json/src/json.hpp

/*!
@mainpage

These pages contain the API documentation of JSON for Modern C++, a C++11
header-only JSON class.

Class @ref nlohmann::basic_json is a good entry point for the documentation.

@copyright The code is licensed under the MIT License
           <http://opensource.org/licenses/MIT>,
           Copyright (c) 2013-2015 Niels Lohmann.
@author Niels Lohmann <http://nlohmann.me>
@see https://github.com/nlohmann/json to download the source code
*/


#ifndef NLOHMANN_JSON_HPP
#define NLOHMANN_JSON_HPP

#include <algorithm>
#include <array>
#include <ciso646>
#include <cmath>
#include <cstdio>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <memory>
#include <sstream>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

// enable ssize_t on MinGW
#ifdef __GNUC__
    #ifdef __MINGW32__
        #include <sys/types.h>
    #endif
#endif

// enable ssize_t for MSVC
#ifdef _MSC_VER
    #include <basetsd.h>
    using ssize_t = SSIZE_T;
#endif

/*!
@brief namespace for Niels Lohmann
@see https://github.com/nlohmann
*/
namespace nlohmann
{


/// namespace with internal helper functions
namespace internals
{
// Helper to determine whether there's a key_type for T.
// http://stackoverflow.com/a/7728728/266378
template<typename T>
struct has_mapped_type
{
  private:
    template<typename C> static char test(typename C::mapped_type*);
    template<typename C> static int  test(...);
  public:
    enum { value = sizeof(test<T>(0)) == sizeof(char) };
};
}

/*!
@brief a class to store JSON values

@tparam ObjectType         type for JSON objects
                           (@c std::map by default)
@tparam ArrayType          type for JSON arrays
                           (@c std::vector by default)
@tparam StringType         type for JSON strings and object keys
                           (@c std::string by default)
@tparam BooleanType        type for JSON booleans
                           (@c bool by default)
@tparam NumberIntegerType  type for JSON integer numbers
                           (@c int64_t by default)
@tparam NumberFloatType    type for JSON floating-point numbers
                           (@c double by default)
@tparam AllocatorType      type of the allocator to use
                           (@c std::allocator by default)

@requirement This class satisfies the Container requirements (see
http://en.cppreference.com/w/cpp/concept/Container):
- basic_json()
- basic_json(const basic_json&)
- reference& operator=(basic_json)
- ~basic_json()
- iterator begin(), const_iterator begin(), const_iterator cbegin()
- iterator end(), const_iterator end(), const_iterator cend()
- bool operator==(const_reference, const_reference), bool operator!=(const_reference, const_reference)
- void swap(reference other)
- size_type size(), size_type max_size()
- bool empty()

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

@see RFC 7159 <http://rfc7159.net/rfc7159>
@see ECMA 404 <http://www.ecma-international.org/publications/standards/Ecma-404.htm>
*/
template <
    template<typename U, typename V, typename... Args> class ObjectType = std::map,
    template<typename U, typename... Args> class ArrayType = std::vector,
    class StringType = std::string,
    class BooleanType = bool,
    class NumberIntegerType = int64_t,
    class NumberFloatType = double,
    template<typename U> class AllocatorType = std::allocator
    >
class basic_json
{
    /////////////////////
    // container types //
    /////////////////////

    /// @name container types
    /// @{

  private:
    /// workaround type for MSVC
    using __basic_json =
        basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberFloatType, AllocatorType>;

  public:
    /// 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
    class iterator;
    /// a const iterator for a basic_json container
    class const_iterator;
    /// a reverse iterator for a basic_json container
    class reverse_iterator;
    /// a const reverse iterator for a basic_json container
    class const_reverse_iterator;

    /// @}


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


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

    /// @name JSON value data types
    /// @{

    /// a type for an object
    using object_t =
        ObjectType<StringType, basic_json, std::less<StringType>, AllocatorType<std::pair<const StringType, basic_json>>>;
    /// a type for an array
    using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
    /// a type for a string
    using string_t = StringType;
    /// a type for a boolean
    using boolean_t = BooleanType;
    /// a type for a number (integer)
    using number_integer_t = NumberIntegerType;
    /// a type for a number (floating-point)
    using number_float_t = NumberFloatType;
    /// a type for list initialization
    using list_init_t = std::initializer_list<basic_json>;

    /// @}


    ///////////////////////////
    // JSON type enumeration //
    ///////////////////////////

    /*!
    @brief the JSON type enumeration

    This enumeration collects the different JSON types. It is internally used
    to distinguish the stored values, and the functions is_null, is_object,
    is_array, is_string, is_boolean, is_number, and is_discarded rely on it.
    */
    enum class value_t : uint8_t
    {
        null,           ///< null value
        object,         ///< object (unordered set of name/value pairs)
        array,          ///< array (ordered collection of values)
        string,         ///< string value
        boolean,        ///< boolean value
        number_integer, ///< number value (integer)
        number_float,   ///< number value (floating-point)
        discarded       ///< (internal) indicates the parser callback chose not to keep the value
    };


  private:
    ////////////////////////
    // JSON value storage //
    ////////////////////////

    /// a JSON value
    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 (floating-point)
        number_float_t number_float;

        /// default constructor (for null values)
        json_value() noexcept = 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 (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::null):
                case (value_t::discarded):
                {
                    break;
                }

                case (value_t::object):
                {
                    AllocatorType<object_t> alloc;
                    object = alloc.allocate(1);
                    alloc.construct(object);
                    break;
                }

                case (value_t::array):
                {
                    AllocatorType<array_t> alloc;
                    array = alloc.allocate(1);
                    alloc.construct(array);
                    break;
                }

                case (value_t::string):
                {
                    AllocatorType<string_t> alloc;
                    string = alloc.allocate(1);
                    alloc.construct(string, "");
                    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_float):
                {
                    number_float = number_float_t(0.0);
                    break;
                }
            }
        }

        /// constructor for strings
        json_value(const string_t& value)
        {
            AllocatorType<string_t> alloc;
            string = alloc.allocate(1);
            alloc.construct(string, value);
        }

        /// constructor for objects
        json_value(const object_t& value)
        {
            AllocatorType<object_t> alloc;
            object = alloc.allocate(1);
            alloc.construct(object, value);
        }

        /// constructor for arrays
        json_value(const array_t& value)
        {
            AllocatorType<array_t> alloc;
            array = alloc.allocate(1);
            alloc.construct(array, value);
        }
    };


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

    /*!
    @brief JSON callback events

    This enumeration lists the parser events that can trigger calling a
    callback function of type @ref parser_callback_t during parsing.
    */
    enum class parse_event_t : uint8_t
    {
        /// the parser read `{` and started to process a JSON object
        object_start,
        /// the parser read `}` and finished processing a JSON object
        object_end,
        /// the parser read `[` and started to process a JSON array
        array_start,
        /// the parser read `]` and finished processing a JSON array
        array_end,
        /// the parser read a key of a value in an object
        key,
        /// the parser finished reading a JSON value
        value
    };

    /*!
    @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(std::istream&, parser_callback_t) or
    @ref parse(const string_t&, parser_callback_t), 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

    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(std::istream&, parser_callback_t) or
    @ref parse(const string_t&, parser_callback_t) for examples
    */
    using parser_callback_t = std::function<bool(
                                  int depth, parse_event_t event, basic_json& parsed)>;

    /*!
    @brief comparison operator for JSON types

    Returns an ordering that is similar to Python:
    - order: null < boolean < number < object < array < string
    - furthermore, each type is not smaller than itself
    */
    friend bool operator<(const value_t lhs, const value_t rhs)
    {
        static constexpr std::array<uint8_t, 7> order = {{
                0, // null
                3, // object
                4, // array
                5, // string
                1, // boolean
                2, // integer
                2  // float
            }
        };

        // discarded values are not comparable
        if (lhs == value_t::discarded or rhs == value_t::discarded)
        {
            return false;
        }

        return order[static_cast<std::size_t>(lhs)] < order[static_cast<std::size_t>(rhs)];
    }


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

    /*!
    @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] value  the type of the value to create

    @complexity Constant.

    @throw std::bad_alloc if allocation for object, array, or string value
    fails

    @liveexample{The following code shows the constructor for different @ref
    value_t values,basic_json__value_t}
    */
    basic_json(const value_t value)
        : m_type(value), m_value(value)
    {}

    /*!
    @brief create a null object (implicitly)

    Create a `null` JSON value. This is the implicit version of the `null`
    value constructor as it takes no parameters.

    @complexity Constant.

    @requirement This function satisfies the Container requirements:
    - The complexity is constant.
    - As postcondition, it holds: `basic_json().empty() == true`.

    @liveexample{The following code shows the constructor for a `null` JSON
    value.,basic_json}

    @sa basic_json(std::nullptr_t)
    @ingroup container
    */
    basic_json() noexcept = default;

    /*!
    @brief create a null object (explicitly)

    Create a `null` JSON value. This is the explicitly version of the `null`
    value constructor as it takes a null pointer as parameter. It allows to
    create `null` values by explicitly assigning a @c nullptr to a JSON value.
    The passed null pointer itself is not read - it is only used to choose the
    right constructor.

    @complexity Constant.

    @liveexample{The following code shows the constructor with null pointer
    parameter.,basic_json__nullptr_t}

    @sa basic_json()
    */
    basic_json(std::nullptr_t) noexcept
        : basic_json(value_t::null)
    {}

    /*!
    @brief create an object (explicit)

    Create an object JSON value with a given content.

    @param[in] value  a value for the object

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for object value fails

    @liveexample{The following code shows the constructor with an @ref object_t
    parameter.,basic_json__object_t}

    @sa basic_json(const CompatibleObjectType&)
    */
    basic_json(const object_t& value)
        : m_type(value_t::object), m_value(value)
    {}

    /*!
    @brief create an object (implicit)

    Create an object JSON value with a given content. This constructor allows
    any type that can be used to construct values of type @ref object_t.
    Examples include the types `std::map` and `std::unordered_map`.

    @tparam CompatibleObjectType an object type whose `key_type` and
    `value_type` is compatible to @ref object_t

    @param[in] value  a value for the object

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for object value fails

    @liveexample{The following code shows the constructor with several
    compatible object type parameters.,basic_json__CompatibleObjectType}

    @sa basic_json(const object_t&)
    */
    template <class CompatibleObjectType, typename
              std::enable_if<
                  std::is_constructible<typename object_t::key_type, typename CompatibleObjectType::key_type>::value and
                  std::is_constructible<basic_json, typename CompatibleObjectType::mapped_type>::value, int>::type
              = 0>
    basic_json(const CompatibleObjectType& value)
        : m_type(value_t::object)
    {
        AllocatorType<object_t> alloc;
        m_value.object = alloc.allocate(1);
        using std::begin;
        using std::end;
        alloc.construct(m_value.object, begin(value), end(value));
    }

    /*!
    @brief create an array (explicit)

    Create an array JSON value with a given content.

    @param[in] value  a value for the array

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for array value fails

    @liveexample{The following code shows the constructor with an @ref array_t
    parameter.,basic_json__array_t}

    @sa basic_json(const CompatibleArrayType&)
    */
    basic_json(const array_t& value)
        : m_type(value_t::array), m_value(value)
    {}

    /*!
    @brief create an array (implicit)

    Create an array JSON value with a given content. This constructor allows
    any type that can be used to construct values of type @ref array_t.
    Examples include the types `std::vector`, `std::list`, and `std::set`.

