!
    @brief add an object to an object
    @copydoc push_back(initializer_list_t)
    */
    reference operator+=(initializer_list_t init)
    {
        push_back(init);
        return *this;
    }

    /*!
    @brief add an object to an array

    Creates a JSON value from the passed parameters @a args to the end of the
    JSON value. If the function is called on a JSON null value, an empty array
    is created before appending the value created from @a args.

    @param[in] args arguments to forward to a constructor of @ref basic_json
    @tparam Args compatible types to create a @ref basic_json object

    @throw type_error.311 when called on a type other than JSON array or
    null; example: `"cannot use emplace_back() with number"`

    @complexity Amortized constant.

    @liveexample{The example shows how `push_back()` can be used to add
    elements to a JSON array. Note how the `null` value was silently converted
    to a JSON array.,emplace_back}

    @since version 2.0.8
    */
    template<class... Args>
    void emplace_back(Args&& ... args)
    {
        // emplace_back only works for null objects or arrays
        if (JSON_UNLIKELY(not(is_null() or is_array())))
        {
            JSON_THROW(type_error::create(311, "cannot use emplace_back() with " + std::string(type_name())));
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
            assert_invariant();
        }

        // add element to array (perfect forwarding)
        m_value.array->emplace_back(std::forward<Args>(args)...);
    }

    /*!
    @brief add an object to an object if key does not exist

    Inserts a new element into a JSON object constructed in-place with the
    given @a args if there is no element with the key in the container. If the
    function is called on a JSON null value, an empty object is created before
    appending the value created from @a args.

    @param[in] args arguments to forward to a constructor of @ref basic_json
    @tparam Args compatible types to create a @ref basic_json object

    @return a pair consisting of an iterator to the inserted element, or the
            already-existing element if no insertion happened, and a bool
            denoting whether the insertion took place.

    @throw type_error.311 when called on a type other than JSON object or
    null; example: `"cannot use emplace() with number"`

    @complexity Logarithmic in the size of the container, O(log(`size()`)).

    @liveexample{The example shows how `emplace()` can be used to add elements
    to a JSON object. Note how the `null` value was silently converted to a
    JSON object. Further note how no value is added if there was already one
    value stored with the same key.,emplace}

    @since version 2.0.8
    */
    template<class... Args>
    std::pair<iterator, bool> emplace(Args&& ... args)
    {
        // emplace only works for null objects or arrays
        if (JSON_UNLIKELY(not(is_null() or is_object())))
        {
            JSON_THROW(type_error::create(311, "cannot use emplace() with " + std::string(type_name())));
        }

        // transform null object into an object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }

        // add element to array (perfect forwarding)
        auto res = m_value.object->emplace(std::forward<Args>(args)...);
        // create result iterator and set iterator to the result of emplace
        auto it = begin();
        it.m_it.object_iterator = res.first;

        // return pair of iterator and boolean
        return {it, res.second};
    }

    /*!
    @brief inserts element

    Inserts element @a val before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] val element to insert
    @return iterator pointing to the inserted @a val.

    @throw type_error.309 if called on JSON values other than arrays;
    example: `"cannot use insert() with string"`
    @throw invalid_iterator.202 if @a pos is not an iterator of *this;
    example: `"iterator does not fit current value"`

    @complexity Constant plus linear in the distance between @a pos and end of
    the container.

    @liveexample{The example shows how `insert()` is used.,insert}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, const basic_json& val)
    {
        // insert only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            // check if iterator pos fits to this JSON value
            if (JSON_UNLIKELY(pos.m_object != this))
            {
                JSON_THROW(invalid_iterator::create(202, "iterator does not fit current value"));
            }

            // insert to array and return iterator
            iterator result(this);
            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, val);
            return result;
        }

        JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
    }

    /*!
    @brief inserts element
    @copydoc insert(const_iterator, const basic_json&)
    */
    iterator insert(const_iterator pos, basic_json&& val)
    {
        return insert(pos, val);
    }

    /*!
    @brief inserts elements

    Inserts @a cnt copies of @a val before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] cnt number of copies of @a val to insert
    @param[in] val element to insert
    @return iterator pointing to the first element inserted, or @a pos if
    `cnt==0`

    @throw type_error.309 if called on JSON values other than arrays; example:
    `"cannot use insert() with string"`
    @throw invalid_iterator.202 if @a pos is not an iterator of *this;
    example: `"iterator does not fit current value"`

    @complexity Linear in @a cnt plus linear in the distance between @a pos
    and end of the container.

    @liveexample{The example shows how `insert()` is used.,insert__count}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, size_type cnt, const basic_json& val)
    {
        // insert only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            // check if iterator pos fits to this JSON value
            if (JSON_UNLIKELY(pos.m_object != this))
            {
                JSON_THROW(invalid_iterator::create(202, "iterator does not fit current value"));
            }

            // insert to array and return iterator
            iterator result(this);
            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
            return result;
        }

        JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
    }

    /*!
    @brief inserts elements

    Inserts elements from range `[first, last)` before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] first begin of the range of elements to insert
    @param[in] last end of the range of elements to insert

    @throw type_error.309 if called on JSON values other than arrays; example:
    `"cannot use insert() with string"`
    @throw invalid_iterator.202 if @a pos is not an iterator of *this;
    example: `"iterator does not fit current value"`
    @throw invalid_iterator.210 if @a first and @a last do not belong to the
    same JSON value; example: `"iterators do not fit"`
    @throw invalid_iterator.211 if @a first or @a last are iterators into
    container for which insert is called; example: `"passed iterators may not
    belong to container"`

    @return iterator pointing to the first element inserted, or @a pos if
    `first==last`

    @complexity Linear in `std::distance(first, last)` plus linear in the
    distance between @a pos and end of the container.

    @liveexample{The example shows how `insert()` is used.,insert__range}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, const_iterator first, const_iterator last)
    {
        // insert only works for arrays
        if (JSON_UNLIKELY(not is_array()))
        {
            JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
        }

        // check if iterator pos fits to this JSON value
        if (JSON_UNLIKELY(pos.m_object != this))
        {
            JSON_THROW(invalid_iterator::create(202, "iterator does not fit current value"));
        }

        // check if range iterators belong to the same JSON object
        if (JSON_UNLIKELY(first.m_object != last.m_object))
        {
            JSON_THROW(invalid_iterator::create(210, "iterators do not fit"));
        }

        if (JSON_UNLIKELY(first.m_object == this))
        {
            JSON_THROW(invalid_iterator::create(211, "passed iterators may not belong to container"));
        }

        // insert to array and return iterator
        iterator result(this);
        result.m_it.array_iterator = m_value.array->insert(
                                         pos.m_it.array_iterator,
                                         first.m_it.array_iterator,
                                         last.m_it.array_iterator);
        return result;
    }

    /*!
    @brief inserts elements

    Inserts elements from initializer list @a ilist before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] ilist initializer list to insert the values from

    @throw type_error.309 if called on JSON values other than arrays; example:
    `"cannot use insert() with string"`
    @throw invalid_iterator.202 if @a pos is not an iterator of *this;
    example: `"iterator does not fit current value"`

    @return iterator pointing to the first element inserted, or @a pos if
    `ilist` is empty

    @complexity Linear in `ilist.size()` plus linear in the distance between
    @a pos and end of the container.

    @liveexample{The example shows how `insert()` is used.,insert__ilist}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, initializer_list_t ilist)
    {
        // insert only works for arrays
        if (JSON_UNLIKELY(not is_array()))
        {
            JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
        }

        // check if iterator pos fits to this JSON value
        if (JSON_UNLIKELY(pos.m_object != this))
        {
            JSON_THROW(invalid_iterator::create(202, "iterator does not fit current value"));
        }

        // insert to array and return iterator
        iterator result(this);
        result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, ilist.begin(), ilist.end());
        return result;
    }

    /*!
    @brief inserts elements

    Inserts elements from range `[first, last)`.

    @param[in] first begin of the range of elements to insert
    @param[in] last end of the range of elements to insert

    @throw type_error.309 if called on JSON values other than objects; example:
    `"cannot use insert() with string"`
    @throw invalid_iterator.202 if iterator @a first or @a last does does not
    point to an object; example: `"iterators first and last must point to
    objects"`
    @throw invalid_iterator.210 if @a first and @a last do not belong to the
    same JSON value; example: `"iterators do not fit"`

    @complexity Logarithmic: `O(N*log(size() + N))`, where `N` is the number
    of elements to insert.

    @liveexample{The example shows how `insert()` is used.,insert__range_object}

    @since version 3.0.0
    */
    void insert(const_iterator first, const_iterator last)
    {
        // insert only works for objects
        if (JSON_UNLIKELY(not is_object()))
        {
            JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
        }

        // check if range iterators belong to the same JSON object
        if (JSON_UNLIKELY(first.m_object != last.m_object))
        {
            JSON_THROW(invalid_iterator::create(210, "iterators do not fit"));
        }

        // passed iterators must belong to objects
        if (JSON_UNLIKELY(not first.m_object->is_object()))
        {
            JSON_THROW(invalid_iterator::create(202, "iterators first and last must point to objects"));
        }

        m_value.object->insert(first.m_it.object_iterator, last.m_it.object_iterator);
    }

    /*!
    @brief updates a JSON object from another object, overwriting existing keys

    Inserts all values from JSON object @a j and overwrites existing keys.

    @param[in] j  JSON object to read values from

    @throw type_error.312 if called on JSON values other than objects; example:
    `"cannot use update() with string"`

    @complexity O(N*log(size() + N)), where N is the number of elements to
                insert.

    @liveexample{The example shows how `update()` is used.,update}

    @sa https://docs.python.org/3.6/library/stdtypes.html#dict.update

    @since version 3.0.0
    */
    void update(const_reference j)
    {
        // implicitly convert null value to an empty object
        if (is_null())
        {
            m_type = value_t::object;
            m_value.object = create<object_t>();
            assert_invariant();
        }

        if (JSON_UNLIKELY(not is_object()))
        {
            JSON_THROW(type_error::create(312, "cannot use update() with " + std::string(type_name())));
        }
        if (JSON_UNLIKELY(not j.is_object()))
        {
            JSON_THROW(type_error::create(312, "cannot use update() with " + std::string(j.type_name())));
        }

        for (auto it = j.cbegin(); it != j.cend(); ++it)
        {
            m_value.object->operator[](it.key()) = it.value();
        }
    }

    /*!
    @brief updates a JSON object from another object, overwriting existing keys

    Inserts all values from from range `[first, last)` and overwrites existing
    keys.

    @param[in] first begin of the range of elements to insert
    @param[in] last end of the range of elements to insert

    @throw type_error.312 if called on JSON values other than objects; example:
    `"cannot use update() with string"`
    @throw invalid_iterator.202 if iterator @a first or @a last does does not
    point to an object; example: `"iterators first and last must point to
    objects"`
    @throw invalid_iterator.210 if @a first and @a last do not belong to the
    same JSON value; example: `"iterators do not fit"`

    @complexity O(N*log(size() + N)), where N is the number of elements to
                insert.