    @tparam CompatibleArrayType an object type whose `value_type` is compatible
    to @ref array_t

    @param[in] value  a value for the array

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for array value fails

    @liveexample{The following code shows the constructor with several
    compatible array type parameters.,basic_json__CompatibleArrayType}

    @sa basic_json(const array_t&)
    */
    template <class CompatibleArrayType, typename
              std::enable_if<
                  not std::is_same<CompatibleArrayType, typename __basic_json::iterator>::value and
                  not std::is_same<CompatibleArrayType, typename __basic_json::const_iterator>::value and
                  not std::is_same<CompatibleArrayType, typename __basic_json::reverse_iterator>::value and
                  not std::is_same<CompatibleArrayType, typename __basic_json::const_reverse_iterator>::value and
                  not std::is_same<CompatibleArrayType, typename array_t::iterator>::value and
                  not std::is_same<CompatibleArrayType, typename array_t::const_iterator>::value and
                  std::is_constructible<basic_json, typename CompatibleArrayType::value_type>::value, int>::type
              = 0>
    basic_json(const CompatibleArrayType& value)
        : m_type(value_t::array)
    {
        AllocatorType<array_t> alloc;
        m_value.array = alloc.allocate(1);
        using std::begin;
        using std::end;
        alloc.construct(m_value.array, begin(value), end(value));
    }

    /*!
    @brief create a string (explicit)

    Create an string JSON value with a given content.

    @param[in] value  a value for the string

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for string value fails

    @liveexample{The following code shows the constructor with an @ref string_t
    parameter.,basic_json__string_t}

    @sa basic_json(const typename string_t::value_type*)
    @sa basic_json(const CompatibleStringType&)
    */
    basic_json(const string_t& value)
        : m_type(value_t::string), m_value(value)
    {}

    /*!
    @brief create a string (explicit)

    Create a string JSON value with a given content.

    @param[in] value  a literal value for the string

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for string value fails

    @liveexample{The following code shows the constructor with string literal
    parameter.,basic_json__string_t_value_type}

    @sa basic_json(const string_t&)
    @sa basic_json(const CompatibleStringType&)
    */
    basic_json(const typename string_t::value_type* value)
        : basic_json(string_t(value))
    {}

    /*!
    @brief create a string (implicit)

    Create a string JSON value with a given content.

    @param[in] value  a value for the string

    @tparam CompatibleStringType an string type which is compatible to @ref
    string_t

    @complexity Linear in the size of the passed @a value.

    @throw std::bad_alloc if allocation for string value fails

    @liveexample{The following code shows the construction of a string value
    from a compatible type.,basic_json__CompatibleStringType}

    @sa basic_json(const string_t&)
    */
    template <class CompatibleStringType, typename
              std::enable_if<
                  std::is_constructible<string_t, CompatibleStringType>::value, int>::type
              = 0>
    basic_json(const CompatibleStringType& value)
        : basic_json(string_t(value))
    {}

    /*!
    @brief create a boolean (explicit)

    Creates a JSON boolean type from a given value.

    @param[in] value  a boolean value to store

    @complexity Constant.

    @liveexample{The example below demonstrates boolean
    values.,basic_json__boolean_t}
    */
    basic_json(boolean_t value)
        : m_type(value_t::boolean), m_value(value)
    {}

    /*!
    @brief create an integer number (explicit)

    Create an interger number JSON value with a given content.

    @tparam T  helper type to compare number_integer_t and int (not visible in)
    the interface.

    @param[in] value  an integer to create a JSON number from

    @note This constructor would have the same signature as @ref
    basic_json(const int value), so we need to switch this one off in case
    number_integer_t is the same as int. This is done via the helper type @a T.

    @complexity Constant.

    @liveexample{The example below shows the construction of a JSON integer
    number value.,basic_json__number_integer_t}

    @sa basic_json(const int)
    */
    template<typename T,
             typename std::enable_if<
                 not (std::is_same<T, int>::value)
                 and std::is_same<T, number_integer_t>::value
                 , int>::type = 0>
    basic_json(const number_integer_t value)
        : m_type(value_t::number_integer), m_value(value)
    {}

    /*!
    @brief create an integer number from an enum type (explicit)

    Create an integer number JSON value with a given content.

    @param[in] value  an integer to create a JSON number from

    @note This constructor allows to pass enums directly to a constructor. As
    C++ has no way of specifying the type of an anonymous enum explicitly, we
    can only rely on the fact that such values implicitly convert to int. As
    int may already be the same type of number_integer_t, we may need to switch
    off the constructor @ref basic_json(const number_integer_t).

    @complexity Constant.

    @liveexample{The example below shows the construction of a JSON integer
    number value from an anonymous enum.,basic_json__const_int}

    @sa basic_json(const number_integer_t)
    */
    basic_json(const int value)
        : m_type(value_t::number_integer),
          m_value(static_cast<number_integer_t>(value))
    {}

    /*!
    @brief create an integer number (implicit)

    Create an integer number JSON value with a given content. This constructor
    allows any type that can be used to construct values of type @ref
    number_integer_t. Examples may include the types `int`, `int32_t`, or
    `short`.

    @tparam CompatibleNumberIntegerType an integer type which is compatible to
    @ref number_integer_t.

    @param[in] value  an integer to create a JSON number from

    @complexity Constant.

    @liveexample{The example below shows the construction of several JSON
    integer number values from compatible
    types.,basic_json__CompatibleIntegerNumberType}

    @sa basic_json(const number_integer_t)
    */
    template<typename CompatibleNumberIntegerType, typename
             std::enable_if<
                 std::is_constructible<number_integer_t, CompatibleNumberIntegerType>::value and
                 std::numeric_limits<CompatibleNumberIntegerType>::is_integer, CompatibleNumberIntegerType>::type
             = 0>
    basic_json(const CompatibleNumberIntegerType value) noexcept
        : m_type(value_t::number_integer),
          m_value(static_cast<number_integer_t>(value))
    {}

    /*!
    @brief create a floating-point number (explicit)

    Create a floating-point number JSON value with a given content.

    @param[in] value  a floating-point value to create a JSON number from

    @note RFC 7159 <http://www.rfc-editor.org/rfc/rfc7159.txt>, section 6
    disallows NaN values:
    > Numeric values that cannot be represented in the grammar below (such
    > as Infinity and NaN) are not permitted.
    In case the parameter @a value is not a number, a JSON null value is
    created instead.

    @complexity Constant.

    @liveexample{The following example creates several floating-point
    values.,basic_json__number_float_t}
    */
    basic_json(const number_float_t value)
        : m_type(value_t::number_float), m_value(value)
    {
        // replace infinity and NAN by null
        if (not std::isfinite(value))
        {
            m_type = value_t::null;
            m_value = json_value();
        }
    }

    /*!
    @brief create an floating-point number (implicit)

    Create an floating-point number JSON value with a given content. This
    constructor allows any type that can be used to construct values of type
    @ref number_float_t. Examples may include the types `float`.

    @tparam CompatibleNumberFloatType a floating-point type which is compatible
    to @ref number_float_t.

    @param[in] value  a floating-point to create a JSON number from

    @note RFC 7159 <http://www.rfc-editor.org/rfc/rfc7159.txt>, section 6
    disallows NaN values:
    > Numeric values that cannot be represented in the grammar below (such
    > as Infinity and NaN) are not permitted.
    In case the parameter @a value is not a number, a JSON null value is
    created instead.

    @complexity Constant.

    @liveexample{The example below shows the construction of several JSON
    floating-point number values from compatible
    types.,basic_json__CompatibleNumberFloatType}

    @sa basic_json(const number_float_t)
    */
    template<typename CompatibleNumberFloatType, typename = typename
             std::enable_if<
                 std::is_constructible<number_float_t, CompatibleNumberFloatType>::value and
                 std::is_floating_point<CompatibleNumberFloatType>::value>::type
             >
    basic_json(const CompatibleNumberFloatType value) noexcept
        : basic_json(number_float_t(value))
    {}

    /*!
    @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 ratioinale is as follows:

    1. The empty initializer list is written as `{}` which is exactly an empty
    JSON object.
    2. C++ has now 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(list_init_t) with an empty
      initializer list in this case
    - arrays whose elements satisfy rule 2: use @ref array(list_init_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(list_init_t) and @ref object(list_init_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 std::domain_error 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

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

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

    @sa basic_json array(list_init_t) - create a JSON array value from an
    initializer list
    @sa basic_json object(list_init_t) - create a JSON object value from an
    initializer list
    */
    basic_json(list_init_t init, bool type_deduction = true,
               value_t manual_type = value_t::array)
    {
        // the initializer list could describe an object
        bool is_object = true;

        // check if each element is an array with two elements whose first element
        // is a string
        for (const auto& element : init)
        {
            if (element.m_type != value_t::array or element.size() != 2
                    or element[0].m_type != value_t::string)
            {
                // we found an element that makes it impossible to use the
                // initializer list as object
                is_object = false;
                break;
            }
        }

        // 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_object = false;
            }

            // if object is wanted but impossible, throw an exception
            if (manual_type == value_t::object and not is_object)
            {
                throw std::domain_error("cannot create object from initializer list");
            }
        }

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

            for (auto& element : init)
            {
                m_value.object->emplace(std::move(*(element[0].m_value.string)), std::move(element[1]));
            }
        }
        else
        {
            // the initializer list describes an array -> create array
            m_type = value_t::array;
            AllocatorType<array_t> alloc;
            m_value.array = alloc.allocate(1);
            alloc.construct(m_value.array, std::move(init));
        }
    }

    /*!
    @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(list_init_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.

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

    @sa basic_json(list_init_t, bool, value_t) - create a JSON value from an
    initializer list
    @sa basic_json object(list_init_t) - create a JSON object value from an
    initializer list
    */
    static basic_json array(list_init_t init = list_init_t())
    {
        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 elments 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 basic_json array(list_init_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(list_init_t, bool, value_t).

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

    @return JSON object value

    @throw std::domain_error if @a init is not a pair whose first elements are
    strings; thrown by @ref basic_json(list_init_t, bool, value_t)

    @complexity Linear in the size of @a init.

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

    @sa basic_json(list_init_t, bool, value_t) - create a JSON value from an
    initializer list
    @sa basic_json array(list_init_t) - create a JSON array value from an
    initializer list
    */
    static basic_json object(list_init_t init = list_init_t())
    {
        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 count copies of a passed
    value. In case @a count is `0`, an empty array is created. As postcondition,
    `std::distance(begin(),end()) == count` holds.

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

    @complexity Linear in @a count.

    @liveexample{The following code shows examples for the @ref
    basic_json(size_type\, const basic_json&)
    constructor.,basic_json__size_type_basic_json}
    */
    basic_json(size_type count, const basic_json& value)
        : m_type(value_t::array)
    {
        AllocatorType<array_t> alloc;
        m_value.array = alloc.allocate(1);
        alloc.construct(m_value.array, count, value);
    }

    /*!
    @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 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, std::out_of_range is thrown.
    - In case of structured types (array, object), the constructor behaves
      as similar versions for `std::vector`.
    - In case of a null type, std::domain_error is thrown.

    @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)

    @throw std::domain_error if iterators are not compatible; that is, do not
    belong to the same JSON value
    @throw std::out_of_range if iterators are for a primitive type (number,
    boolean, or string) where an out of range error can be detected easily
    @throw std::bad_alloc if allocation for object, array, or string fails
    @throw std::domain_error if called with a null value

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

    @liveexample{The example below shows several ways to create JSON values by
    specifying a subrange with iterators.,basic_json__InputIt_InputIt}
    */
    template <class InputIT, typename
              std::enable_if<
                  std::is_same<InputIT, typename __basic_json::iterator>::value or
                  std::is_same<InputIT, typename __basic_json::const_iterator>::value
                  , int>::type
              = 0>
    basic_json(InputIT first, InputIT last) : m_type(first.m_object->m_type)
    {
        // make sure iterator fits the current value
        if (first.m_object != last.m_object)
        {
            throw std::domain_error("iterators are not compatible");
        }

        // check if iterator range is complete for primitive values
        switch (m_type)
        {
            case value_t::number_integer:
            case value_t::number_float:
            case value_t::boolean:
            case value_t::string:
            {
                if (first.m_it.generic_iterator != 0 or last.m_it.generic_iterator != 1)
                {
                    throw std::out_of_range("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_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:
            {
                AllocatorType<object_t> alloc;
                m_value.object = alloc.allocate(1);
                alloc.construct(m_value.object, first.m_it.object_iterator, last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                AllocatorType<array_t> alloc;
                m_value.array = alloc.allocate(1);
                alloc.construct(m_value.array, first.m_it.array_iterator, last.m_it.array_iterator);
                break;
            }

            default:
            {
                throw std::domain_error("cannot use construct with iterators from " + first.m_object->type_name());
            }
        }
    }

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

    /*!
    @brief copy constructor

    Creates a copy of a given JSON value.

    @param[in] other  the JSON value to copy

    @complexity Linear in the size of @a other.

    @requirement This function satisfies the Container requirements:
    - The complexity is linear.
    - As postcondition, it holds: `other == basic_json(other)`.

    @throw std::bad_alloc if allocation for object, array, or string fails.

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

    @ingroup container
    */
    basic_json(const basic_json& other)
        : m_type(other.m_type)
    {
        switch (m_type)
        {
            case (value_t::null):
            case (value_t::discarded):
            {
                break;
            }

            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_float):
            {
                m_value = other.m_value.number_float;
                break;
            }
        }
    }

    /*!
    @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 @a other is a JSON null value

    @complexity Constant.

    @liveexample{The code below shows the move constructor explicitly called
    via std::move.,basic_json__moveconstructor}
    */
    basic_json(basic_json&& other) noexcept
        : m_type(std::move(other.m_type)),
          m_value(std::move(other.m_value))
    {
        // invalidate payload
        other.m_type = value_t::null;
        other.m_value = {};
    }

    /*!
    @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 satisfies the 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}

    @ingroup container
    */
    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
    )
    {
        using std::swap;
        std::swap(m_type, other.m_type);
        std::swap(m_value, other.m_value);
        return *this;
    }

    /*!
    @brief destructor

    Destroys the JSON value and frees all allocated memory.

    @complexity Linear.

    @requirement This function satisfies the Container requirements:
    - The complexity is linear.
    - All stored elements are destroyed and all memory is freed.

    @ingroup container
    */
    ~basic_json()
    {
        switch (m_type)
        {
            case (value_t::object):
            {
                AllocatorType<object_t> alloc;
                alloc.destroy(m_value.object);
                alloc.deallocate(m_value.object, 1);
                m_value.object = nullptr;
                break;
            }

            case (value_t::array):
            {
                AllocatorType<array_t> alloc;
                alloc.destroy(m_value.array);
                alloc.deallocate(m_value.array, 1);
                m_value.array = nullptr;
                break;
            }

            case (value_t::string):
            {
                AllocatorType<string_t> alloc;
                alloc.destroy(m_value.string);
                alloc.deallocate(m_value.string, 1);
                m_value.string = nullptr;
                break;
            }

            default:
            {
                // all other types need no specific destructor
                break;
            }
        }
    }


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

    /// @name object inspection
    /// @{

    /*!
    @brief serialization

    Serialization function for JSON values. The function tries to mimick
    Python's @p json.dumps() function, and currently supports its @p indent
    parameter.

    @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

    @return string containing the serialization of the JSON value

    @complexity Linear.

    @liveexample{The following example shows the effect of different @a indent
    parameters to the result of the serializaion.,dump}

    @see https://docs.python.org/2/library/json.html#json.dump
    */
    string_t dump(const int indent = -1) const
    {
        std::stringstream ss;

        if (indent >= 0)
        {
            dump(ss, true, static_cast<unsigned int>(indent));
        }
        else
        {
            dump(ss, false, 0);
        }

        return ss.str();
    }

    /*!
    @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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref type() for all JSON
    types.,type}
    */
    value_t type() const noexcept
    {
        return m_type;
    }

    /*!
    @brief return whether type is primitive

    This function returns true iff 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.

    @liveexample{The following code exemplifies @ref is_primitive for all JSON
    types.,is_primitive}
    */
    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 iff the JSON type is structured (array or
    object).

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_structured for all JSON
    types.,is_structured}
    */
    bool is_structured() const noexcept
    {
        return is_array() or is_object();
    }

    /*!
    @brief return whether value is null

    This function returns true iff the JSON value is null.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_null for all JSON
    types.,is_null}
    */
    bool is_null() const noexcept
    {
        return m_type == value_t::null;
    }

    /*!
    @brief return whether value is a boolean

    This function returns true iff the JSON value is a boolean.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_boolean for all JSON
    types.,is_boolean}
    */
    bool is_boolean() const noexcept
    {
        return m_type == value_t::boolean;
    }

    /*!
    @brief return whether value is a number

    This function returns true iff the JSON value is a number. This includes
    both integer and floating-point values.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_number for all JSON
    types.,is_number}
    */
    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 iff the JSON value is an integer number. This
    excludes floating-point values.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_number_integer for all
    JSON types.,is_number_integer}
    */
    bool is_number_integer() const noexcept
    {
        return m_type == value_t::number_integer;
    }

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

    This function returns true iff the JSON value is a floating-point number.
    This excludes integer values.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_number_float for all
    JSON types.,is_number_float}
    */
    bool is_number_float() const noexcept
    {
        return m_type == value_t::number_float;
    }

    /*!
    @brief return whether value is an object

    This function returns true iff the JSON value is an object.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_object for all JSON
    types.,is_object}
    */
    bool is_object() const noexcept
    {
        return m_type == value_t::object;
    }

    /*!
    @brief return whether value is an array

    This function returns true iff the JSON value is an array.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_array for all JSON
    types.,is_array}
    */
    bool is_array() const noexcept
    {
        return m_type == value_t::array;
    }

    /*!
    @brief return whether value is a string

    This function returns true iff the JSON value is a string.

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

    @complexity Constant.

    @liveexample{The following code exemplifies @ref is_string for all JSON
    types.,is_string}
    */
    bool is_string() const noexcept
    {
        return m_type == value_t::string;
    }

    /*!
    @brief return whether value is discarded

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

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

    @complexity Constant.

    @todo Add example.
    */
    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.

    @liveexample{The following code exemplifies the value_t operator for all
    JSON types.,operator__value_t}
    */
    operator value_t() const noexcept
    {
        return m_type;
    }

    /// @}

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

    /// get an object (explicit)
    template <class T, typename
              std::enable_if<
                  std::is_convertible<typename object_t::key_type, typename T::key_type>::value and
                  std::is_convertible<__basic_json, typename T::mapped_type>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        switch (m_type)
        {
            case (value_t::object):
            {
                return T(m_value.object->begin(), m_value.object->end());
            }
            default:
            {
                throw std::domain_error("type must be object, but is " + type_name());
            }
        }
    }

    /// get an object (explicit)
    object_t get_impl(object_t*) const
    {
        switch (m_type)
        {
            case (value_t::object):
            {
                return *(m_value.object);
            }
            default:
            {
                throw std::domain_error("type must be object, but is " + type_name());
            }
        }
    }

    /// get an array (explicit)
    template <class T, typename
              std::enable_if<
                  std::is_convertible<__basic_json, typename T::value_type>::value and
                  not std::is_same<__basic_json, typename T::value_type>::value and
                  not std::is_arithmetic<T>::value and
                  not std::is_convertible<std::string, T>::value and
                  not internals::has_mapped_type<T>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        switch (m_type)
        {
            case (value_t::array):
            {
                T to_vector;
                std::transform(m_value.array->begin(), m_value.array->end(),
                               std::inserter(to_vector, to_vector.end()), [](basic_json i)
                {
                    return i.get<typename T::value_type>();
                });
                return to_vector;
            }
            default:
            {
                throw std::domain_error("type must be array, but is " + type_name());
            }
        }
    }

    /// get an array (explicit)
    template <class T, typename
              std::enable_if<
                  std::is_convertible<__basic_json, T>::value and
                  not std::is_same<__basic_json, T>::value
                  , int>::type = 0>
    std::vector<T> get_impl(std::vector<T>*) const
    {
        switch (m_type)
        {
            case (value_t::array):
            {
                std::vector<T> to_vector;
                to_vector.reserve(m_value.array->size());
                std::transform(m_value.array->begin(), m_value.array->end(),
                               std::inserter(to_vector, to_vector.end()), [](basic_json i)
                {
                    return i.get<T>();
                });
                return to_vector;
            }
            default:
            {
                throw std::domain_error("type must be array, but is " + type_name());
            }
        }
    }

    /// get an array (explicit)
    template <class T, typename
              std::enable_if<
                  std::is_same<basic_json, typename T::value_type>::value and
                  not internals::has_mapped_type<T>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        switch (m_type)
        {
            case (value_t::array):
            {
                return T(m_value.array->begin(), m_value.array->end());
            }
            default:
            {
                throw std::domain_error("type must be array, but is " + type_name());
            }
        }
    }

    /// get an array (explicit)
    array_t get_impl(array_t*) const
    {
        switch (m_type)
        {
            case (value_t::array):
            {
                return *(m_value.array);
            }
            default:
            {
                throw std::domain_error("type must be array, but is " + type_name());
            }
        }
    }

    /// get a string (explicit)
    template <typename T, typename
              std::enable_if<
                  std::is_convertible<string_t, T>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        switch (m_type)
        {
            case (value_t::string):
            {
                return *m_value.string;
            }
            default:
            {
                throw std::domain_error("type must be string, but is " + type_name());
            }
        }
    }

    /// get a number (explicit)
    template<typename T, typename
             std::enable_if<
                 std::is_arithmetic<T>::value
                 , int>::type = 0>
    T get_impl(T*) const
    {
        switch (m_type)
        {
            case (value_t::number_integer):
            {
                return static_cast<T>(m_value.number_integer);
            }
            case (value_t::number_float):
            {
                return static_cast<T>(m_value.number_float);
            }
            default:
            {
                throw std::domain_error("type must be number, but is " + type_name());
            }
        }
    }

    /// get a boolean (explicit)
    boolean_t get_impl(boolean_t*) const
    {
        switch (m_type)
        {
            case (value_t::boolean):
            {
                return m_value.boolean;
            }
            default:
            {
                throw std::domain_error("type must be boolean, but is " + type_name());
            }
        }
    }

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

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

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

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

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

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

  public:

    /// @name value access
    /// @{

    /*!
    @brief get a value (explicit)

    Explicit type conversion between the JSON value and a compatible value.

    @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

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

    @throw std::domain_error in case passed type @a ValueType is incompatible
    to JSON

    @complexity Linear in the size of the JSON value.

    @liveexample{The example below shows serveral 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++
    assiciative containers such as `std::unordered_map<std::string\,
    json>`.,get__ValueType_const}

    @internal
    The idea of using a casted null pointer to choose the correct
    implementation is from <http://stackoverflow.com/a/8315197/266378>.
    @endinternal

    @sa @ref operator ValueType() const for implicit conversion
    @sa @ref get() for pointer-member access
    */
    template<typename ValueType, typename
             std::enable_if<
                 not std::is_pointer<ValueType>::value
                 , int>::type = 0>
    ValueType get() const
    {
        return get_impl(static_cast<ValueType*>(nullptr));
    }

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

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

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

    Implict 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, 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_ptr}
    */
    template<typename PointerType, typename
             std::enable_if<
                 std::is_pointer<PointerType>::value
                 , int>::type = 0>
    PointerType get_ptr() const noexcept
    {
        // get_impl_ptr will only work with non-const and non-volatile pointer
        // types. Therefore, we case away all cv properties to be able to
        // select the correct function. The cv properties will then be added
        // again by the const const cast to PointerType.
        return const_cast<PointerType>(get_impl_ptr(
                                           static_cast<typename std::add_pointer<typename std::remove_cv<typename std::remove_pointer<PointerType>::type>::type>::type>
                                           (nullptr)));
    }

    /*!
    @brief get a value (implicit)

    Implict 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

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

    @throw std::domain_error in case passed type @a ValueType is incompatible
    to JSON, thrown by @ref get() const

    @complexity Linear in the size of the JSON value.