    @liveexample{The example shows how `update()` is used__range.,update}

    @sa https://docs.python.org/3.6/library/stdtypes.html#dict.update

    @since version 3.0.0
    */
    void update(const_iterator first, const_iterator last)
    {
        // implicitly convert null value to an empty object
        if (is_null())
        {
            m_type = value_t::object;
            m_value.object = create<object_t>();
            assert_invariant();
        }

        if (JSON_UNLIKELY(not is_object()))
        {
            JSON_THROW(type_error::create(312, "cannot use update() with " + std::string(type_name())));
        }

        // check if range iterators belong to the same JSON object
        if (JSON_UNLIKELY(first.m_object != last.m_object))
        {
            JSON_THROW(invalid_iterator::create(210, "iterators do not fit"));
        }

        // passed iterators must belong to objects
        if (JSON_UNLIKELY(not first.m_object->is_object()
                          or not first.m_object->is_object()))
        {
            JSON_THROW(invalid_iterator::create(202, "iterators first and last must point to objects"));
        }

        for (auto it = first; it != last; ++it)
        {
            m_value.object->operator[](it.key()) = it.value();
        }
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of the JSON value with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other JSON value to exchange the contents with

    @complexity Constant.

    @liveexample{The example below shows how JSON values can be swapped with
    `swap()`.,swap__reference}

    @since version 1.0.0
    */
    void swap(reference other) noexcept (
        std::is_nothrow_move_constructible<value_t>::value and
        std::is_nothrow_move_assignable<value_t>::value and
        std::is_nothrow_move_constructible<json_value>::value and
        std::is_nothrow_move_assignable<json_value>::value
    )
    {
        std::swap(m_type, other.m_type);
        std::swap(m_value, other.m_value);
        assert_invariant();
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of a JSON array with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other array to exchange the contents with

    @throw type_error.310 when JSON value is not an array; example: `"cannot
    use swap() with string"`

    @complexity Constant.

    @liveexample{The example below shows how arrays can be swapped with
    `swap()`.,swap__array_t}

    @since version 1.0.0
    */
    void swap(array_t& other)
    {
        // swap only works for arrays
        if (JSON_LIKELY(is_array()))
        {
            std::swap(*(m_value.array), other);
        }
        else
        {
            JSON_THROW(type_error::create(310, "cannot use swap() with " + std::string(type_name())));
        }
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of a JSON object with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other object to exchange the contents with

    @throw type_error.310 when JSON value is not an object; example:
    `"cannot use swap() with string"`

    @complexity Constant.

    @liveexample{The example below shows how objects can be swapped with
    `swap()`.,swap__object_t}

    @since version 1.0.0
    */
    void swap(object_t& other)
    {
        // swap only works for objects
        if (JSON_LIKELY(is_object()))
        {
            std::swap(*(m_value.object), other);
        }
        else
        {
            JSON_THROW(type_error::create(310, "cannot use swap() with " + std::string(type_name())));
        }
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of a JSON string with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other string to exchange the contents with

    @throw type_error.310 when JSON value is not a string; example: `"cannot
    use swap() with boolean"`

    @complexity Constant.

    @liveexample{The example below shows how strings can be swapped with
    `swap()`.,swap__string_t}

    @since version 1.0.0
    */
    void swap(string_t& other)
    {
        // swap only works for strings
        if (JSON_LIKELY(is_string()))
        {
            std::swap(*(m_value.string), other);
        }
        else
        {
            JSON_THROW(type_error::create(310, "cannot use swap() with " + std::string(type_name())));
        }
    }

    /// @}

  public:
    //////////////////////////////////////////
    // lexicographical comparison operators //
    //////////////////////////////////////////

    /// @name lexicographical comparison operators
    /// @{

    /*!
    @brief comparison: equal

    Compares two JSON values for equality according to the following rules:
    - Two JSON values are equal if (1) they are from the same type and (2)
      their stored values are the same according to their respective
      `operator==`.
    - Integer and floating-point numbers are automatically converted before
      comparison. Note than two NaN values are always treated as unequal.
    - Two JSON null values are equal.

    @note Floating-point inside JSON values numbers are compared with
    `json::number_float_t::operator==` which is `double::operator==` by
    default. To compare floating-point while respecting an epsilon, an alternative
    [comparison function](https://github.com/mariokonrad/marnav/blob/master/src/marnav/math/floatingpoint.hpp#L34-#L39)
    could be used, for instance
    @code {.cpp}
    template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>
    inline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept
    {
        return std::abs(a - b) <= epsilon;
    }
    @endcode

    @note NaN values never compare equal to themselves or to other NaN values.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether the values @a lhs and @a rhs are equal

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__equal}

    @since version 1.0.0
    */
    friend bool operator==(const_reference lhs, const_reference rhs) noexcept
    {
        const auto lhs_type = lhs.type();
        const auto rhs_type = rhs.type();

        if (lhs_type == rhs_type)
        {
            switch (lhs_type)
            {
                case value_t::array:
                    return (*lhs.m_value.array == *rhs.m_value.array);

                case value_t::object:
                    return (*lhs.m_value.object == *rhs.m_value.object);

                case value_t::null:
                    return true;

                case value_t::string:
                    return (*lhs.m_value.string == *rhs.m_value.string);

                case value_t::boolean:
                    return (lhs.m_value.boolean == rhs.m_value.boolean);

                case value_t::number_integer:
                    return (lhs.m_value.number_integer == rhs.m_value.number_integer);

                case value_t::number_unsigned:
                    return (lhs.m_value.number_unsigned == rhs.m_value.number_unsigned);

                case value_t::number_float:
                    return (lhs.m_value.number_float == rhs.m_value.number_float);

                default:
                    return false;
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return (static_cast<number_float_t>(lhs.m_value.number_integer) == rhs.m_value.number_float);
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_integer)
        {
            return (lhs.m_value.number_float == static_cast<number_float_t>(rhs.m_value.number_integer));
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_float)
        {
            return (static_cast<number_float_t>(lhs.m_value.number_unsigned) == rhs.m_value.number_float);
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_unsigned)
        {
            return (lhs.m_value.number_float == static_cast<number_float_t>(rhs.m_value.number_unsigned));
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_integer)
        {
            return (static_cast<number_integer_t>(lhs.m_value.number_unsigned) == rhs.m_value.number_integer);
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_unsigned)
        {
            return (lhs.m_value.number_integer == static_cast<number_integer_t>(rhs.m_value.number_unsigned));
        }

        return false;
    }

    /*!
    @brief comparison: equal
    @copydoc operator==(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator==(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs == basic_json(rhs));
    }

    /*!
    @brief comparison: equal
    @copydoc operator==(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator==(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) == rhs);
    }

    /*!
    @brief comparison: not equal

    Compares two JSON values for inequality by calculating `not (lhs == rhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether the values @a lhs and @a rhs are not equal

    @complexity Linear.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__notequal}

    @since version 1.0.0
    */
    friend bool operator!=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs == rhs);
    }

    /*!
    @brief comparison: not equal
    @copydoc operator!=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator!=(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs != basic_json(rhs));
    }

    /*!
    @brief comparison: not equal
    @copydoc operator!=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator!=(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) != rhs);
    }

    /*!
    @brief comparison: less than

    Compares whether one JSON value @a lhs is less than another JSON value @a
    rhs according to the following rules:
    - If @a lhs and @a rhs have the same type, the values are compared using
      the default `<` operator.
    - Integer and floating-point numbers are automatically converted before
      comparison
    - In case @a lhs and @a rhs have different types, the values are ignored
      and the order of the types is considered, see
      @ref operator<(const value_t, const value_t).

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is less than @a rhs

    @complexity Linear.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__less}

    @since version 1.0.0
    */
    friend bool operator<(const_reference lhs, const_reference rhs) noexcept
    {
        const auto lhs_type = lhs.type();
        const auto rhs_type = rhs.type();

        if (lhs_type == rhs_type)
        {
            switch (lhs_type)
            {
                case value_t::array:
                    return (*lhs.m_value.array) < (*rhs.m_value.array);

                case value_t::object:
                    return *lhs.m_value.object < *rhs.m_value.object;

                case value_t::null:
                    return false;

                case value_t::string:
                    return *lhs.m_value.string < *rhs.m_value.string;

                case value_t::boolean:
                    return lhs.m_value.boolean < rhs.m_value.boolean;

                case value_t::number_integer:
                    return lhs.m_value.number_integer < rhs.m_value.number_integer;

                case value_t::number_unsigned:
                    return lhs.m_value.number_unsigned < rhs.m_value.number_unsigned;

                case value_t::number_float:
                    return lhs.m_value.number_float < rhs.m_value.number_float;

                default:
                    return false;
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_integer) < rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_integer)
        {
            return lhs.m_value.number_float < static_cast<number_float_t>(rhs.m_value.number_integer);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_unsigned) < rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_float < static_cast<number_float_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_integer < static_cast<number_integer_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_integer)
        {
            return static_cast<number_integer_t>(lhs.m_value.number_unsigned) < rhs.m_value.number_integer;
        }

        // We only reach this line if we cannot compare values. In that case,
        // we compare types. Note we have to call the operator explicitly,
        // because MSVC has problems otherwise.
        return operator<(lhs_type, rhs_type);
    }

    /*!
    @brief comparison: less than
    @copydoc operator<(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator<(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs < basic_json(rhs));
    }

    /*!
    @brief comparison: less than
    @copydoc operator<(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator<(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) < rhs);
    }

    /*!
    @brief comparison: less than or equal

    Compares whether one JSON value @a lhs is less than or equal to another
    JSON value by calculating `not (rhs < lhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is less than or equal to @a rhs

    @complexity Linear.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__greater}

    @since version 1.0.0
    */
    friend bool operator<=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (rhs < lhs);
    }

    /*!
    @brief comparison: less than or equal
    @copydoc operator<=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator<=(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs <= basic_json(rhs));
    }

    /*!
    @brief comparison: less than or equal
    @copydoc operator<=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator<=(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) <= rhs);
    }

    /*!
    @brief comparison: greater than

    Compares whether one JSON value @a lhs is greater than another
    JSON value by calculating `not (lhs <= rhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is greater than to @a rhs

    @complexity Linear.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__lessequal}

    @since version 1.0.0
    */
    friend bool operator>(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs <= rhs);
    }

    /*!
    @brief comparison: greater than
    @copydoc operator>(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator>(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs > basic_json(rhs));
    }

    /*!
    @brief comparison: greater than
    @copydoc operator>(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator>(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) > rhs);
    }

    /*!
    @brief comparison: greater than or equal

    Compares whether one JSON value @a lhs is greater than or equal to another
    JSON value by calculating `not (lhs < rhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is greater than or equal to @a rhs

    @complexity Linear.

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__greaterequal}

    @since version 1.0.0
    */
    friend bool operator>=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs < rhs);
    }

    /*!
    @brief comparison: greater than or equal
    @copydoc operator>=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator>=(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs >= basic_json(rhs));
    }

    /*!
    @brief comparison: greater than or equal
    @copydoc operator>=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator>=(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) >= rhs);
    }

    /// @}

    ///////////////////
    // serialization //
    ///////////////////

    /// @name serialization
    /// @{

    /*!
    @brief serialize to stream

    Serialize the given JSON value @a j to the output stream @a o. The JSON
    value will be serialized using the @ref dump member function.

    - The indentation of the output can be controlled with the member variable
      `width` of the output stream @a o. For instance, using the manipulator
      `std::setw(4)` on @a o sets the indentation level to `4` and the
      serialization result is the same as calling `dump(4)`.