    @liveexample{The example below shows serveral 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++
    assiciative containers such as `std::unordered_map<std::string\,
    json>`.,operator__ValueType}
    */
    template<typename ValueType, typename
             std::enable_if<
                 not std::is_pointer<ValueType>::value
                 , int>::type = 0>
    operator ValueType() const
    {
        // delegate the call to get<>() const
        return get<ValueType>();
    }

    /// @}


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

    /// @name element access
    /// @{

    /*!
    @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 std::domain_error if JSON is not an array
    @throw std::out_of_range if the index @a idx is out of range of the array;
    that is, `idx >= size()`

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read and
    written using at.,at__size_type}
    */
    reference at(size_type idx)
    {
        // at only works for arrays
        if (m_type != value_t::array)
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }

        return m_value.array->at(idx);
    }

    /*!
    @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 std::domain_error if JSON is not an array
    @throw std::out_of_range if the index @a idx is out of range of the array;
    that is, `idx >= size()`

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read using
    at.,at__size_type_const}
    */
    const_reference at(size_type idx) const
    {
        // at only works for arrays
        if (m_type != value_t::array)
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }

        return m_value.array->at(idx);
    }

    /*!
    @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 std::domain_error if JSON is not an object
    @throw std::out_of_range if the key @a key is is not stored in the object;
    that is, `find(key) == end()`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using at.,at__object_t_key_type}
    */
    reference at(const typename object_t::key_type& key)
    {
        // at only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }

        return m_value.object->at(key);
    }

    /*!
    @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 std::domain_error if JSON is not an object
    @throw std::out_of_range if the key @a key is is not stored in the object;
    that is, `find(key) == end()`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    at.,at__object_t_key_type_const}
    */
    const_reference at(const typename object_t::key_type& key) const
    {
        // at only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }

        return m_value.object->at(key);
    }

    /*!
    @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 std::domain_error if JSON is not an array or null

    @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}
    */
    reference operator[](size_type idx)
    {
        // implicitly convert null to object
        if (m_type == value_t::null)
        {
            m_type = value_t::array;
            AllocatorType<array_t> alloc;
            m_value.array = alloc.allocate(1);
            alloc.construct(m_value.array);
        }

        // [] only works for arrays
        if (m_type != value_t::array)
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }

        for (size_t i = m_value.array->size(); i <= idx; ++i)
        {
            m_value.array->push_back(basic_json());
        }

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

    /*!
    @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 std::domain_error if JSON is not an array

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read using
    the [] operator.,operatorarray__size_type_const}
    */
    const_reference operator[](size_type idx) const
    {
        // at only works for arrays
        if (m_type != value_t::array)
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }

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

    /*!
    @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 std::domain_error if JSON is not an object or null

    @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}
    */
    reference operator[](const typename object_t::key_type& key)
    {
        // implicitly convert null to object
        if (m_type == value_t::null)
        {
            m_type = value_t::object;
            AllocatorType<object_t> alloc;
            m_value.object = alloc.allocate(1);
            alloc.construct(m_value.object);
        }

        // [] only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }

        return m_value.object->operator[](key);
    }

    /*!
    @brief access specified object element

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

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

    @return reference to the element at key @a key

    @throw std::domain_error if JSON is not an object or null

    @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}
    */
    const_reference operator[](const typename object_t::key_type& key) const
    {
        // at only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }

        return m_value.object->operator[](key);
    }

    /*!
    @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.

    @note This function is required for compatibility reasons with Clang.

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

    @return reference to the element at key @a key

    @throw std::domain_error if JSON is not an object or null

    @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}
    */
    template<typename T, std::size_t n>
    reference operator[](const T (&key)[n])
    {
        // implicitly convert null to object
        if (m_type == value_t::null)
        {
            m_type = value_t::object;
            m_value = value_t::object;
        }

        // at only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }

        return m_value.object->operator[](key);
    }

    /*!
    @brief access specified object element

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

    @note This function is required for compatibility reasons with Clang.

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

    @return reference to the element at key @a key

    @throw std::domain_error if JSON is not an object or null

    @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}
    */
    template<typename T, std::size_t n>
    const_reference operator[](const T (&key)[n]) const
    {
        // at only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }

        return m_value.object->operator[](key);
    }

    /*!
    @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 cast of number, string, or boolean values, a
    reference to the value is returned.

    @complexity Constant.

    @note Calling `front` on an empty container is undefined.

    @throw std::out_of_range when called on null value

    @liveexample{The following code shows an example for @ref front.,front}
    */
    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 `{ auto tmp =
    c.end(); --tmp; return *tmp; }`.

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

    @complexity Constant.

    @note Calling `back` on an empty container is undefined.

    @throw std::out_of_range when called on null value.

    @liveexample{The following code shows an example for @ref back.,back}
    */
    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. Invalidates iterators and
    references at or after the point of the erase, including the end()
    iterator. The iterator @a pos must be valid and dereferenceable. Thus the
    end() iterator (which is valid, but is not dereferencable) 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 InteratorType an @ref iterator or @ref const_iterator

    @throw std::domain_error if called on a `null` value
    @throw std::domain_error if called on an iterator which does not belong to
    the current JSON value
    @throw std::out_of_range if called on a primitive type with invalid iterator
    (i.e., any iterator which is not end())

    @complexity The complexity depends on the type:
    - objects: amortized constant
    - arrays: linear in distance between 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}
    */
    template <class InteratorType, typename
              std::enable_if<
                  std::is_same<InteratorType, typename __basic_json::iterator>::value or
                  std::is_same<InteratorType, typename __basic_json::const_iterator>::value
                  , int>::type
              = 0>
    InteratorType erase(InteratorType pos)
    {
        // make sure iterator fits the current value
        if (this != pos.m_object)
        {
            throw std::domain_error("iterator does not fit current value");
        }

        InteratorType result = end();

        switch (m_type)
        {
            case value_t::number_integer:
            case value_t::number_float:
            case value_t::boolean:
            case value_t::string:
            {
                if (pos.m_it.generic_iterator != 0)
                {
                    throw std::out_of_range("iterator out of range");
                }

                if (m_type == value_t::string)
                {
                    delete m_value.string;
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                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:
            {
                throw std::domain_error("cannot use erase() with " + type_name());
            }
        }

        return result;
    }

    /*!
    @brief remove elements given an iterator range

    Removes the element specified by the range `[first; last)`. Invalidates
    iterators and references at or after the point of the erase, including the
    end() iterator. 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 InteratorType an @ref iterator or @ref const_iterator

    @throw std::domain_error if called on a `null` value
    @throw std::domain_error if called on iterators which does not belong to
    the current JSON value
    @throw std::out_of_range if called on a primitive type with invalid iterators
    (i.e., if `first != begin()` and `last != end()`)

    @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}
    */
    template <class InteratorType, typename
              std::enable_if<
                  std::is_same<InteratorType, typename basic_json::iterator>::value or
                  std::is_same<InteratorType, typename basic_json::const_iterator>::value
                  , int>::type
              = 0>
    InteratorType erase(InteratorType first, InteratorType last)
    {
        // make sure iterator fits the current value
        if (this != first.m_object or this != last.m_object)
        {
            throw std::domain_error("iterators do not fit current value");
        }

        InteratorType result = end();

        switch (m_type)
        {
            case value_t::number_integer:
            case value_t::number_float:
            case value_t::boolean:
            case value_t::string:
            {
                if (first.m_it.generic_iterator != 0 or last.m_it.generic_iterator != 1)
                {
                    throw std::out_of_range("iterators out of range");
                }

                if (m_type == value_t::string)
                {
                    delete m_value.string;
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                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:
            {
                throw std::domain_error("cannot use erase with " + 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 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).

    @throw std::domain_error when called on a type other than JSON object

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

    @liveexample{The example shows the effect of erase.,erase__key_type}
    */
    size_type erase(const typename object_t::key_type& key)
    {
        // this erase only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use erase() with " + type_name());
        }

        return m_value.object->erase(key);
    }

    /*!
    @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 std::domain_error when called on a type other than JSON array
    @throw std::out_of_range when `idx >= size()`

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

    @liveexample{The example shows the effect of erase.,erase__size_type}
    */
    void erase(const size_type idx)
    {
        // this erase only works for arrays
        if (m_type != value_t::array)
        {
            throw std::domain_error("cannot use erase() with " + type_name());
        }

        if (idx >= size())
        {
            throw std::out_of_range("index out of range");
        }

        m_value.array->erase(m_value.array->begin() + static_cast<difference_type>(idx));
    }

    /// find an element in an object
    iterator find(typename object_t::key_type key)
    {
        auto result = end();

        if (m_type == value_t::object)
        {
            result.m_it.object_iterator = m_value.object->find(key);
        }

        return result;
    }

    /// find an element in an object
    const_iterator find(typename object_t::key_type key) const
    {
        auto result = cend();

        if (m_type == value_t::object)
        {
            result.m_it.object_iterator = m_value.object->find(key);
        }

        return result;
    }

    /// returns the number of occurrences of a key in an object
    size_type count(typename object_t::key_type key) const
    {
        // return 0 for all nonobject types
        return (m_type == value_t::object) ? m_value.object->count(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 satisfies the Container requirements:
    - The complexity is constant.

    @liveexample{The following code shows an example for @ref begin.,begin}

    @ingroup container
    */
    iterator begin()
    {
        iterator result(this);
        result.set_begin();
        return result;
    }

    /*!
    @copydoc basic_json::cbegin()
    @ingroup container
    */
    const_iterator begin() const
    {
        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 satisfies the 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 @ref cbegin.,cbegin}

    @ingroup container
    */
    const_iterator cbegin() const
    {
        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 satisfies the Container requirements:
    - The complexity is constant.

    @liveexample{The following code shows an example for @ref end.,end}

    @ingroup container
    */
    iterator end()
    {
        iterator result(this);
        result.set_end();
        return result;
    }

    /*!
    @copydoc basic_json::cend()
    @ingroup container
    */
    const_iterator end() const
    {
        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 satisfies the 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 @ref cend.,cend}

    @ingroup container
    */
    const_iterator cend() const
    {
        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 satisfies the ReversibleContainer requirements:
    - The complexity is constant.
    - Has the semantics of `reverse_iterator(end())`.

    @liveexample{The following code shows an example for @ref rbegin.,rbegin}

    @ingroup reversiblecontainer
    */
    reverse_iterator rbegin()
    {
        return reverse_iterator(end());
    }

    /*!
    @copydoc basic_json::crbegin()
    @ingroup reversiblecontainer
    */
    const_reverse_iterator rbegin() const
    {
        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 satisfies the ReversibleContainer requirements:
    - The complexity is constant.
    - Has the semantics of `reverse_iterator(begin())`.

    @liveexample{The following code shows an example for @ref rend.,rend}

    @ingroup reversiblecontainer
    */
    reverse_iterator rend()
    {
        return reverse_iterator(begin());
    }

    /*!
    @copydoc basic_json::crend()
    @ingroup reversiblecontainer
    */
    const_reverse_iterator rend() const
    {
        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 satisfies the 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 @ref crbegin.,crbegin}

    @ingroup reversiblecontainer
    */
    const_reverse_iterator crbegin() const
    {
        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 satisfies the 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 @ref crend.,crend}

    @ingroup reversiblecontainer
    */
    const_reverse_iterator crend() const
    {
        return const_reverse_iterator(cbegin());
    }

    /// @}


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

    /// @name capacity
    /// @{

    /*!
    @brief checks whether the container is empty

    Checks if a JSON value has no elements.

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

    @requirement This function satisfies the Container requirements:
    - The complexity is constant.
    - Has the semantics of `begin() == end()`.

    @liveexample{The following code uses @ref empty to check if a @ref json
    object contains any elements.,empty}

    @ingroup container
    */
    bool empty() const noexcept
    {
        switch (m_type)
        {
            case (value_t::null):
            {
                return true;
            }

            case (value_t::array):
            {
                return m_value.array->empty();
            }

            case (value_t::object):
            {
                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        | @c 0
            boolean     | @c 1
            string      | @c 1
            number      | @c 1
            object      | result of function object_t::size()
            array       | result of function array_t::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.

    @requirement This function satisfies the Container requirements:
    - The complexity is constant.
    - Has the semantics of `std::distance(begin(), end())`.

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

    @ingroup container
    */
    size_type size() const noexcept
    {
        switch (m_type)
        {
            case (value_t::null):
            {
                return 0;
            }

            case (value_t::array):
            {
                return m_value.array->size();
            }

            case (value_t::object):
            {
                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        | @c 0
            boolean     | @c 1
            string      | @c 1
            number      | @c 1
            object      | result of function object_t::max_size()
            array       | result of function array_t::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.

    @requirement This function satisfies the Container requirements:
    - The complexity is constant.
    - Has the semantics of returning `b.size()` where `b` is the largest
      possible JSON value.

    @liveexample{The following code calls @ref max_size on the different value
    types. Note the output is implementation specific.,max_size}

    @ingroup container
    */
    size_type max_size() const noexcept
    {
        switch (m_type)
        {
            case (value_t::null):
            {
                return 0;
            }

            case (value_t::array):
            {
                return m_value.array->max_size();
            }

            case (value_t::object):
            {
                return m_value.object->max_size();
            }

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

    /// @}


    ///////////////
    // 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:

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

    @note Floating-point numbers are set to `0.0` which will be serialized to
    `0`. The vale type remains @ref number_float_t.

    @complexity Linear in the size of the JSON value.

    @liveexample{The example below shows the effect of @ref clear to different
    JSON types.,clear}
    */
    void clear() noexcept
    {
        switch (m_type)
        {
            case (value_t::null):
            case (value_t::discarded):
            {
                break;
            }

            case (value_t::number_integer):
            {
                m_value.number_integer = 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;
            }
        }
    }

    /// add an object to an array
    void push_back(basic_json&& value)
    {
        // push_back only works for null objects or arrays
        if (not(m_type == value_t::null or m_type == value_t::array))
        {
            throw std::domain_error("cannot use push_back() with " + type_name());
        }

        // transform null object into an array
        if (m_type == value_t::null)
        {
            m_type = value_t::array;
            m_value = value_t::array;
        }

        // add element to array (move semantics)
        m_value.array->push_back(std::move(value));
        // invalidate object
        value.m_type = value_t::null;
    }

    /// add an object to an array
    reference operator+=(basic_json&& value)
    {
        push_back(std::move(value));
        return *this;
    }

    /// add an object to an array
    void push_back(const basic_json& value)
    {
        // push_back only works for null objects or arrays
        if (not(m_type == value_t::null or m_type == value_t::array))
        {
            throw std::domain_error("cannot use push_back() with " + type_name());
        }

        // transform null object into an array
        if (m_type == value_t::null)
        {
            m_type = value_t::array;
            m_value = value_t::array;
        }

        // add element to array
        m_value.array->push_back(value);
    }

    /// add an object to an array
    reference operator+=(const basic_json& value)
    {
        push_back(value);
        return *this;
    }

    /// add an object to an object
    void push_back(const typename object_t::value_type& value)
    {
        // push_back only works for null objects or objects
        if (not(m_type == value_t::null or m_type == value_t::object))
        {
            throw std::domain_error("cannot use push_back() with " + type_name());
        }

        // transform null object into an object
        if (m_type == value_t::null)
        {
            m_type = value_t::object;
            m_value = value_t::object;
        }

        // add element to array
        m_value.object->insert(value);
    }

    /// add an object to an object
    reference operator+=(const typename object_t::value_type& value)
    {
        push_back(value);
        return operator[](value.first);
    }

    /*!
    @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 arrays can be
    swapped.,swap__reference}

    @ingroup container
    */
    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);
    }

    /*!
    @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 std::domain_error when JSON value is not an array

    @complexity Constant.

    @liveexample{The example below shows how JSON values can be
    swapped.,swap__array_t}

    @ingroup container
    */
    void swap(array_t& other)
    {
        // swap only works for arrays
        if (m_type != value_t::array)
        {
            throw std::domain_error("cannot use swap() with " + type_name());
        }

        // swap arrays
        std::swap(*(m_value.array), other);
    }

    /// swaps the contents
    void swap(object_t& other)
    {
        // swap only works for objects
        if (m_type != value_t::object)
        {
            throw std::domain_error("cannot use swap() with " + type_name());
        }

        // swap arrays
        std::swap(*(m_value.object), other);
    }

    /// swaps the contents
    void swap(string_t& other)
    {
        // swap only works for strings
        if (m_type != value_t::string)
        {
            throw std::domain_error("cannot use swap() with " + type_name());
        }

        // swap arrays
        std::swap(*(m_value.string), other);
    }

    /// @}


    //////////////////////////////////////////
    // 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.
    - Integer and floating-point numbers are automatically converted before
      comparison. Floating-point numbers are compared indirectly: two
      floating-point numbers `f1` and `f2` are considered equal if neither
      `f1 > f2` nor `f2 > f1` holds.
    - Two JSON null values are equal.

    @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

    @complexity Linear.

    @todo Add example.

    @ingroup container
    */
    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_float):
                    return approx(lhs.m_value.number_float, rhs.m_value.number_float);
                case (value_t::discarded):
                    return false;
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return approx(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 approx(lhs.m_value.number_float,
                          static_cast<number_float_t>(rhs.m_value.number_integer));
        }
        return false;
    }

    /*!
    @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.

    @todo Add example.

    @ingroup container
    */
    friend bool operator!=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (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.

    @todo Add example.
    */
    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_float):
                    return lhs.m_value.number_float < rhs.m_value.number_float;
                case (value_t::discarded):
                    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);
        }

        // We only reach this line if we cannot compare values. In that case,
        // we compare types.
        return lhs_type < rhs_type;
    }

    /*!
    @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.

    @todo Add example.
    */
    friend bool operator<=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (rhs < lhs);
    }

    /*!
    @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.

    @todo Add example.
    */
    friend bool operator>(const_reference lhs, const_reference rhs) noexcept
    {
        return not (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.

    @todo Add example.
    */
    friend bool operator>=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (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)`.

    @param[in,out] o  stream to serialize to
    @param[in] j  JSON value to serialize

    @return the stream @a o

    @complexity Linear.

    @liveexample{The example below shows the serialization with different
    parameters to `width` to adjust the indentation level.,operator_serialize}
    */
    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
        j.dump(o, pretty_print, static_cast<unsigned int>(indentation));
        return o;
    }

    /*!
    @brief serialize to stream
    @copydoc operator<<(std::ostream&, const basic_json&)
    */
    friend std::ostream& operator>>(const basic_json& j, std::ostream& o)
    {
        return o << j;
    }

    /// @}


    /////////////////////
    // deserialization //
    /////////////////////

    /// @name deserialization
    /// @{

    /*!
    @brief deserialize from string

    @param[in] s  string to read a serialized JSON value 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

    @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.

    @liveexample{The example below demonstrates the parse function with and
    without callback function.,parse__string__parser_callback_t}

    @sa parse(std::istream&, parser_callback_t) for a version that reads from
    an input stream
    */
    static basic_json parse(const string_t& s, parser_callback_t cb = nullptr)
    {
        return parser(s, cb).parse();
    }

    /*!
    @brief deserialize from stream

    @param[in,out] i  stream to read a serialized JSON value 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

    @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.

    @liveexample{The example below demonstrates the parse function with and
    without callback function.,parse__istream__parser_callback_t}

    @sa parse(const string_t&, parser_callback_t) for a version that reads
    from a string
    */
    static basic_json parse(std::istream& i, parser_callback_t cb = nullptr)
    {
        return parser(i, cb).parse();
    }

    /*!
    @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 std::invalid_argument in case of parse errors

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser.

    @liveexample{The example below shows how a JSON value is constructed by
    reading a serialization from a stream.,operator_deserialize}

    @sa parse(std::istream&, parser_callback_t) for a variant with a parser
    callback function to filter values while parsing
    */
    friend std::istream& operator<<(basic_json& j, std::istream& i)
    {
        j = parser(i).parse();
        return i;
    }

    /*!
    @brief deserialize from stream
    @copydoc operator<<(basic_json&, std::istream&)
    */
    friend std::istream& operator>>(std::istream& i, basic_json& j)
    {
        j = parser(i).parse();
        return i;
    }

    /// @}


  private:
    ///////////////////////////
    // convenience functions //
    ///////////////////////////

    /// return the type as string
    string_t type_name() const
    {
        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";
            }
        }
    }

    /*!
    @brief escape a string

    Escape a string by replacing certain special characters by a sequence of an
    escape character (backslash) and another character and other control
    characters by a sequence of "\u" followed by a four-digit hex
    representation.

    @param[out] o  the stream to write the escaped string to
    @param[in] s  the string to escape
    */
    static void escape_string(std::ostream& o, const string_t& s)
    {
        for (const auto c : s)
        {
            switch (c)
            {
                // quotation mark (0x22)
                case '"':
                {
                    o << "\\\"";
                    break;
                }

                // reverse solidus (0x5c)
                case '\\':
                {
                    o << "\\\\";
                    break;
                }

                // backspace (0x08)
                case '\b':
                {
                    o << "\\b";
                    break;
                }

                // formfeed (0x0c)
                case '\f':
                {
                    o << "\\f";
                    break;
                }

                // newline (0x0a)
                case '\n':
                {
                    o << "\\n";
                    break;
                }

                // carriage return (0x0d)
                case '\r':
                {
                    o << "\\r";
                    break;
                }

                // horizontal tab (0x09)
                case '\t':
                {
                    o << "\\t";
                    break;
                }

                default:
                {
                    if (c >= 0 and c <= 0x1f)
                    {
                        // control characters (everything between 0x00 and 0x1f)
                        // -> create four-digit hex representation
                        o << "\\u" << std::hex << std::setw(4) << std::setfill('0') << int(c);
                    }
                    else
                    {
                        // all other characters are added as-is
                        o << c;
                    }
                    break;
                }
            }
        }
    }

    /*!
    @brief internal implementation of the serialization function

    This function is called by the public member function dump and organizes
    the serializaion internally. The indentation level is propagated as
    additional parameter. In case of arrays and objects, the function is called
    recursively. Note that

    - strings and object keys are escaped using escape_string()
    - integer numbers are converted implictly via operator<<
    - floating-point numbers are converted to a string using "%g" format