    - The indentation character can be controlled with the member variable
      `fill` of the output stream @a o. For instance, the manipulator
      `std::setfill('\\t')` sets indentation to use a tab character rather than
      the default space character.

    @param[in,out] o  stream to serialize to
    @param[in] j  JSON value to serialize

    @return the stream @a o

    @throw type_error.316 if a string stored inside the JSON value is not
                          UTF-8 encoded

    @complexity Linear.

    @liveexample{The example below shows the serialization with different
    parameters to `width` to adjust the indentation level.,operator_serialize}

    @since version 1.0.0; indentation character added in version 3.0.0
    */
    friend std::ostream& operator<<(std::ostream& o, const basic_json& j)
    {
        // read width member and use it as indentation parameter if nonzero
        const bool pretty_print = (o.width() > 0);
        const auto indentation = (pretty_print ? o.width() : 0);

        // reset width to 0 for subsequent calls to this stream
        o.width(0);

        // do the actual serialization
        serializer s(detail::output_adapter<char>(o), o.fill());
        s.dump(j, pretty_print, false, static_cast<unsigned int>(indentation));
        return o;
    }

    /*!
    @brief serialize to stream
    @deprecated This stream operator is deprecated and will be removed in a
                future version of the library. Please use
                @ref operator<<(std::ostream&, const basic_json&)
                instead; that is, replace calls like `j >> o;` with `o << j;`.
    @since version 1.0.0; deprecated since version 3.0.0
    */
    JSON_DEPRECATED
    friend std::ostream& operator>>(const basic_json& j, std::ostream& o)
    {
        return o << j;
    }

    /// @}


    /////////////////////
    // deserialization //
    /////////////////////

    /// @name deserialization
    /// @{

    /*!
    @brief deserialize from a compatible input

    This function reads from a compatible input. Examples are:
    - an array of 1-byte values
    - strings with character/literal type with size of 1 byte
    - input streams
    - container with contiguous storage of 1-byte values. Compatible container
      types include `std::vector`, `std::string`, `std::array`,
      `std::valarray`, and `std::initializer_list`. Furthermore, C-style
      arrays can be used with `std::begin()`/`std::end()`. User-defined
      containers can be used as long as they implement random-access iterators
      and a contiguous storage.

    @pre Each element of the container has a size of 1 byte. Violating this
    precondition yields undefined behavior. **This precondition is enforced
    with a static assertion.**

    @pre The container storage is contiguous. Violating this precondition
    yields undefined behavior. **This precondition is enforced with an
    assertion.**
    @pre Each element of the container has a size of 1 byte. Violating this
    precondition yields undefined behavior. **This precondition is enforced
    with a static assertion.**

    @warning There is no way to enforce all preconditions at compile-time. If
             the function is called with a noncompliant container and with
             assertions switched off, the behavior is undefined and will most
             likely yield segmentation violation.

    @param[in] i  input to read from
    @param[in] cb  a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)

    @return result of the deserialization

    @throw parse_error.101 if a parse error occurs; example: `""unexpected end
    of input; expected string literal""`
    @throw parse_error.102 if to_unicode fails or surrogate error
    @throw parse_error.103 if to_unicode fails

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.

    @liveexample{The example below demonstrates the `parse()` function reading
    from an array.,parse__array__parser_callback_t}

    @liveexample{The example below demonstrates the `parse()` function with
    and without callback function.,parse__string__parser_callback_t}

    @liveexample{The example below demonstrates the `parse()` function with
    and without callback function.,parse__istream__parser_callback_t}

    @liveexample{The example below demonstrates the `parse()` function reading
    from a contiguous container.,parse__contiguouscontainer__parser_callback_t}

    @since version 2.0.3 (contiguous containers)
    */
    static basic_json parse(detail::input_adapter i,
                            const parser_callback_t cb = nullptr,
                            const bool allow_exceptions = true)
    {
        basic_json result;
        parser(i, cb, allow_exceptions).parse(true, result);
        return result;
    }

    /*!
    @copydoc basic_json parse(detail::input_adapter, const parser_callback_t)
    */
    static basic_json parse(detail::input_adapter& i,
                            const parser_callback_t cb = nullptr,
                            const bool allow_exceptions = true)
    {
        basic_json result;
        parser(i, cb, allow_exceptions).parse(true, result);
        return result;
    }

    static bool accept(detail::input_adapter i)
    {
        return parser(i).accept(true);
    }

    static bool accept(detail::input_adapter& i)
    {
        return parser(i).accept(true);
    }

    /*!
    @brief deserialize from an iterator range with contiguous storage

    This function reads from an iterator range of a container with contiguous
    storage of 1-byte values. Compatible container types include
    `std::vector`, `std::string`, `std::array`, `std::valarray`, and
    `std::initializer_list`. Furthermore, C-style arrays can be used with
    `std::begin()`/`std::end()`. User-defined containers can be used as long
    as they implement random-access iterators and a contiguous storage.

    @pre The iterator range is contiguous. Violating this precondition yields
    undefined behavior. **This precondition is enforced with an assertion.**
    @pre Each element in the range has a size of 1 byte. Violating this
    precondition yields undefined behavior. **This precondition is enforced
    with a static assertion.**

    @warning There is no way to enforce all preconditions at compile-time. If
             the function is called with noncompliant iterators and with
             assertions switched off, the behavior is undefined and will most
             likely yield segmentation violation.

    @tparam IteratorType iterator of container with contiguous storage
    @param[in] first  begin of the range to parse (included)
    @param[in] last  end of the range to parse (excluded)
    @param[in] cb  a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)
    @param[in] allow_exceptions  whether to throw exceptions in case of a
    parse error (optional, true by default)

    @return result of the deserialization

    @throw parse_error.101 in case of an unexpected token
    @throw parse_error.102 if to_unicode fails or surrogate error
    @throw parse_error.103 if to_unicode fails

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.

    @liveexample{The example below demonstrates the `parse()` function reading
    from an iterator range.,parse__iteratortype__parser_callback_t}

    @since version 2.0.3
    */
    template<class IteratorType, typename std::enable_if<
                 std::is_base_of<
                     std::random_access_iterator_tag,
                     typename std::iterator_traits<IteratorType>::iterator_category>::value, int>::type = 0>
    static basic_json parse(IteratorType first, IteratorType last,
                            const parser_callback_t cb = nullptr,
                            const bool allow_exceptions = true)
    {
        basic_json result;
        parser(detail::input_adapter(first, last), cb, allow_exceptions).parse(true, result);
        return result;
    }

    template<class IteratorType, typename std::enable_if<
                 std::is_base_of<
                     std::random_access_iterator_tag,
                     typename std::iterator_traits<IteratorType>::iterator_category>::value, int>::type = 0>
    static bool accept(IteratorType first, IteratorType last)
    {
        return parser(detail::input_adapter(first, last)).accept(true);
    }

    /*!
    @brief deserialize from stream
    @deprecated This stream operator is deprecated and will be removed in a
                future version of the library. Please use
                @ref operator>>(std::istream&, basic_json&)
                instead; that is, replace calls like `j << i;` with `i >> j;`.
    @since version 1.0.0; deprecated since version 3.0.0
    */
    JSON_DEPRECATED
    friend std::istream& operator<<(basic_json& j, std::istream& i)
    {
        return operator>>(i, j);
    }

    /*!
    @brief deserialize from stream

    Deserializes an input stream to a JSON value.

    @param[in,out] i  input stream to read a serialized JSON value from
    @param[in,out] j  JSON value to write the deserialized input to

    @throw parse_error.101 in case of an unexpected token
    @throw parse_error.102 if to_unicode fails or surrogate error
    @throw parse_error.103 if to_unicode fails

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser.

    @note A UTF-8 byte order mark is silently ignored.

    @liveexample{The example below shows how a JSON value is constructed by
    reading a serialization from a stream.,operator_deserialize}

    @sa parse(std::istream&, const parser_callback_t) for a variant with a
    parser callback function to filter values while parsing

    @since version 1.0.0
    */
    friend std::istream& operator>>(std::istream& i, basic_json& j)
    {
        parser(detail::input_adapter(i)).parse(false, j);
        return i;
    }

    /// @}

    ///////////////////////////
    // convenience functions //
    ///////////////////////////

    /*!
    @brief return the type as string

    Returns the type name as string to be used in error messages - usually to
    indicate that a function was called on a wrong JSON type.

    @return a string representation of a the @a m_type member:
            Value type  | return value
            ----------- | -------------
            null        | `"null"`
            boolean     | `"boolean"`
            string      | `"string"`
            number      | `"number"` (for all number types)
            object      | `"object"`
            array       | `"array"`
            discarded   | `"discarded"`

    @exceptionsafety No-throw guarantee: this function never throws exceptions.

    @complexity Constant.

    @liveexample{The following code exemplifies `type_name()` for all JSON
    types.,type_name}

    @sa @ref type() -- return the type of the JSON value
    @sa @ref operator value_t() -- return the type of the JSON value (implicit)

    @since version 1.0.0, public since 2.1.0, `const char*` and `noexcept`
    since 3.0.0
    */
    const char* type_name() const noexcept
    {
        {
            switch (m_type)
            {
                case value_t::null:
                    return "null";
                case value_t::object:
                    return "object";
                case value_t::array:
                    return "array";
                case value_t::string:
                    return "string";
                case value_t::boolean:
                    return "boolean";
                case value_t::discarded:
                    return "discarded";
                default:
                    return "number";
            }
        }
    }


  private:
    //////////////////////
    // member variables //
    //////////////////////

    /// the type of the current element
    value_t m_type = value_t::null;

    /// the value of the current element
    json_value m_value = {};

    //////////////////////////////////////////
    // binary serialization/deserialization //
    //////////////////////////////////////////

    /// @name binary serialization/deserialization support
    /// @{

  public:
    /*!
    @brief create a CBOR serialization of a given JSON value

    Serializes a given JSON value @a j to a byte vector using the CBOR (Concise
    Binary Object Representation) serialization format. CBOR is a binary
    serialization format which aims to be more compact than JSON itself, yet
    more efficient to parse.

    The library uses the following mapping from JSON values types to
    CBOR types according to the CBOR specification (RFC 7049):

    JSON value type | value/range                                | CBOR type                          | first byte
    --------------- | ------------------------------------------ | ---------------------------------- | ---------------
    null            | `null`                                     | Null                               | 0xF6
    boolean         | `true`                                     | True                               | 0xF5
    boolean         | `false`                                    | False                              | 0xF4
    number_integer  | -9223372036854775808..-2147483649          | Negative integer (8 bytes follow)  | 0x3B
    number_integer  | -2147483648..-32769                        | Negative integer (4 bytes follow)  | 0x3A
    number_integer  | -32768..-129                               | Negative integer (2 bytes follow)  | 0x39
    number_integer  | -128..-25                                  | Negative integer (1 byte follow)   | 0x38
    number_integer  | -24..-1                                    | Negative integer                   | 0x20..0x37
    number_integer  | 0..23                                      | Integer                            | 0x00..0x17
    number_integer  | 24..255                                    | Unsigned integer (1 byte follow)   | 0x18
    number_integer  | 256..65535                                 | Unsigned integer (2 bytes follow)  | 0x19
    number_integer  | 65536..4294967295                          | Unsigned integer (4 bytes follow)  | 0x1A
    number_integer  | 4294967296..18446744073709551615           | Unsigned integer (8 bytes follow)  | 0x1B
    number_unsigned | 0..23                                      | Integer                            | 0x00..0x17
    number_unsigned | 24..255                                    | Unsigned integer (1 byte follow)   | 0x18
    number_unsigned | 256..65535                                 | Unsigned integer (2 bytes follow)  | 0x19
    number_unsigned | 65536..4294967295                          | Unsigned integer (4 bytes follow)  | 0x1A
    number_unsigned | 4294967296..18446744073709551615           | Unsigned integer (8 bytes follow)  | 0x1B
    number_float    | *any value*                                | Double-Precision Float             | 0xFB
    string          | *length*: 0..23                            | UTF-8 string                       | 0x60..0x77
    string          | *length*: 23..255                          | UTF-8 string (1 byte follow)       | 0x78
    string          | *length*: 256..65535                       | UTF-8 string (2 bytes follow)      | 0x79
    string          | *length*: 65536..4294967295                | UTF-8 string (4 bytes follow)      | 0x7A
    string          | *length*: 4294967296..18446744073709551615 | UTF-8 string (8 bytes follow)      | 0x7B
    array           | *size*: 0..23                              | array                              | 0x80..0x97
    array           | *size*: 23..255                            | array (1 byte follow)              | 0x98
    array           | *size*: 256..65535                         | array (2 bytes follow)             | 0x99
    array           | *size*: 65536..4294967295                  | array (4 bytes follow)             | 0x9A
    array           | *size*: 4294967296..18446744073709551615   | array (8 bytes follow)             | 0x9B
    object          | *size*: 0..23                              | map                                | 0xA0..0xB7
    object          | *size*: 23..255                            | map (1 byte follow)                | 0xB8
    object          | *size*: 256..65535                         | map (2 bytes follow)               | 0xB9
    object          | *size*: 65536..4294967295                  | map (4 bytes follow)               | 0xBA
    object          | *size*: 4294967296..18446744073709551615   | map (8 bytes follow)               | 0xBB

    @note The mapping is **complete** in the sense that any JSON value type
          can be converted to a CBOR value.

    @note If NaN or Infinity are stored inside a JSON number, they are
          serialized properly. This behavior differs from the @ref dump()
          function which serializes NaN or Infinity to `null`.

    @note The following CBOR types are not used in the conversion:
          - byte strings (0x40..0x5F)
          - UTF-8 strings terminated by "break" (0x7F)
          - arrays terminated by "break" (0x9F)
          - maps terminated by "break" (0xBF)
          - date/time (0xC0..0xC1)
          - bignum (0xC2..0xC3)
          - decimal fraction (0xC4)
          - bigfloat (0xC5)
          - tagged items (0xC6..0xD4, 0xD8..0xDB)
          - expected conversions (0xD5..0xD7)
          - simple values (0xE0..0xF3, 0xF8)
          - undefined (0xF7)
          - half and single-precision floats (0xF9-0xFA)
          - break (0xFF)

    @param[in] j  JSON value to serialize
    @return MessagePack serialization as byte vector

    @complexity Linear in the size of the JSON value @a j.

    @liveexample{The example shows the serialization of a JSON value to a byte
    vector in CBOR format.,to_cbor}

    @sa http://cbor.io
    @sa @ref from_cbor(detail::input_adapter, const bool strict) for the
        analogous deserialization
    @sa @ref to_msgpack(const basic_json&) for the related MessagePack format
    @sa @ref to_ubjson(const basic_json&, const bool, const bool) for the
             related UBJSON format

    @since version 2.0.9
    */
    static std::vector<uint8_t> to_cbor(const basic_json& j)
    {
        std::vector<uint8_t> result;
        to_cbor(j, result);
        return result;
    }

    static void to_cbor(const basic_json& j, detail::output_adapter<uint8_t> o)
    {
        binary_writer<uint8_t>(o).write_cbor(j);
    }

    static void to_cbor(const basic_json& j, detail::output_adapter<char> o)
    {
        binary_writer<char>(o).write_cbor(j);
    }

    /*!
    @brief create a MessagePack serialization of a given JSON value

    Serializes a given JSON value @a j to a byte vector using the MessagePack
    serialization format. MessagePack is a binary serialization format which
    aims to be more compact than JSON itself, yet more efficient to parse.

    The library uses the following mapping from JSON values types to
    MessagePack types according to the MessagePack specification:

    JSON value type | value/range                       | MessagePack type | first byte
    --------------- | --------------------------------- | ---------------- | ----------
    null            | `null`                            | nil              | 0xC0
    boolean         | `true`                            | true             | 0xC3
    boolean         | `false`                           | false            | 0xC2
    number_integer  | -9223372036854775808..-2147483649 | int64            | 0xD3
    number_integer  | -2147483648..-32769               | int32            | 0xD2
    number_integer  | -32768..-129                      | int16            | 0xD1
    number_integer  | -128..-33                         | int8             | 0xD0
    number_integer  | -32..-1                           | negative fixint  | 0xE0..0xFF
    number_integer  | 0..127                            | positive fixint  | 0x00..0x7F
    number_integer  | 128..255                          | uint 8           | 0xCC
    number_integer  | 256..65535                        | uint 16          | 0xCD
    number_integer  | 65536..4294967295                 | uint 32          | 0xCE
    number_integer  | 4294967296..18446744073709551615  | uint 64          | 0xCF
    number_unsigned | 0..127                            | positive fixint  | 0x00..0x7F
    number_unsigned | 128..255                          | uint 8           | 0xCC
    number_unsigned | 256..65535                        | uint 16          | 0xCD
    number_unsigned | 65536..4294967295                 | uint 32          | 0xCE
    number_unsigned | 4294967296..18446744073709551615  | uint 64          | 0xCF
    number_float    | *any value*                       | float 64         | 0xCB
    string          | *length*: 0..31                   | fixstr           | 0xA0..0xBF
    string          | *length*: 32..255                 | str 8            | 0xD9
    string          | *length*: 256..65535              | str 16           | 0xDA
    string          | *length*: 65536..4294967295       | str 32           | 0xDB
    array           | *size*: 0..15                     | fixarray         | 0x90..0x9F
    array           | *size*: 16..65535                 | array 16         | 0xDC
    array           | *size*: 65536..4294967295         | array 32         | 0xDD
    object          | *size*: 0..15                     | fix map          | 0x80..0x8F
    object          | *size*: 16..65535                 | map 16           | 0xDE
    object          | *size*: 65536..4294967295         | map 32           | 0xDF

    @note The mapping is **complete** in the sense that any JSON value type
          can be converted to a MessagePack value.

    @note The following values can **not** be converted to a MessagePack value:
          - strings with more than 4294967295 bytes
          - arrays with more than 4294967295 elements
          - objects with more than 4294967295 elements

    @note The following MessagePack types are not used in the conversion:
          - bin 8 - bin 32 (0xC4..0xC6)
          - ext 8 - ext 32 (0xC7..0xC9)
          - float 32 (0xCA)
          - fixext 1 - fixext 16 (0xD4..0xD8)

    @note Any MessagePack output created @ref to_msgpack can be successfully
          parsed by @ref from_msgpack.

    @note If NaN or Infinity are stored inside a JSON number, they are
          serialized properly. This behavior differs from the @ref dump()
          function which serializes NaN or Infinity to `null`.

    @param[in] j  JSON value to serialize
    @return MessagePack serialization as byte vector

    @complexity Linear in the size of the JSON value @a j.

    @liveexample{The example shows the serialization of a JSON value to a byte
    vector in MessagePack format.,to_msgpack}

    @sa http://msgpack.org
    @sa @ref from_msgpack(const std::vector<uint8_t>&, const size_t) for the
        analogous deserialization
    @sa @ref to_cbor(const basic_json& for the related CBOR format
    @sa @ref to_ubjson(const basic_json&, const bool, const bool) for the
             related UBJSON format

    @since version 2.0.9
    */
    static std::vector<uint8_t> to_msgpack(const basic_json& j)
    {
        std::vector<uint8_t> result;
        to_msgpack(j, result);
        return result;
    }

    static void to_msgpack(const basic_json& j, detail::output_adapter<uint8_t> o)
    {
        binary_writer<uint8_t>(o).write_msgpack(j);
    }

    static void to_msgpack(const basic_json& j, detail::output_adapter<char> o)
    {
        binary_writer<char>(o).write_msgpack(j);
    }

    /*!
    @brief create a UBJSON serialization of a given JSON value

    Serializes a given JSON value @a j to a byte vector using the UBJSON
    (Universal Binary JSON) serialization format. UBJSON aims to be more compact
    than JSON itself, yet more efficient to parse.

    The library uses the following mapping from JSON values types to
    UBJSON types according to the UBJSON specification:

    JSON value type | value/range                       | UBJSON type | marker
    --------------- | --------------------------------- | ----------- | ------
    null            | `null`                            | null        | `Z`
    boolean         | `true`                            | true        | `T`
    boolean         | `false`                           | false       | `F`
    number_integer  | -9223372036854775808..-2147483649 | int64       | `L`
    number_integer  | -2147483648..-32769               | int32       | `l`
    number_integer  | -32768..-129                      | int16       | `I`
    number_integer  | -128..127                         | int8        | `i`
    number_integer  | 128..255                          | uint8       | `U`
    number_integer  | 256..32767                        | int16       | `I`
    number_integer  | 32768..2147483647                 | int32       | `l`
    number_integer  | 2147483648..9223372036854775807   | int64       | `L`
    number_unsigned | 0..127                            | int8        | `i`
    number_unsigned | 128..255                          | uint8       | `U`
    number_unsigned | 256..32767                        | int16       | `I`
    number_unsigned | 32768..2147483647                 | int32       | `l`
    number_unsigned | 2147483648..9223372036854775807   | int64       | `L`
    number_float    | *any value*                       | float64     | `D`
    string          | *with shortest length indicator*  | string      | `S`
    array           | *see notes on optimized format*   | array       | `[`
    object          | *see notes on optimized format*   | map         | `{`

    @note The mapping is **complete** in the sense that any JSON value type
          can be converted to a UBJSON value.

    @note The following values can **not** be converted to a UBJSON value:
          - strings with more than 9223372036854775807 bytes (theoretical)
          - unsigned integer numbers above 9223372036854775807

    @note The following markers are not used in the conversion:
          - `Z`: no-op values are not created.
          - `C`: single-byte strings are serialized with `S` markers.

    @note Any UBJSON output created @ref to_ubjson can be successfully parsed
          by @ref from_ubjson.

    @note If NaN or Infinity are stored inside a JSON number, they are
          serialized properly. This behavior differs from the @ref dump()
          function which serializes NaN or Infinity to `null`.

    @note The optimized formats for containers are supported: Parameter
          @a use_size adds size information to the beginning of a container and
          removes the closing marker. Parameter @a use_type further checks
          whether all elements of a container have the same type and adds the
          type marker to the beginning of the container. The @a use_type
          parameter must only be used together with @a use_size = true. Note
          that @a use_size = true alone may result in larger representations -
          the benefit of this parameter is that the receiving side is
          immediately informed on the number of elements of the container.

    @param[in] j  JSON value to serialize
    @param[in] use_size  whether to add size annotations to container types
    @param[in] use_type  whether to add type annotations to container types
                         (must be combined with @a use_size = true)
    @return UBJSON serialization as byte vector

    @complexity Linear in the size of the JSON value @a j.