    @param[out] o              stream to write to
    @param[in] pretty_print    whether the output shall be pretty-printed
    @param[in] indent_step     the indent level
    @param[in] current_indent  the current indent level (only used internally)
    */
    void dump(std::ostream& o, const bool pretty_print, const unsigned int indent_step,
              const unsigned int current_indent = 0) const
    {
        // variable to hold indentation for recursive calls
        unsigned int new_indent = current_indent;

        switch (m_type)
        {
            case (value_t::object):
            {
                if (m_value.object->empty())
                {
                    o << "{}";
                    return;
                }

                o << "{";

                // increase indentation
                if (pretty_print)
                {
                    new_indent += indent_step;
                    o << "\n";
                }

                for (auto i = m_value.object->cbegin(); i != m_value.object->cend(); ++i)
                {
                    if (i != m_value.object->cbegin())
                    {
                        o << (pretty_print ? ",\n" : ",");
                    }
                    o << string_t(new_indent, ' ') << "\"";
                    escape_string(o, i->first);
                    o << "\":" << (pretty_print ? " " : "");
                    i->second.dump(o, pretty_print, indent_step, new_indent);
                }

                // decrease indentation
                if (pretty_print)
                {
                    new_indent -= indent_step;
                    o << "\n";
                }

                o << string_t(new_indent, ' ') + "}";
                return;
            }

            case (value_t::array):
            {
                if (m_value.array->empty())
                {
                    o << "[]";
                    return;
                }

                o << "[";

                // increase indentation
                if (pretty_print)
                {
                    new_indent += indent_step;
                    o << "\n";
                }

                for (auto i = m_value.array->cbegin(); i != m_value.array->cend(); ++i)
                {
                    if (i != m_value.array->cbegin())
                    {
                        o << (pretty_print ? ",\n" : ",");
                    }
                    o << string_t(new_indent, ' ');
                    i->dump(o, pretty_print, indent_step, new_indent);
                }

                // decrease indentation
                if (pretty_print)
                {
                    new_indent -= indent_step;
                    o << "\n";
                }

                o << string_t(new_indent, ' ') << "]";
                return;
            }

            case (value_t::string):
            {
                o << string_t("\"");
                escape_string(o, *m_value.string);
                o << "\"";
                return;
            }

            case (value_t::boolean):
            {
                o << (m_value.boolean ? "true" : "false");
                return;
            }

            case (value_t::number_integer):
            {
                o << m_value.number_integer;
                return;
            }

            case (value_t::number_float):
            {
                // 15 digits of precision allows round-trip IEEE 754
                // string->double->string; to be safe, we read this value from
                // std::numeric_limits<number_float_t>::digits10
                o << std::setprecision(std::numeric_limits<number_float_t>::digits10) << m_value.number_float;
                return;
            }

            case (value_t::discarded):
            {
                o << "<discarded>";
                return;
            }

            default:
            {
                o << "null";
                return;
            }
        }
    }

    /// "equality" comparison for floating point numbers
    template<typename T>
    static bool approx(const T a, const T b)
    {
        return not (a > b or a < b);
    }


  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 = {};


  private:
    ///////////////
    // iterators //
    ///////////////

    /// an iterator value
    union internal_iterator
    {
        /// iterator for JSON objects
        typename object_t::iterator object_iterator;
        /// iterator for JSON arrays
        typename array_t::iterator array_iterator;
        /// generic iterator for all other types
        difference_type generic_iterator;

        /// default constructor
        internal_iterator() : generic_iterator(-1) {}
    };

  public:
    /// a random access iterator for the basic_json class
    class iterator : public std::iterator<std::random_access_iterator_tag, basic_json>
    {
        // allow basic_json class to access m_it
        friend class basic_json;

      public:
        /// the type of the values when the iterator is dereferenced
        using value_type = typename basic_json::value_type;
        /// a type to represent differences between iterators
        using difference_type = typename basic_json::difference_type;
        /// defines a pointer to the type iterated over (value_type)
        using pointer = typename basic_json::pointer;
        /// defines a reference to the type iterated over (value_type)
        using reference = typename basic_json::reference;
        /// the category of the iterator
        using iterator_category = std::bidirectional_iterator_tag;

        /// default constructor
        iterator() = default;

        /// constructor for a given JSON instance
        iterator(pointer object) : m_object(object)
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = typename object_t::iterator();
                    break;
                }
                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = typename array_t::iterator();
                    break;
                }
                default:
                {
                    m_it.generic_iterator = -1;
                    break;
                }
            }
        }

        /// copy constructor
        iterator(const iterator& other) noexcept
            : m_object(other.m_object), m_it(other.m_it)
        {}

        /// copy assignment
        iterator& operator=(iterator other) noexcept (
            std::is_nothrow_move_constructible<pointer>::value and
            std::is_nothrow_move_assignable<pointer>::value and
            std::is_nothrow_move_constructible<internal_iterator>::value and
            std::is_nothrow_move_assignable<internal_iterator>::value
        )
        {
            std::swap(m_object, other.m_object);
            std::swap(m_it, other.m_it);
            return *this;
        }

      private:
        /// set the iterator to the first value
        void set_begin()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = m_object->m_value.object->begin();
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = m_object->m_value.array->begin();
                    break;
                }

                case (basic_json::value_t::null):
                {
                    // set to end so begin()==end() is true: null is empty
                    m_it.generic_iterator = 1;
                    break;
                }

                default:
                {
                    m_it.generic_iterator = 0;
                    break;
                }
            }
        }

        /// set the iterator past the last value
        void set_end()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = m_object->m_value.object->end();
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = m_object->m_value.array->end();
                    break;
                }

                default:
                {
                    m_it.generic_iterator = 1;
                    break;
                }
            }
        }

      public:
        /// return a reference to the value pointed to by the iterator
        reference operator*()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return m_it.object_iterator->second;
                }

                case (basic_json::value_t::array):
                {
                    return *m_it.array_iterator;
                }

                case (basic_json::value_t::null):
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.generic_iterator == 0)
                    {
                        return *m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        /// dereference the iterator
        pointer operator->()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return &(m_it.object_iterator->second);
                }

                case (basic_json::value_t::array):
                {
                    return &*m_it.array_iterator;
                }

                case (basic_json::value_t::null):
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.generic_iterator == 0)
                    {
                        return m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        /// post-increment (it++)
        iterator operator++(int)
        {
            auto result = *this;

            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator++;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator++;
                    break;
                }

                default:
                {
                    m_it.generic_iterator++;
                    break;
                }
            }

            return result;
        }

        /// pre-increment (++it)
        iterator& operator++()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    ++m_it.object_iterator;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    ++m_it.array_iterator;
                    break;
                }

                default:
                {
                    ++m_it.generic_iterator;
                    break;
                }
            }

            return *this;
        }

        /// post-decrement (it--)
        iterator operator--(int)
        {
            auto result = *this;

            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator--;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator--;
                    break;
                }

                default:
                {
                    m_it.generic_iterator--;
                    break;
                }
            }

            return result;
        }

        /// pre-decrement (--it)
        iterator& operator--()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    --m_it.object_iterator;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    --m_it.array_iterator;
                    break;
                }

                default:
                {
                    --m_it.generic_iterator;
                    break;
                }
            }

            return *this;
        }

        /// comparison: equal
        bool operator==(const iterator& other) const
        {
            // if objects are not the same, the comparison is undefined
            if (m_object != other.m_object)
            {
                throw std::domain_error("cannot compare iterators of different containers");
            }

            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return (m_it.object_iterator == other.m_it.object_iterator);
                }

                case (basic_json::value_t::array):
                {
                    return (m_it.array_iterator == other.m_it.array_iterator);
                }

                default:
                {
                    return (m_it.generic_iterator == other.m_it.generic_iterator);
                }
            }
        }

        /// comparison: not equal
        bool operator!=(const iterator& other) const
        {
            return not operator==(other);
        }

        /// comparison: smaller
        bool operator<(const iterator& other) const
        {
            // if objects are not the same, the comparison is undefined
            if (m_object != other.m_object)
            {
                throw std::domain_error("cannot compare iterators of different containers");
            }

            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator< for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    return (m_it.array_iterator < other.m_it.array_iterator);
                }

                default:
                {
                    return (m_it.generic_iterator < other.m_it.generic_iterator);
                }
            }
        }

        /// comparison: less than or equal
        bool operator<=(const iterator& other) const
        {
            return not other.operator < (*this);
        }

        /// comparison: greater than
        bool operator>(const iterator& other) const
        {
            return not operator<=(other);
        }

        /// comparison: greater than or equal
        bool operator>=(const iterator& other) const
        {
            return not operator<(other);
        }

        /// add to iterator
        iterator& operator+=(difference_type i)
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator+= for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator += i;
                    break;
                }

                default:
                {
                    m_it.generic_iterator += i;
                    break;
                }
            }

            return *this;
        }

        /// subtract from iterator
        iterator& operator-=(difference_type i)
        {
            return operator+=(-i);
        }

        /// add to iterator
        iterator operator+(difference_type i)
        {
            auto result = *this;
            result += i;
            return result;
        }

        /// subtract from iterator
        iterator operator-(difference_type i)
        {
            auto result = *this;
            result -= i;
            return result;
        }

        /// return difference
        difference_type operator-(const iterator& other) const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator- for object iterators");
                    return 0;
                }

                case (basic_json::value_t::array):
                {
                    return m_it.array_iterator - other.m_it.array_iterator;
                }

                default:
                {
                    return m_it.generic_iterator - other.m_it.generic_iterator;
                }
            }
        }

        /// access to successor
        reference operator[](difference_type n)
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator[] for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    return *(m_it.array_iterator + n);
                }

                case (basic_json::value_t::null):
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.generic_iterator == -n)
                    {
                        return *m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        /// return the key of an object iterator
        typename object_t::key_type key() const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return m_it.object_iterator->first;
                }

                default:
                {
                    throw std::domain_error("cannot use key() for non-object iterators");
                }
            }
        }

        /// return the key of an iterator
        reference value()
        {
            return operator*();
        }

      private:
        /// associated JSON instance
        pointer m_object = nullptr;
        /// the actual iterator of the associated instance
        internal_iterator m_it = internal_iterator();
    };

    /// a const random access iterator for the basic_json class
    class const_iterator : public std::iterator<std::random_access_iterator_tag, const basic_json>
    {
        // allow basic_json class to access m_it
        friend class basic_json;

      public:
        /// the type of the values when the iterator is dereferenced
        using value_type = typename basic_json::value_type;
        /// a type to represent differences between iterators
        using difference_type = typename basic_json::difference_type;
        /// defines a pointer to the type iterated over (value_type)
        using pointer = typename basic_json::const_pointer;
        /// defines a reference to the type iterated over (value_type)
        using reference = typename basic_json::const_reference;
        /// the category of the iterator
        using iterator_category = std::bidirectional_iterator_tag;

        /// default constructor
        const_iterator() = default;

        /// constructor for a given JSON instance
        const_iterator(pointer object) : m_object(object)
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = typename object_t::iterator();
                    break;
                }
                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = typename array_t::iterator();
                    break;
                }
                default:
                {
                    m_it.generic_iterator = -1;
                    break;
                }
            }
        }

        /// copy constructor given a nonconst iterator
        const_iterator(const iterator& other) : m_object(other.m_object)
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = other.m_it.object_iterator;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = other.m_it.array_iterator;
                    break;
                }

                default:
                {
                    m_it.generic_iterator = other.m_it.generic_iterator;
                    break;
                }
            }
        }

        /// copy constructor
        const_iterator(const const_iterator& other) noexcept
            : m_object(other.m_object), m_it(other.m_it)
        {}

        /// copy assignment
        const_iterator& operator=(const_iterator other) noexcept(
            std::is_nothrow_move_constructible<pointer>::value and
            std::is_nothrow_move_assignable<pointer>::value and
            std::is_nothrow_move_constructible<internal_iterator>::value and
            std::is_nothrow_move_assignable<internal_iterator>::value
        )
        {
            std::swap(m_object, other.m_object);
            std::swap(m_it, other.m_it);
            return *this;
        }

      private:
        /// set the iterator to the first value
        void set_begin()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = m_object->m_value.object->begin();
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = m_object->m_value.array->begin();
                    break;
                }

                case (basic_json::value_t::null):
                {
                    // set to end so begin()==end() is true: null is empty
                    m_it.generic_iterator = 1;
                    break;
                }

                default:
                {
                    m_it.generic_iterator = 0;
                    break;
                }
            }
        }

        /// set the iterator past the last value
        void set_end()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    m_it.object_iterator = m_object->m_value.object->end();
                    break;
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator = m_object->m_value.array->end();
                    break;
                }

                default:
                {
                    m_it.generic_iterator = 1;
                    break;
                }
            }
        }

      public:
        /// return a reference to the value pointed to by the iterator
        reference operator*() const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return m_it.object_iterator->second;
                }

                case (basic_json::value_t::array):
                {
                    return *m_it.array_iterator;
                }

                case (basic_json::value_t::null):
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.generic_iterator == 0)
                    {
                        return *m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        /// dereference the iterator
        pointer operator->() const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return &(m_it.object_iterator->second);
                }

                case (basic_json::value_t::array):
                {
                    return &*m_it.array_iterator;
                }

                default:
                {
                    if (m_it.generic_iterator == 0)
                    {
                        return m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        /// post-increment (it++)
        const_iterator operator++(int)
        {
            auto result = *this;
            ++(*this);

            return result;
        }

        /// pre-increment (++it)
        const_iterator& operator++()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    ++m_it.object_iterator;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    ++m_it.array_iterator;
                    break;
                }

                default:
                {
                    ++m_it.generic_iterator;
                    break;
                }
            }

            return *this;
        }

        /// post-decrement (it--)
        const_iterator operator--(int)
        {
            auto result = *this;
            --(*this);

            return result;
        }

        /// pre-decrement (--it)
        const_iterator& operator--()
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    --m_it.object_iterator;
                    break;
                }

                case (basic_json::value_t::array):
                {
                    --m_it.array_iterator;
                    break;
                }

                default:
                {
                    --m_it.generic_iterator;
                    break;
                }
            }

            return *this;
        }

        /// comparison: equal
        bool operator==(const const_iterator& other) const
        {
            // if objects are not the same, the comparison is undefined
            if (m_object != other.m_object)
            {
                throw std::domain_error("cannot compare iterators of different containers");
            }

            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return (m_it.object_iterator == other.m_it.object_iterator);
                }

                case (basic_json::value_t::array):
                {
                    return (m_it.array_iterator == other.m_it.array_iterator);
                }

                default:
                {
                    return (m_it.generic_iterator == other.m_it.generic_iterator);
                }
            }
        }

        /// comparison: not equal
        bool operator!=(const const_iterator& other) const
        {
            return not operator==(other);
        }

        /// comparison: smaller
        bool operator<(const const_iterator& other) const
        {
            // if objects are not the same, the comparison is undefined
            if (m_object != other.m_object)
            {
                throw std::domain_error("cannot compare iterators of different containers");
            }

            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator< for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    return (m_it.array_iterator < other.m_it.array_iterator);
                }

                default:
                {
                    return (m_it.generic_iterator < other.m_it.generic_iterator);
                }
            }
        }

        /// comparison: less than or equal
        bool operator<=(const const_iterator& other) const
        {
            return not other.operator < (*this);
        }

        /// comparison: greater than
        bool operator>(const const_iterator& other) const
        {
            return not operator<=(other);
        }

        /// comparison: greater than or equal
        bool operator>=(const const_iterator& other) const
        {
            return not operator<(other);
        }

        /// add to iterator
        const_iterator& operator+=(difference_type i)
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator+= for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    m_it.array_iterator += i;
                    break;
                }

                default:
                {
                    m_it.generic_iterator += i;
                    break;
                }
            }

            return *this;
        }

        /// subtract from iterator
        const_iterator& operator-=(difference_type i)
        {
            return operator+=(-i);
        }

        /// add to iterator
        const_iterator operator+(difference_type i)
        {
            auto result = *this;
            result += i;
            return result;
        }

        /// subtract from iterator
        const_iterator operator-(difference_type i)
        {
            auto result = *this;
            result -= i;
            return result;
        }

        /// return difference
        difference_type operator-(const const_iterator& other) const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator- for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    return m_it.array_iterator - other.m_it.array_iterator;
                }

                default:
                {
                    return m_it.generic_iterator - other.m_it.generic_iterator;
                }
            }
        }

        /// access to successor
        reference operator[](difference_type n) const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    throw std::domain_error("cannot use operator[] for object iterators");
                }

                case (basic_json::value_t::array):
                {
                    return *(m_it.array_iterator + n);
                }

                case (basic_json::value_t::null):
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.generic_iterator == -n)
                    {
                        return *m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        /// return the key of an object iterator
        typename object_t::key_type key() const
        {
            switch (m_object->m_type)
            {
                case (basic_json::value_t::object):
                {
                    return m_it.object_iterator->first;
                }

                default:
                {
                    throw std::domain_error("cannot use key() for non-object iterators");
                }
            }
        }

        /// return the value of an iterator
        reference value() const
        {
            return operator*();
        }

      private:
        /// associated JSON instance
        pointer m_object = nullptr;
        /// the actual iterator of the associated instance
        internal_iterator m_it = internal_iterator();
    };

    /// a reverse random access iterator for the basic_json class
    class reverse_iterator : public std::reverse_iterator<typename basic_json::iterator>
    {
      public:
        reverse_iterator(const typename
                         std::reverse_iterator<typename basic_json::iterator>::iterator_type&
                         it)
            : std::reverse_iterator<basic_json::iterator>(it) {}

        /// return the key of an object iterator
        typename object_t::key_type key() const
        {
            return this->base().key();
        }

        /// return the value of an iterator
        reference value() const
        {
            return this->base().operator * ();
        }
    };

    /// a const reverse random access iterator for the basic_json class
    class const_reverse_iterator : public std::reverse_iterator<typename basic_json::const_iterator>
    {
      public:
        const_reverse_iterator(const typename
                               std::reverse_iterator<typename basic_json::const_iterator>::iterator_type& it)
            : std::reverse_iterator<basic_json::const_iterator>(it) {}

        /// return the key of an object iterator
        typename object_t::key_type key() const
        {
            return this->base().key();
        }

        /// return the value of an iterator
        const_reference value() const
        {
            return this->base().operator * ();
        }
    };


  private:
    //////////////////////
    // lexer and parser //
    //////////////////////

    /*!
    @brief lexical analysis

    This class organizes the lexical analysis during JSON deserialization. The
    core of it is a scanner generated by re2c <http://re2c.org> that processes
    a buffer and recognizes tokens according to RFC 7159 and ECMA-404.
    */
    class lexer
    {
      public:
        /// token types for the parser
        enum class token_type
        {
            uninitialized,    ///< indicating the scanner is uninitialized
            literal_true,     ///< the "true" literal
            literal_false,    ///< the "false" literal
            literal_null,     ///< the "null" literal
            value_string,     ///< a string - use get_string() for actual value
            value_number,     ///< a number - use get_number() for actual value
            begin_array,      ///< the character for array begin "["
            begin_object,     ///< the character for object begin "{"
            end_array,        ///< the character for array end "]"
            end_object,       ///< the character for object end "}"
            name_separator,   ///< the name separator ":"
            value_separator,  ///< the value separator ","
            parse_error,      ///< indicating a parse error
            end_of_input      ///< indicating the end of the input buffer
        };

        /// the char type to use in the lexer
        using lexer_char_t = unsigned char;

        /// constructor with a given buffer
        explicit lexer(const string_t& s) noexcept
            : m_stream(nullptr), m_buffer(s)
        {
            m_content = reinterpret_cast<const lexer_char_t*>(s.c_str());
            m_start = m_cursor = m_content;
            m_limit = m_content + s.size();
        }
        explicit lexer(std::istream* s) noexcept
            : m_stream(s), m_buffer()
        {
            getline(*m_stream, m_buffer);
            m_content = reinterpret_cast<const lexer_char_t*>(m_buffer.c_str());
            m_start = m_cursor = m_content;
            m_limit = m_content + m_buffer.size();
        }

        /// default constructor
        lexer() = default;

        // switch of unwanted functions
        lexer(const lexer&) = delete;
        lexer operator=(const lexer&) = delete;

        /*!
        @brief create a string from a Unicode code point

        @param[in] codepoint1  the code point (can be high surrogate)
        @param[in] codepoint2  the code point (can be low surrogate or 0)
        @return string representation of the code point
        @throw std::out_of_range if code point is >0x10ffff
        @throw std::invalid_argument if the low surrogate is invalid

        @see <http://en.wikipedia.org/wiki/UTF-8#Sample_code>
        */
        static string_t to_unicode(const std::size_t codepoint1,
                                   const std::size_t codepoint2 = 0)
        {
            string_t result;

            // calculate the codepoint from the given code points
            std::size_t codepoint = codepoint1;

            // check if codepoint1 is a high surrogate
            if (codepoint1 >= 0xD800 and codepoint1 <= 0xDBFF)
            {
                // check if codepoint2 is a low surrogate
                if (codepoint2 >= 0xDC00 and codepoint2 <= 0xDFFF)
                {
                    codepoint =
                        // high surrogate occupies the most significant 22 bits
                        (codepoint1 << 10)
                        // low surrogate occupies the least significant 15 bits
                        + codepoint2
                        // there is still the 0xD800, 0xDC00 and 0x10000 noise
                        // in the result so we have to substract with:
                        // (0xD800 << 10) + DC00 - 0x10000 = 0x35FDC00
                        - 0x35FDC00;
                }
                else
                {
                    throw std::invalid_argument("missing or wrong low surrogate");
                }
            }

            if (codepoint < 0x80)
            {
                // 1-byte characters: 0xxxxxxx (ASCII)
                result.append(1, static_cast<typename string_t::value_type>(codepoint));
            }
            else if (codepoint <= 0x7ff)
            {
                // 2-byte characters: 110xxxxx 10xxxxxx
                result.append(1, static_cast<typename string_t::value_type>(0xC0 | ((codepoint >> 6) & 0x1F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | (codepoint & 0x3F)));
            }
            else if (codepoint <= 0xffff)
            {
                // 3-byte characters: 1110xxxx 10xxxxxx 10xxxxxx
                result.append(1, static_cast<typename string_t::value_type>(0xE0 | ((codepoint >> 12) & 0x0F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | ((codepoint >> 6) & 0x3F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | (codepoint & 0x3F)));
            }
            else if (codepoint <= 0x10ffff)
            {
                // 4-byte characters: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
                result.append(1, static_cast<typename string_t::value_type>(0xF0 | ((codepoint >> 18) & 0x07)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | ((codepoint >> 12) & 0x3F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | ((codepoint >> 6) & 0x3F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | (codepoint & 0x3F)));
            }
            else
            {
                throw std::out_of_range("code points above 0x10FFFF are invalid");
            }

            return result;
        }

        /// return name of values of type token_type
        static std::string token_type_name(token_type t)
        {
            switch (t)
            {
                case (token_type::uninitialized):
                    return "<uninitialized>";
                case (token_type::literal_true):
                    return "true literal";
                case (token_type::literal_false):
                    return "false literal";
                case (token_type::literal_null):
                    return "null literal";
                case (token_type::value_string):
                    return "string literal";
                case (token_type::value_number):
                    return "number literal";
                case (token_type::begin_array):
                    return "[";
                case (token_type::begin_object):
                    return "{";
                case (token_type::end_array):
                    return "]";
                case (token_type::end_object):
                    return "}";
                case (token_type::name_separator):
                    return ":";
                case (token_type::value_separator):
                    return ",";
                case (token_type::end_of_input):
                    return "<end of input>";
                default:
                    return "<parse error>";
            }
        }

        /*!
        This function implements a scanner for JSON. It is specified using
        regular expressions that try to follow RFC 7159 and ECMA-404 as close
        as possible. These regular expressions are then translated into a
        deterministic finite automaton (DFA) by the tool re2c
        <http://re2c.org>. As a result, the translated code for this function
        consists of a large block of code with goto jumps.