    @liveexample{The example shows the serialization of a JSON value to a byte
    vector in UBJSON format.,to_ubjson}

    @sa http://ubjson.org
    @sa @ref from_ubjson(detail::input_adapter, const bool strict) for the
        analogous deserialization
    @sa @ref to_cbor(const basic_json& for the related CBOR format
    @sa @ref to_msgpack(const basic_json&) for the related MessagePack format

    @since version 3.1.0
    */
    static std::vector<uint8_t> to_ubjson(const basic_json& j,
                                          const bool use_size = false,
                                          const bool use_type = false)
    {
        std::vector<uint8_t> result;
        to_ubjson(j, result, use_size, use_type);
        return result;
    }

    static void to_ubjson(const basic_json& j, detail::output_adapter<uint8_t> o,
                          const bool use_size = false, const bool use_type = false)
    {
        binary_writer<uint8_t>(o).write_ubjson(j, use_size, use_type);
    }

    static void to_ubjson(const basic_json& j, detail::output_adapter<char> o,
                          const bool use_size = false, const bool use_type = false)
    {
        binary_writer<char>(o).write_ubjson(j, use_size, use_type);
    }

    /*!
    @brief create a JSON value from an input in CBOR format

    Deserializes a given input @a i to a JSON value using the CBOR (Concise
    Binary Object Representation) serialization format.

    The library maps CBOR types to JSON value types as follows:

    CBOR type              | JSON value type | first byte
    ---------------------- | --------------- | ----------
    Integer                | number_unsigned | 0x00..0x17
    Unsigned integer       | number_unsigned | 0x18
    Unsigned integer       | number_unsigned | 0x19
    Unsigned integer       | number_unsigned | 0x1A
    Unsigned integer       | number_unsigned | 0x1B
    Negative integer       | number_integer  | 0x20..0x37
    Negative integer       | number_integer  | 0x38
    Negative integer       | number_integer  | 0x39
    Negative integer       | number_integer  | 0x3A
    Negative integer       | number_integer  | 0x3B
    Negative integer       | number_integer  | 0x40..0x57
    UTF-8 string           | string          | 0x60..0x77
    UTF-8 string           | string          | 0x78
    UTF-8 string           | string          | 0x79
    UTF-8 string           | string          | 0x7A
    UTF-8 string           | string          | 0x7B
    UTF-8 string           | string          | 0x7F
    array                  | array           | 0x80..0x97
    array                  | array           | 0x98
    array                  | array           | 0x99
    array                  | array           | 0x9A
    array                  | array           | 0x9B
    array                  | array           | 0x9F
    map                    | object          | 0xA0..0xB7
    map                    | object          | 0xB8
    map                    | object          | 0xB9
    map                    | object          | 0xBA
    map                    | object          | 0xBB
    map                    | object          | 0xBF
    False                  | `false`         | 0xF4
    True                   | `true`          | 0xF5
    Nill                   | `null`          | 0xF6
    Half-Precision Float   | number_float    | 0xF9
    Single-Precision Float | number_float    | 0xFA
    Double-Precision Float | number_float    | 0xFB

    @warning The mapping is **incomplete** in the sense that not all CBOR
             types can be converted to a JSON value. The following CBOR types
             are not supported and will yield parse errors (parse_error.112):
             - byte strings (0x40..0x5F)
             - date/time (0xC0..0xC1)
             - bignum (0xC2..0xC3)
             - decimal fraction (0xC4)
             - bigfloat (0xC5)
             - tagged items (0xC6..0xD4, 0xD8..0xDB)
             - expected conversions (0xD5..0xD7)
             - simple values (0xE0..0xF3, 0xF8)
             - undefined (0xF7)

    @warning CBOR allows map keys of any type, whereas JSON only allows
             strings as keys in object values. Therefore, CBOR maps with keys
             other than UTF-8 strings are rejected (parse_error.113).

    @note Any CBOR output created @ref to_cbor can be successfully parsed by
          @ref from_cbor.

    @param[in] i  an input in CBOR format convertible to an input adapter
    @param[in] strict  whether to expect the input to be consumed until EOF
                       (true by default)
    @return deserialized JSON value

    @throw parse_error.110 if the given input ends prematurely or the end of
    file was not reached when @a strict was set to true
    @throw parse_error.112 if unsupported features from CBOR were
    used in the given input @a v or if the input is not valid CBOR
    @throw parse_error.113 if a string was expected as map key, but not found

    @complexity Linear in the size of the input @a i.

    @liveexample{The example shows the deserialization of a byte vector in CBOR
    format to a JSON value.,from_cbor}

    @sa http://cbor.io
    @sa @ref to_cbor(const basic_json&) for the analogous serialization
    @sa @ref from_msgpack(detail::input_adapter, const bool) for the
        related MessagePack format
    @sa @ref from_ubjson(detail::input_adapter, const bool) for the related
        UBJSON format

    @since version 2.0.9; parameter @a start_index since 2.1.1; changed to
           consume input adapters, removed start_index parameter, and added
           @a strict parameter since 3.0.0
    */
    static basic_json from_cbor(detail::input_adapter i,
                                const bool strict = true)
    {
        return binary_reader(i).parse_cbor(strict);
    }

    /*!
    @copydoc from_cbor(detail::input_adapter, const bool)
    */
    template<typename A1, typename A2,
             detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
    static basic_json from_cbor(A1 && a1, A2 && a2, const bool strict = true)
    {
        return binary_reader(detail::input_adapter(std::forward<A1>(a1), std::forward<A2>(a2))).parse_cbor(strict);
    }

    /*!
    @brief create a JSON value from an input in MessagePack format

    Deserializes a given input @a i to a JSON value using the MessagePack
    serialization format.

    The library maps MessagePack types to JSON value types as follows:

    MessagePack type | JSON value type | first byte
    ---------------- | --------------- | ----------
    positive fixint  | number_unsigned | 0x00..0x7F
    fixmap           | object          | 0x80..0x8F
    fixarray         | array           | 0x90..0x9F
    fixstr           | string          | 0xA0..0xBF
    nil              | `null`          | 0xC0
    false            | `false`         | 0xC2
    true             | `true`          | 0xC3
    float 32         | number_float    | 0xCA
    float 64         | number_float    | 0xCB
    uint 8           | number_unsigned | 0xCC
    uint 16          | number_unsigned | 0xCD
    uint 32          | number_unsigned | 0xCE
    uint 64          | number_unsigned | 0xCF
    int 8            | number_integer  | 0xD0
    int 16           | number_integer  | 0xD1
    int 32           | number_integer  | 0xD2
    int 64           | number_integer  | 0xD3
    str 8            | string          | 0xD9
    str 16           | string          | 0xDA
    str 32           | string          | 0xDB
    array 16         | array           | 0xDC
    array 32         | array           | 0xDD
    map 16           | object          | 0xDE
    map 32           | object          | 0xDF
    negative fixint  | number_integer  | 0xE0-0xFF

    @warning The mapping is **incomplete** in the sense that not all
             MessagePack types can be converted to a JSON value. The following
             MessagePack types are not supported and will yield parse errors:
              - bin 8 - bin 32 (0xC4..0xC6)
              - ext 8 - ext 32 (0xC7..0xC9)
              - fixext 1 - fixext 16 (0xD4..0xD8)

    @note Any MessagePack output created @ref to_msgpack can be successfully
          parsed by @ref from_msgpack.

    @param[in] i  an input in MessagePack format convertible to an input
                  adapter
    @param[in] strict  whether to expect the input to be consumed until EOF
                       (true by default)

    @throw parse_error.110 if the given input ends prematurely or the end of
    file was not reached when @a strict was set to true
    @throw parse_error.112 if unsupported features from MessagePack were
    used in the given input @a i or if the input is not valid MessagePack
    @throw parse_error.113 if a string was expected as map key, but not found

    @complexity Linear in the size of the input @a i.

    @liveexample{The example shows the deserialization of a byte vector in
    MessagePack format to a JSON value.,from_msgpack}

    @sa http://msgpack.org
    @sa @ref to_msgpack(const basic_json&) for the analogous serialization
    @sa @ref from_cbor(detail::input_adapter, const bool) for the related CBOR
        format
    @sa @ref from_ubjson(detail::input_adapter, const bool) for the related
        UBJSON format

    @since version 2.0.9; parameter @a start_index since 2.1.1; changed to
           consume input adapters, removed start_index parameter, and added
           @a strict parameter since 3.0.0
    */
    static basic_json from_msgpack(detail::input_adapter i,
                                   const bool strict = true)
    {
        return binary_reader(i).parse_msgpack(strict);
    }

    /*!
    @copydoc from_msgpack(detail::input_adapter, const bool)
    */
    template<typename A1, typename A2,
             detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
    static basic_json from_msgpack(A1 && a1, A2 && a2, const bool strict = true)
    {
        return binary_reader(detail::input_adapter(std::forward<A1>(a1), std::forward<A2>(a2))).parse_msgpack(strict);
    }

    /*!
    @brief create a JSON value from an input in UBJSON format

    Deserializes a given input @a i to a JSON value using the UBJSON (Universal
    Binary JSON) serialization format.

    The library maps UBJSON types to JSON value types as follows:

    UBJSON type | JSON value type                         | marker
    ----------- | --------------------------------------- | ------
    no-op       | *no value, next value is read*          | `N`
    null        | `null`                                  | `Z`
    false       | `false`                                 | `F`
    true        | `true`                                  | `T`
    float32     | number_float                            | `d`
    float64     | number_float                            | `D`
    uint8       | number_unsigned                         | `U`
    int8        | number_integer                          | `i`
    int16       | number_integer                          | `I`
    int32       | number_integer                          | `l`
    int64       | number_integer                          | `L`
    string      | string                                  | `S`
    char        | string                                  | `C`
    array       | array (optimized values are supported)  | `[`
    object      | object (optimized values are supported) | `{`

    @note The mapping is **complete** in the sense that any UBJSON value can
          be converted to a JSON value.

    @param[in] i  an input in UBJSON format convertible to an input adapter
    @param[in] strict  whether to expect the input to be consumed until EOF
                       (true by default)

    @throw parse_error.110 if the given input ends prematurely or the end of
    file was not reached when @a strict was set to true
    @throw parse_error.112 if a parse error occurs
    @throw parse_error.113 if a string could not be parsed successfully

    @complexity Linear in the size of the input @a i.

    @liveexample{The example shows the deserialization of a byte vector in
    UBJSON format to a JSON value.,from_ubjson}

    @sa http://ubjson.org
    @sa @ref to_ubjson(const basic_json&, const bool, const bool) for the
             analogous serialization
    @sa @ref from_cbor(detail::input_adapter, const bool) for the related CBOR
        format
    @sa @ref from_msgpack(detail::input_adapter, const bool) for the related
        MessagePack format

    @since version 3.1.0
    */
    static basic_json from_ubjson(detail::input_adapter i,
                                  const bool strict = true)
    {
        return binary_reader(i).parse_ubjson(strict);
    }

    template<typename A1, typename A2,
             detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
    static basic_json from_ubjson(A1 && a1, A2 && a2, const bool strict = true)
    {
        return binary_reader(detail::input_adapter(std::forward<A1>(a1), std::forward<A2>(a2))).parse_ubjson(strict);
    }

    /// @}

    //////////////////////////
    // JSON Pointer support //
    //////////////////////////

    /// @name JSON Pointer functions
    /// @{

    /*!
    @brief access specified element via JSON Pointer

    Uses a JSON pointer to retrieve a reference to the respective JSON value.
    No bound checking is performed. Similar to @ref operator[](const typename
    object_t::key_type&), `null` values are created in arrays and objects if
    necessary.

    In particular:
    - If the JSON pointer points to an object key that does not exist, it
      is created an filled with a `null` value before a reference to it
      is returned.
    - If the JSON pointer points to an array index that does not exist, it
      is created an filled with a `null` value before a reference to it
      is returned. All indices between the current maximum and the given
      index are also filled with `null`.
    - The special value `-` is treated as a synonym for the index past the
      end.

    @param[in] ptr  a JSON pointer

    @return reference to the element pointed to by @a ptr

    @complexity Constant.

    @throw parse_error.106   if an array index begins with '0'
    @throw parse_error.109   if an array index was not a number
    @throw out_of_range.404  if the JSON pointer can not be resolved

    @liveexample{The behavior is shown in the example.,operatorjson_pointer}

    @since version 2.0.0
    */
    reference operator[](const json_pointer& ptr)
    {
        return ptr.get_unchecked(this);
    }

    /*!
    @brief access specified element via JSON Pointer

    Uses a JSON pointer to retrieve a reference to the respective JSON value.
    No bound checking is performed. The function does not change the JSON
    value; no `null` values are created. In particular, the the special value
    `-` yields an exception.

    @param[in] ptr  JSON pointer to the desired element

    @return const reference to the element pointed to by @a ptr

    @complexity Constant.

    @throw parse_error.106   if an array index begins with '0'
    @throw parse_error.109   if an array index was not a number
    @throw out_of_range.402  if the array index '-' is used
    @throw out_of_range.404  if the JSON pointer can not be resolved

    @liveexample{The behavior is shown in the example.,operatorjson_pointer_const}

    @since version 2.0.0
    */
    const_reference operator[](const json_pointer& ptr) const
    {
        return ptr.get_unchecked(this);
    }

    /*!
    @brief access specified element via JSON Pointer

    Returns a reference to the element at with specified JSON pointer @a ptr,
    with bounds checking.

    @param[in] ptr  JSON pointer to the desired element

    @return reference to the element pointed to by @a ptr

    @throw parse_error.106 if an array index in the passed JSON pointer @a ptr
    begins with '0'. See example below.

    @throw parse_error.109 if an array index in the passed JSON pointer @a ptr
    is not a number. See example below.

    @throw out_of_range.401 if an array index in the passed JSON pointer @a ptr
    is out of range. See example below.

    @throw out_of_range.402 if the array index '-' is used in the passed JSON
    pointer @a ptr. As `at` provides checked access (and no elements are
    implicitly inserted), the index '-' is always invalid. See example below.

    @throw out_of_range.403 if the JSON pointer describes a key of an object
    which cannot be found. See example below.

    @throw out_of_range.404 if the JSON pointer @a ptr can not be resolved.
    See example below.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Constant.

    @since version 2.0.0

    @liveexample{The behavior is shown in the example.,at_json_pointer}
    */
    reference at(const json_pointer& ptr)
    {
        return ptr.get_checked(this);
    }

    /*!
    @brief access specified element via JSON Pointer

    Returns a const reference to the element at with specified JSON pointer @a
    ptr, with bounds checking.

    @param[in] ptr  JSON pointer to the desired element

    @return reference to the element pointed to by @a ptr

    @throw parse_error.106 if an array index in the passed JSON pointer @a ptr
    begins with '0'. See example below.

    @throw parse_error.109 if an array index in the passed JSON pointer @a ptr
    is not a number. See example below.

    @throw out_of_range.401 if an array index in the passed JSON pointer @a ptr
    is out of range. See example below.

    @throw out_of_range.402 if the array index '-' is used in the passed JSON
    pointer @a ptr. As `at` provides checked access (and no elements are
    implicitly inserted), the index '-' is always invalid. See example below.

    @throw out_of_range.403 if the JSON pointer describes a key of an object
    which cannot be found. See example below.

    @throw out_of_range.404 if the JSON pointer @a ptr can not be resolved.
    See example below.

    @exceptionsafety Strong guarantee: if an exception is thrown, there are no
    changes in the JSON value.

    @complexity Constant.

    @since version 2.0.0

    @liveexample{The behavior is shown in the example.,at_json_pointer_const}
    */
    const_reference at(const json_pointer& ptr) const
    {
        return ptr.get_checked(this);
    }

    /*!
    @brief return flattened JSON value

    The function creates a JSON object whose keys are JSON pointers (see [RFC
    6901](https://tools.ietf.org/html/rfc6901)) and whose values are all
    primitive. The original JSON value can be restored using the @ref
    unflatten() function.

    @return an object that maps JSON pointers to primitive values

    @note Empty objects and arrays are flattened to `null` and will not be
          reconstructed correctly by the @ref unflatten() function.

    @complexity Linear in the size the JSON value.

    @liveexample{The following code shows how a JSON object is flattened to an
    object whose keys consist of JSON pointers.,flatten}

    @sa @ref unflatten() for the reverse function

    @since version 2.0.0
    */
    basic_json flatten() const
    {
        basic_json result(value_t::object);
        json_pointer::flatten("", *this, result);
        return result;
    }

    /*!
    @brief unflatten a previously flattened JSON value

    The function restores the arbitrary nesting of a JSON value that has been
    flattened before using the @ref flatten() function. The JSON value must
    meet certain constraints:
    1. The value must be an object.
    2. The keys must be JSON pointers (see
       [RFC 6901](https://tools.ietf.org/html/rfc6901))
    3. The mapped values must be primitive JSON types.

    @return the original JSON from a flattened version

    @note Empty objects and arrays are flattened by @ref flatten() to `null`
          values and can not unflattened to their original type. Apart from
          this example, for a JSON value `j`, the following is always true:
          `j == j.flatten().unflatten()`.

    @complexity Linear in the size the JSON value.

    @throw type_error.314  if value is not an object
    @throw type_error.315  if object values are not primitive

    @liveexample{The following code shows how a flattened JSON object is
    unflattened into the original nested JSON object.,unflatten}

    @sa @ref flatten() for the reverse function

    @since version 2.0.0
    */
    basic_json unflatten() const
    {
        return json_pointer::unflatten(*this);
    }

    /// @}

    //////////////////////////
    // JSON Patch functions //
    //////////////////////////

    /// @name JSON Patch functions
    /// @{

    /*!
    @brief applies a JSON patch

    [JSON Patch](http://jsonpatch.com) defines a JSON document structure for
    expressing a sequence of operations to apply to a JSON) document. With
    this function, a JSON Patch is applied to the current JSON value by
    executing all operations from the patch.

    @param[in] json_patch  JSON patch document
    @return patched document

    @note The application of a patch is atomic: Either all operations succeed
          and the patched document is returned or an exception is thrown. In
          any case, the original value is not changed: the patch is applied
          to a copy of the value.

    @throw parse_error.104 if the JSON patch does not consist of an array of
    objects

    @throw parse_error.105 if the JSON patch is malformed (e.g., mandatory
    attributes are missing); example: `"operation add must have member path"`

    @throw out_of_range.401 if an array index is out of range.

    @throw out_of_range.403 if a JSON pointer inside the patch could not be
    resolved successfully in the current JSON value; example: `"key baz not
    found"`

    @throw out_of_range.405 if JSON pointer has no parent ("add", "remove",
    "move")

    @throw other_error.501 if "test" operation was unsuccessful

    @complexity Linear in the size of the JSON value and the length of the
    JSON patch. As usually only a fraction of the JSON value is affected by
    the patch, the complexity can usually be neglected.

    @liveexample{The following code shows how a JSON patch is applied to a
    value.,patch}

    @sa @ref diff -- create a JSON patch by comparing two JSON values

    @sa [RFC 6902 (JSON Patch)](https://tools.ietf.org/html/rfc6902)
    @sa [RFC 6901 (JSON Pointer)](https://tools.ietf.org/html/rfc6901)

    @since version 2.0.0
    */
    basic_json patch(const basic_json& json_patch) const
    {
        // make a working copy to apply the patch to
        basic_json result = *this;

        // the valid JSON Patch operations
        enum class patch_operations {add, remove, replace, move, copy, test, invalid};

        const auto get_op = [](const std::string & op)
        {
            if (op == "add")
            {
                return patch_operations::add;
            }
            if (op == "remove")
            {
                return patch_operations::remove;
            }
            if (op == "replace")
            {
                return patch_operations::replace;
            }
            if (op == "move")
            {
                return patch_operations::move;
            }
            if (op == "copy")
            {
                return patch_operations::copy;
            }
            if (op == "test")
            {
                return patch_operations::test;
            }

            return patch_operations::invalid;
        };

        // wrapper for "add" operation; add value at ptr
        const auto operation_add = [&result](json_pointer & ptr, basic_json val)
        {
            // adding to the root of the target document means replacing it
            if (ptr.is_root())
            {
                result = val;
            }
            else
            {
                // make sure the top element of the pointer exists
                json_pointer top_pointer = ptr.top();
                if (top_pointer != ptr)
                {
                    result.at(top_pointer);
                }

                // get reference to parent of JSON pointer ptr
                const auto last_path = ptr.pop_back();
                basic_json& parent = result[ptr];

                switch (parent.m_type)
                {
                    case value_t::null:
                    case value_t::object:
                    {
                        // use operator[] to add value
                        parent[last_path] = val;
                        break;
                    }

                    case value_t::array:
                    {
                        if (last_path == "-")
                        {
                            // special case: append to back
                            parent.push_back(val);
                        }
                        else
                        {
                            const auto idx = json_pointer::array_index(last_path);
                            if (JSON_UNLIKELY(static_cast<size_type>(idx) > parent.size()))
                            {
                                // avoid undefined behavior
                                JSON_THROW(out_of_range::create(401, "array index " + std::to_string(idx) + " is out of range"));
                            }
                            else
                            {
                                // default case: insert add offset
                                parent.insert(parent.begin() + static_cast<difference_type>(idx), val);
                            }
                        }
                        break;
                    }

                    default:
                    {
                        // if there exists a parent it cannot be primitive
                        assert(false);  // LCOV_EXCL_LINE
                    }
                }
            }
        };

        // wrapper for "remove" operation; remove value at ptr
        const auto operation_remove = [&result](json_pointer & ptr)
        {
            // get reference to parent of JSON pointer ptr
            const auto last_path = ptr.pop_back();
            basic_json& parent = result.at(ptr);

            // remove child
            if (parent.is_object())
            {
                // perform range check
                auto it = parent.find(last_path);
                if (JSON_LIKELY(it != parent.end()))
                {
                    parent.erase(it);
                }
                else
                {
                    JSON_THROW(out_of_range::create(403, "key '" + last_path + "' not found"));
                }
            }
            else if (parent.is_array())
            {
                // note erase performs range check
                parent.erase(static_cast<size_type>(json_pointer::array_index(last_path)));
            }
        };

        // type check: top level value must be an array
        if (JSON_UNLIKELY(not json_patch.is_array()))
        {
            JSON_THROW(parse_error::create(104, 0, "JSON patch must be an array of objects"));
        }

        // iterate and apply the operations
        for (const auto& val : json_patch)
        {
            // wrapper to get a value for an operation
            const auto get_value = [&val](const std::string & op,
                                          const std::string & member,
                                          bool string_type) -> basic_json &
            {
                // find value
                auto it = val.m_value.object->find(member);

                // context-sensitive error message
                const auto error_msg = (op == "op") ? "operation" : "operation '" + op + "'";

                // check if desired value is present
                if (JSON_UNLIKELY(it == val.m_value.object->end()))
                {
                    JSON_THROW(parse_error::create(105, 0, error_msg + " must have member '" + member + "'"));
                }

                // check if result is of type string
                if (JSON_UNLIKELY(string_type and not it->second.is_string()))
                {
                    JSON_THROW(parse_error::create(105, 0, error_msg + " must have string member '" + member + "'"));
                }

                // no error: return value
                return it->second;
            };

            // type check: every element of the array must be an object
            if (JSON_UNLIKELY(not val.is_object()))
            {
                JSON_THROW(parse_error::create(104, 0, "JSON patch must be an array of objects"));
            }

            // collect mandatory members
            const std::string op = get_value("op", "op", true);
            const std::string path = get_value(op, "path", true);
            json_pointer ptr(path);

            switch (get_op(op))
            {
                case patch_operations::add:
                {
                    operation_add(ptr, get_value("add", "value", false));
                    break;
                }

                case patch_operations::remove:
                {
                    operation_remove(ptr);
                    break;
                }

                case patch_operations::replace:
                {
                    // the "path" location must exist - use at()
                    result.at(ptr) = get_value("replace", "value", false);
                    break;
                }

                case patch_operations::move:
                {
                    const std::string from_path = get_value("move", "from", true);
                    json_pointer from_ptr(from_path);

                    // the "from" location must exist - use at()
                    basic_json v = result.at(from_ptr);

                    // The move operation is functionally identical to a
                    // "remove" operation on the "from" location, followed
                    // immediately by an "add" operation at the target
                    // location with the value that was just removed.
                    operation_remove(from_ptr);
                    operation_add(ptr, v);
                    break;
                }

                case patch_operations::copy:
                {
                    const std::string from_path = get_value("copy", "from", true);
                    const json_pointer from_ptr(from_path);

                    // the "from" location must exist - use at()
                    basic_json v = result.at(from_ptr);

                    // The copy is functionally identical to an "add"
                    // operation at the target location using the value
                    // specified in the "from" member.
                    operation_add(ptr, v);
                    break;
                }

                case patch_operations::test:
                {
                    bool success = false;
                    JSON_TRY
                    {
                        // check if "value" matches the one at "path"
                        // the "path" location must exist - use at()
                        success = (result.at(ptr) == get_value("test", "value", false));
                    }
                    JSON_CATCH (out_of_range&)
                    {
                        // ignore out of range errors: success remains false
                    }

                    // throw an exception if test fails
                    if (JSON_UNLIKELY(not success))
                    {
                        JSON_THROW(other_error::create(501, "unsuccessful: " + val.dump()));
                    }

                    break;
                }

                case patch_operations::invalid:
                {
                    // op must be "add", "remove", "replace", "move", "copy", or
                    // "test"
                    JSON_THROW(parse_error::create(105, 0, "operation value '" + op + "' is invalid"));
                }
            }
        }

        return result;
    }

    /*!
    @brief creates a diff as a JSON patch

    Creates a [JSON Patch](http://jsonpatch.com) so that value @a source can
    be changed into the value @a target by calling @ref patch function.

    @invariant For two JSON values @a source and @a target, the following code
    yields always `true`:
    @code {.cpp}
    source.patch(diff(source, target)) == target;
    @endcode

    @note Currently, only `remove`, `add`, and `replace` operations are
          generated.

    @param[in] source  JSON value to compare from
    @param[in] target  JSON value to compare against
    @param[in] path    helper value to create JSON pointers

    @return a JSON patch to convert the @a source to @a target

    @complexity Linear in the lengths of @a source and @a target.

    @liveexample{The following code shows how a JSON patch is created as a
    diff for two JSON values.,diff}

    @sa @ref patch -- apply a JSON patch
    @sa @ref merge_patch -- apply a JSON Merge Patch

    @sa [RFC 6902 (JSON Patch)](https://tools.ietf.org/html/rfc6902)

    @since version 2.0.0
    */
    static basic_json diff(const basic_json& source, const basic_json& target,
                           const std::string& path = "")
    {
        // the patch
        basic_json result(value_t::array);

        // if the values are the same, return empty patch
        if (source == target)
        {
            return result;
        }

        if (source.type() != target.type())
        {
            // different types: replace value
            result.push_back(
            {
                {"op", "replace"}, {"path", path}, {"value", target}
            });
        }
        else
        {
            switch (source.type())
            {
                case value_t::array:
                {
                    // first pass: traverse common elements
                    std::size_t i = 0;
                    while (i < source.size() and i < target.size())
                    {
                        // recursive call to compare array values at index i
                        auto temp_diff = diff(source[i], target[i], path + "/" + std::to_string(i));
                        result.insert(result.end(), temp_diff.begin(), temp_diff.end());
                        ++i;
                    }

                    // i now reached the end of at least one array
                    // in a second pass, traverse the remaining elements

                    // remove my remaining elements
                    const auto end_index = static_cast<difference_type>(result.size());
                    while (i < source.size())
                    {
                        // add operations in reverse order to avoid invalid
                        // indices
                        result.insert(result.begin() + end_index, object(
                        {
                            {"op", "remove"},
                            {"path", path + "/" + std::to_string(i)}
                        }));
                        ++i;
                    }

                    // add other remaining elements
                    while (i < target.size())
                    {
                        result.push_back(
                        {
                            {"op", "add"},
                            {"path", path + "/" + std::to_string(i)},
                            {"value", target[i]}
                        });
                        ++i;
                    }

                    break;
                }

                case value_t::object:
                {
                    // first pass: traverse this object's elements
                    for (auto it = source.cbegin(); it != source.cend(); ++it)
                    {
                        // escape the key name to be used in a JSON patch
                        const auto key = json_pointer::escape(it.key());

                        if (target.find(it.key()) != target.end())
                        {
                            // recursive call to compare object values at key it
                            auto temp_diff = diff(it.value(), target[it.key()], path + "/" + key);
                            result.insert(result.end(), temp_diff.begin(), temp_diff.end());
                        }
                        else
                        {
                            // found a key that is not in o -> remove it
                            result.push_back(object(
                            {
                                {"op", "remove"}, {"path", path + "/" + key}
                            }));
                        }
                    }

                    // second pass: traverse other object's elements
                    for (auto it = target.cbegin(); it != target.cend(); ++it)
                    {
                        if (source.find(it.key()) == source.end())
                        {
                            // found a key that is not in this -> add it
                            const auto key = json_pointer::escape(it.key());
                            result.push_back(
                            {
                                {"op", "add"}, {"path", path + "/" + key},
                                {"value", it.value()}
                            });
                        }
                    }

                    break;
                }

                default:
                {
                    // both primitive type: replace value
                    result.push_back(
                    {
                        {"op", "replace"}, {"path", path}, {"value", target}
                    });
                    break;
                }
            }
        }

        return result;
    }

    /// @}

    ////////////////////////////////
    // JSON Merge Patch functions //
    ////////////////////////////////

    /// @name JSON Merge Patch functions
    /// @{

    /*!
    @brief applies a JSON Merge Patch

    The merge patch format is primarily intended for use with the HTTP PATCH
    method as a means of describing a set of modifications to a target
    resource's content. This function applies a merge patch to the current
    JSON value.

    The function implements the following algorithm from Section 2 of
    [RFC 7396 (JSON Merge Patch)](https://tools.ietf.org/html/rfc7396):

    ```
    define MergePatch(Target, Patch):
      if Patch is an Object:
        if Target is not an Object:
          Target = {} // Ignore the contents and set it to an empty Object
        for each Name/Value pair in Patch:
          if Value is null:
            if Name exists in Target:
              remove the Name/Value pair from Target
          else:
            Target[Name] = MergePatch(Target[Name], Value)
        return Target
      else:
        return Patch
    ```

    Thereby, `Target` is the current object; that is, the patch is applied to
    the current value.

    @param[in] patch  the patch to apply

    @complexity Linear in the lengths of @a patch.

    @liveexample{The following code shows how a JSON Merge Patch is applied to
    a JSON document.,merge_patch}

    @sa @ref patch -- apply a JSON patch
    @sa [RFC 7396 (JSON Merge Patch)](https://tools.ietf.org/html/rfc7396)

    @since version 3.0.0
    */
    void merge_patch(const basic_json& patch)
    {
        if (patch.is_object())
        {
            if (not is_object())
            {
                *this = object();
            }
            for (auto it = patch.begin(); it != patch.end(); ++it)
            {
                if (it.value().is_null())
                {
                    erase(it.key());
                }
                else
                {
                    operator[](it.key()).merge_patch(it.value());
                }
            }
        }
        else
        {
            *this = patch;
        }
    }

    /// @}
};

//////////////////
// json_pointer //
//////////////////

NLOHMANN_BASIC_JSON_TPL_DECLARATION
NLOHMANN_BASIC_JSON_TPL&
json_pointer::get_and_create(NLOHMANN_BASIC_JSON_TPL& j) const
{
    using size_type = typename NLOHMANN_BASIC_JSON_TPL::size_type;
    auto result = &j;

    // in case no reference tokens exist, return a reference to the JSON value
    // j which will be overwritten by a primitive value
    for (const auto& reference_token : reference_tokens)
    {
        switch (result->m_type)
        {
            case detail::value_t::null:
            {
                if (reference_token == "0")
                {
                    // start a new array if reference token is 0
                    result = &result->operator[](0);
                }
                else
                {
                    // start a new object otherwise
                    result = &result->operator[](reference_token);
                }
                break;
            }

            case detail::value_t::object:
            {
                // create an entry in the object
                result = &result->operator[](reference_token);
                break;
            }

            case detail::value_t::array:
            {
                // create an entry in the array
                JSON_TRY
                {
                    result = &result->operator[](static_cast<size_type>(array_index(reference_token)));
                }
                JSON_CATCH(std::invalid_argument&)
                {
                    JSON_THROW(detail::parse_error::create(109, 0, "array index '" + reference_token + "' is not a number"));
                }
                break;
            }

            /*
            The following code is only reached if there exists a reference
            token _and_ the current value is primitive. In this case, we have
            an error situation, because primitive values may only occur as
            single value; that is, with an empty list of reference tokens.
            */
            default:
                JSON_THROW(detail::type_error::create(313, "invalid value to unflatten"));
        }
    }

    return *result;
}

NLOHMANN_BASIC_JSON_TPL_DECLARATION
NLOHMANN_BASIC_JSON_TPL&
json_pointer::get_unchecked(NLOHMANN_BASIC_JSON_TPL* ptr) const
{
    using size_type = typename NLOHMANN_BASIC_JSON_TPL::size_type;
    for (const auto& reference_token : reference_tokens)
    {
        // convert null values to arrays or objects before continuing
        if (ptr->m_type == detail::value_t::null)
        {
            // check if reference token is a number
            const bool nums =
                std::all_of(reference_token.begin(), reference_token.end(),
                            [](const char x)
            {
                return (x >= '0' and x <= '9');
            });

            // change value to array for numbers or "-" or to object otherwise
            *ptr = (nums or reference_token == "-")
                   ? detail::value_t::array
                   : detail::value_t::object;
        }

        switch (ptr->m_type)
        {
            case detail::value_t::object:
            {
                // use unchecked object access
                ptr = &ptr->operator[](reference_token);
                break;
            }

            case detail::value_t::array:
            {
                // error condition (cf. RFC 6901, Sect. 4)
                if (JSON_UNLIKELY(reference_token.size() > 1 and reference_token[0] == '0'))
                {
                    JSON_THROW(detail::parse_error::create(106, 0,
                                                           "array index '" + reference_token +
                                                           "' must not begin with '0'"));
                }

                if (reference_token == "-")
                {
                    // explicitly treat "-" as index beyond the end
                    ptr = &ptr->operator[](ptr->m_value.array->size());
                }
                else
                {
                    // convert array index to number; unchecked access
                    JSON_TRY
                    {
                        ptr = &ptr->operator[](
                            static_cast<size_type>(array_index(reference_token)));
                    }
                    JSON_CATCH(std::invalid_argument&)
                    {
                        JSON_THROW(detail::parse_error::create(109, 0, "array index '" + reference_token + "' is not a number"));
                    }
                }
                break;
            }

            default:
                JSON_THROW(detail::out_of_range::create(404, "unresolved reference token '" + reference_token + "'"));
        }
    }

    return *ptr;
}

NLOHMANN_BASIC_JSON_TPL_DECLARATION
NLOHMANN_BASIC_JSON_TPL&
json_pointer::get_checked(NLOHMANN_BASIC_JSON_TPL* ptr) const
{
    using size_type = typename NLOHMANN_BASIC_JSON_TPL::size_type;
    for (const auto& reference_token : reference_tokens)
    {
        switch (ptr->m_type)
        {
            case detail::value_t::object:
            {
                // note: at performs range check
                ptr = &ptr->at(reference_token);
                break;
            }

            case detail::value_t::array:
            {
                if (JSON_UNLIKELY(reference_token == "-"))
                {
                    // "-" always fails the range check
                    JSON_THROW(detail::out_of_range::create(402,
                                                            "array index '-' (" + std::to_string(ptr->m_value.array->size()) +
                                                            ") is out of range"));
                }

                // error condition (cf. RFC 6901, Sect. 4)
                if (JSON_UNLIKELY(reference_token.size() > 1 and reference_token[0] == '0'))
                {
                    JSON_THROW(detail::parse_error::create(106, 0,
                                                           "array index '" + reference_token +
                                                           "' must not begin with '0'"));
                }

                // note: at performs range check
                JSON_TRY
                {
                    ptr = &ptr->at(static_cast<size_type>(array_index(reference_token)));
                }
                JSON_CATCH(std::invalid_argument&)
                {
                    JSON_THROW(detail::parse_error::create(109, 0, "array index '" + reference_token + "' is not a number"));
                }
                break;
            }

            default:
                JSON_THROW(detail::out_of_range::create(404, "unresolved reference token '" + reference_token + "'"));
        }
    }

    return *ptr;
}

NLOHMANN_BASIC_JSON_TPL_DECLARATION
const NLOHMANN_BASIC_JSON_TPL&
json_pointer::get_unchecked(const NLOHMANN_BASIC_JSON_TPL* ptr) const
{
    using size_type = typename NLOHMANN_BASIC_JSON_TPL::size_type;
    for (const auto& reference_token : reference_tokens)
    {
        switch (ptr->m_type)
        {
            case detail::value_t::object:
            {
                // use unchecked object access
                ptr = &ptr->operator[](reference_token);
                break;
            }

            case detail::value_t::array:
            {
                if (JSON_UNLIKELY(reference_token == "-"))
                {
                    // "-" cannot be used for const access
                    JSON_THROW(detail::out_of_range::create(402,
                                                            "array index '-' (" + std::to_string(ptr->m_value.array->size()) +
                                                            ") is out of range"));
                }

                // error condition (cf. RFC 6901, Sect. 4)
                if (JSON_UNLIKELY(reference_token.size() > 1 and reference_token[0] == '0'))
                {
                    JSON_THROW(detail::parse_error::create(106, 0,
                                                           "array index '" + reference_token +
                                                           "' must not begin with '0'"));
                }

                // use unchecked array access
                JSON_TRY
                {
                    ptr = &ptr->operator[](
                        static_cast<size_type>(array_index(reference_token)));
                }
                JSON_CATCH(std::invalid_argument&)
                {
                    JSON_THROW(detail::parse_error::create(109, 0, "array index '" + reference_token + "' is not a number"));
                }
                break;
            }

            default:
                JSON_THROW(detail::out_of_range::create(404, "unresolved reference token '" + reference_token + "'"));
        }
    }

    return *ptr;
}

NLOHMANN_BASIC_JSON_TPL_DECLARATION
const NLOHMANN_BASIC_JSON_TPL&
json_pointer::get_checked(const NLOHMANN_BASIC_JSON_TPL* ptr) const
{
    using size_type = typename NLOHMANN_BASIC_JSON_TPL::size_type;
    for (const auto& reference_token : reference_tokens)
    {
        switch (ptr->m_type)
        {
            case detail::value_t::object:
            {
                // note: at performs range check
                ptr = &ptr->at(reference_token);
                break;
            }

            case detail::value_t::array:
            {
                if (JSON_UNLIKELY(reference_token == "-"))
                {
                    // "-" always fails the range check
                    JSON_THROW(detail::out_of_range::create(402,
                                                            "array index '-' (" + std::to_string(ptr->m_value.array->size()) +
                                                            ") is out of range"));
                }

                // error condition (cf. RFC 6901, Sect. 4)
                if (JSON_UNLIKELY(reference_token.size() > 1 and reference_token[0] == '0'))
                {
                    JSON_THROW(detail::parse_error::create(106, 0,
                                                           "array index '" + reference_token +
                                                           "' must not begin with '0'"));
                }

                // note: at performs range check
                JSON_TRY
                {
                    ptr = &ptr->at(static_cast<size_type>(array_index(reference_token)));
                }
                JSON_CATCH(std::invalid_argument&)
                {
                    JSON_THROW(detail::parse_error::create(109, 0, "array index '" + reference_token + "' is not a number"));
                }
                break;
            }

            default:
                JSON_THROW(detail::out_of_range::create(404, "unresolved reference token '" + reference_token + "'"));
        }
    }

    return *ptr;
}

NLOHMANN_BASIC_JSON_TPL_DECLARATION
void json_pointer::flatten(const std::string& reference_string,
                           const NLOHMANN_BASIC_JSON_TPL& value,
                           NLOHMANN_BASIC_JSON_TPL& result)
{
    switch (value.m_type)
    {
        case detail::value_t::array:
        {
            if (value.m_value.array->empty())
            {
                // flatten empty array as null
                result[reference_string] = nullptr;
            }
            else
            {
                // iterate array and use index as reference string
                for (std::size_t i = 0; i < value.m_value.array->size(); ++i)
                {
                    flatten(reference_string + "/" + std::to_string(i),
                            value.m_value.array->operator[](i), result);
                }
            }
            break;
        }

        case detail::value_t::object:
        {
            if (value.m_value.object->empty())
            {
                // flatten empty object as null
                result[reference_string] = nullptr;
            }
            else
            {
                // iterate object and use keys as reference string
                for (const auto& element : *value.m_value.object)
                {
                    flatten(reference_string + "/" + escape(element.first), element.second, result);
                }
            }
            break;
        }

        default:
        {
            // add primitive value with its reference string
            result[reference_string] = value;
            break;
        }
    }
}

NLOHMANN_BASIC_JSON_TPL_DECLARATION
NLOHMANN_BASIC_JSON_TPL
json_pointer::unflatten(const NLOHMANN_BASIC_JSON_TPL& value)
{
    if (JSON_UNLIKELY(not value.is_object()))
    {
        JSON_THROW(detail::type_error::create(314, "only objects can be unflattened"));
    }

    NLOHMANN_BASIC_JSON_TPL result;

    // iterate the JSON object values
    for (const auto& element : *value.m_value.object)
    {
        if (JSON_UNLIKELY(not element.second.is_primitive()))
        {
            JSON_THROW(detail::type_error::create(315, "values in object must be primitive"));
        }

        // assign value to reference pointed to by JSON pointer; Note that if
        // the JSON pointer is "" (i.e., points to the whole value), function
        // get_and_create returns a reference to result itself. An assignment
        // will then create a primitive value.
        json_pointer(element.first).get_and_create(result) = element.second;
    }

    return result;
}

inline bool operator==(json_pointer const& lhs, json_pointer const& rhs) noexcept
{
    return (lhs.reference_tokens == rhs.reference_tokens);
}

inline bool operator!=(json_pointer const& lhs, json_pointer const& rhs) noexcept
{
    return not (lhs == rhs);
}
} // namespace nlohmann

///////////////////////
// nonmember support //
///////////////////////

// specialization of std::swap, and std::hash
namespace std
{
/*!
@brief exchanges the values of two JSON objects

@since version 1.0.0
*/
template<>
inline void swap(nlohmann::json& j1,
                 nlohmann::json& j2) noexcept(
                     is_nothrow_move_constructible<nlohmann::json>::value and
                     is_nothrow_move_assignable<nlohmann::json>::value
                 )
{
    j1.swap(j2);
}

/// hash value for JSON objects
template<>
struct hash<nlohmann::json>
{
    /*!
    @brief return a hash value for a JSON object

    @since version 1.0.0
    */
    std::size_t operator()(const nlohmann::json& j) const
    {
        // a naive hashing via the string representation
        const auto& h = hash<nlohmann::json::string_t>();
        return h(j.dump());
    }
};

/// specialization for std::less<value_t>
/// @note: do not remove the space after '<',
///        see https://github.com/nlohmann/json/pull/679
template<>
struct less< ::nlohmann::detail::value_t>
{
    /*!
    @brief compare two value_t enum values
    @since version 3.0.0
    */
    bool operator()(nlohmann::detail::value_t lhs,
                    nlohmann::detail::value_t rhs) const noexcept
    {
        return nlohmann::detail::operator<(lhs, rhs);
    }
};

} // namespace std

/*!
@brief user-defined string literal for JSON values

This operator implements a user-defined string literal for JSON objects. It
can be used by adding `"_json"` to a string literal and returns a JSON object
if no parse error occurred.

@param[in] s  a string representation of a JSON object
@param[in] n  the length of string @a s
@return a JSON object

@since version 1.0.0
*/
inline nlohmann::json operator "" _json(const char* s, std::size_t n)
{
    return nlohmann::json::parse(s, s + n);
}

/*!
@brief user-defined string literal for JSON pointer

This operator implements a user-defined string literal for JSON Pointers. It
can be used by adding `"_json_pointer"` to a string literal and returns a JSON pointer
object if no parse error occurred.

@param[in] s  a string representation of a JSON Pointer
@param[in] n  the length of string @a s
@return a JSON pointer object

@since version 2.0.0
*/
inline nlohmann::json::json_pointer operator "" _json_pointer(const char* s, std::size_t n)
{
    return nlohmann::json::json_pointer(std::string(s, n));
}

#include "detail/macro_unscope.hpp"

#endif