        @return the class of the next token read from the buffer
        */
        token_type scan() noexcept
        {
            // pointer for backtracking information
            m_marker = nullptr;

            // remember the begin of the token
            m_start = m_cursor;


            {
                lexer_char_t yych;
                unsigned int yyaccept = 0;
                static const unsigned char yybm[] =
                {
                    0,   0,   0,   0,   0,   0,   0,   0,
                    0,  32,  32,   0,   0,  32,   0,   0,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    96,  64,   0,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    192, 192, 192, 192, 192, 192, 192, 192,
                    192, 192,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,   0,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                };

                if ((m_limit - m_cursor) < 5)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '9')
                {
                    if (yych <= ' ')
                    {
                        if (yych <= '\n')
                        {
                            if (yych <= 0x00)
                            {
                                goto basic_json_parser_27;
                            }
                            if (yych <= 0x08)
                            {
                                goto basic_json_parser_29;
                            }
                            if (yych >= '\n')
                            {
                                goto basic_json_parser_4;
                            }
                        }
                        else
                        {
                            if (yych == '\r')
                            {
                                goto basic_json_parser_2;
                            }
                            if (yych <= 0x1F)
                            {
                                goto basic_json_parser_29;
                            }
                        }
                    }
                    else
                    {
                        if (yych <= ',')
                        {
                            if (yych == '"')
                            {
                                goto basic_json_parser_26;
                            }
                            if (yych <= '+')
                            {
                                goto basic_json_parser_29;
                            }
                            goto basic_json_parser_14;
                        }
                        else
                        {
                            if (yych <= '-')
                            {
                                goto basic_json_parser_22;
                            }
                            if (yych <= '/')
                            {
                                goto basic_json_parser_29;
                            }
                            if (yych <= '0')
                            {
                                goto basic_json_parser_23;
                            }
                            goto basic_json_parser_25;
                        }
                    }
                }
                else
                {
                    if (yych <= 'm')
                    {
                        if (yych <= '\\')
                        {
                            if (yych <= ':')
                            {
                                goto basic_json_parser_16;
                            }
                            if (yych == '[')
                            {
                                goto basic_json_parser_6;
                            }
                            goto basic_json_parser_29;
                        }
                        else
                        {
                            if (yych <= ']')
                            {
                                goto basic_json_parser_8;
                            }
                            if (yych == 'f')
                            {
                                goto basic_json_parser_21;
                            }
                            goto basic_json_parser_29;
                        }
                    }
                    else
                    {
                        if (yych <= 'z')
                        {
                            if (yych <= 'n')
                            {
                                goto basic_json_parser_18;
                            }
                            if (yych == 't')
                            {
                                goto basic_json_parser_20;
                            }
                            goto basic_json_parser_29;
                        }
                        else
                        {
                            if (yych <= '{')
                            {
                                goto basic_json_parser_10;
                            }
                            if (yych == '}')
                            {
                                goto basic_json_parser_12;
                            }
                            goto basic_json_parser_29;
                        }
                    }
                }
basic_json_parser_2:
                ++m_cursor;
                yych = *m_cursor;
                goto basic_json_parser_5;
basic_json_parser_3:
                {
                    return scan();
                }
basic_json_parser_4:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
basic_json_parser_5:
                if (yybm[0 + yych] & 32)
                {
                    goto basic_json_parser_4;
                }
                goto basic_json_parser_3;
basic_json_parser_6:
                ++m_cursor;
                {
                    return token_type::begin_array;
                }
basic_json_parser_8:
                ++m_cursor;
                {
                    return token_type::end_array;
                }
basic_json_parser_10:
                ++m_cursor;
                {
                    return token_type::begin_object;
                }
basic_json_parser_12:
                ++m_cursor;
                {
                    return token_type::end_object;
                }
basic_json_parser_14:
                ++m_cursor;
                {
                    return token_type::value_separator;
                }
basic_json_parser_16:
                ++m_cursor;
                {
                    return token_type::name_separator;
                }
basic_json_parser_18:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 'u')
                {
                    goto basic_json_parser_59;
                }
basic_json_parser_19:
                {
                    return token_type::parse_error;
                }
basic_json_parser_20:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 'r')
                {
                    goto basic_json_parser_55;
                }
                goto basic_json_parser_19;
basic_json_parser_21:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 'a')
                {
                    goto basic_json_parser_50;
                }
                goto basic_json_parser_19;
basic_json_parser_22:
                yych = *++m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_19;
                }
                if (yych <= '0')
                {
                    goto basic_json_parser_49;
                }
                if (yych <= '9')
                {
                    goto basic_json_parser_40;
                }
                goto basic_json_parser_19;
basic_json_parser_23:
                yyaccept = 1;
                yych = *(m_marker = ++m_cursor);
                if (yych <= 'D')
                {
                    if (yych == '.')
                    {
                        goto basic_json_parser_42;
                    }
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_43;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_43;
                    }
                }
basic_json_parser_24:
                {
                    return token_type::value_number;
                }
basic_json_parser_25:
                yyaccept = 1;
                yych = *(m_marker = ++m_cursor);
                goto basic_json_parser_41;
basic_json_parser_26:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych <= 0x0F)
                {
                    goto basic_json_parser_19;
                }
                goto basic_json_parser_31;
basic_json_parser_27:
                ++m_cursor;
                {
                    return token_type::end_of_input;
                }
basic_json_parser_29:
                yych = *++m_cursor;
                goto basic_json_parser_19;
basic_json_parser_30:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
basic_json_parser_31:
                if (yybm[0 + yych] & 64)
                {
                    goto basic_json_parser_30;
                }
                if (yych <= 0x0F)
                {
                    goto basic_json_parser_32;
                }
                if (yych <= '"')
                {
                    goto basic_json_parser_34;
                }
                goto basic_json_parser_33;
basic_json_parser_32:
                m_cursor = m_marker;
                if (yyaccept == 0)
                {
                    goto basic_json_parser_19;
                }
                else
                {
                    goto basic_json_parser_24;
                }
basic_json_parser_33:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= 'e')
                {
                    if (yych <= '/')
                    {
                        if (yych == '"')
                        {
                            goto basic_json_parser_30;
                        }
                        if (yych <= '.')
                        {
                            goto basic_json_parser_32;
                        }
                        goto basic_json_parser_30;
                    }
                    else
                    {
                        if (yych <= '\\')
                        {
                            if (yych <= '[')
                            {
                                goto basic_json_parser_32;
                            }
                            goto basic_json_parser_30;
                        }
                        else
                        {
                            if (yych == 'b')
                            {
                                goto basic_json_parser_30;
                            }
                            goto basic_json_parser_32;
                        }
                    }
                }
                else
                {
                    if (yych <= 'q')
                    {
                        if (yych <= 'f')
                        {
                            goto basic_json_parser_30;
                        }
                        if (yych == 'n')
                        {
                            goto basic_json_parser_30;
                        }
                        goto basic_json_parser_32;
                    }
                    else
                    {
                        if (yych <= 's')
                        {
                            if (yych <= 'r')
                            {
                                goto basic_json_parser_30;
                            }
                            goto basic_json_parser_32;
                        }
                        else
                        {
                            if (yych <= 't')
                            {
                                goto basic_json_parser_30;
                            }
                            if (yych <= 'u')
                            {
                                goto basic_json_parser_36;
                            }
                            goto basic_json_parser_32;
                        }
                    }
                }
basic_json_parser_34:
                ++m_cursor;
                {
                    return token_type::value_string;
                }
basic_json_parser_36:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych >= ':')
                    {
                        goto basic_json_parser_32;
                    }
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_37;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych >= 'g')
                    {
                        goto basic_json_parser_32;
                    }
                }
basic_json_parser_37:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych >= ':')
                    {
                        goto basic_json_parser_32;
                    }
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_38;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych >= 'g')
                    {
                        goto basic_json_parser_32;
                    }
                }
basic_json_parser_38:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych >= ':')
                    {
                        goto basic_json_parser_32;
                    }
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_39;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych >= 'g')
                    {
                        goto basic_json_parser_32;
                    }
                }
basic_json_parser_39:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych <= '9')
                    {
                        goto basic_json_parser_30;
                    }
                    goto basic_json_parser_32;
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_30;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych <= 'f')
                    {
                        goto basic_json_parser_30;
                    }
                    goto basic_json_parser_32;
                }
basic_json_parser_40:
                yyaccept = 1;
                m_marker = ++m_cursor;
                if ((m_limit - m_cursor) < 3)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
basic_json_parser_41:
                if (yybm[0 + yych] & 128)
                {
                    goto basic_json_parser_40;
                }
                if (yych <= 'D')
                {
                    if (yych != '.')
                    {
                        goto basic_json_parser_24;
                    }
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_43;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_43;
                    }
                    goto basic_json_parser_24;
                }
basic_json_parser_42:
                yych = *++m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_32;
                }
                if (yych <= '9')
                {
                    goto basic_json_parser_47;
                }
                goto basic_json_parser_32;
basic_json_parser_43:
                yych = *++m_cursor;
                if (yych <= ',')
                {
                    if (yych != '+')
                    {
                        goto basic_json_parser_32;
                    }
                }
                else
                {
                    if (yych <= '-')
                    {
                        goto basic_json_parser_44;
                    }
                    if (yych <= '/')
                    {
                        goto basic_json_parser_32;
                    }
                    if (yych <= '9')
                    {
                        goto basic_json_parser_45;
                    }
                    goto basic_json_parser_32;
                }
basic_json_parser_44:
                yych = *++m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_32;
                }
                if (yych >= ':')
                {
                    goto basic_json_parser_32;
                }
basic_json_parser_45:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_24;
                }
                if (yych <= '9')
                {
                    goto basic_json_parser_45;
                }
                goto basic_json_parser_24;
basic_json_parser_47:
                yyaccept = 1;
                m_marker = ++m_cursor;
                if ((m_limit - m_cursor) < 3)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= 'D')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_24;
                    }
                    if (yych <= '9')
                    {
                        goto basic_json_parser_47;
                    }
                    goto basic_json_parser_24;
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_43;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_43;
                    }
                    goto basic_json_parser_24;
                }
basic_json_parser_49:
                yyaccept = 1;
                yych = *(m_marker = ++m_cursor);
                if (yych <= 'D')
                {
                    if (yych == '.')
                    {
                        goto basic_json_parser_42;
                    }
                    goto basic_json_parser_24;
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_43;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_43;
                    }
                    goto basic_json_parser_24;
                }
basic_json_parser_50:
                yych = *++m_cursor;
                if (yych != 'l')
                {
                    goto basic_json_parser_32;
                }
                yych = *++m_cursor;
                if (yych != 's')
                {
                    goto basic_json_parser_32;
                }
                yych = *++m_cursor;
                if (yych != 'e')
                {
                    goto basic_json_parser_32;
                }
                ++m_cursor;
                {
                    return token_type::literal_false;
                }
basic_json_parser_55:
                yych = *++m_cursor;
                if (yych != 'u')
                {
                    goto basic_json_parser_32;
                }
                yych = *++m_cursor;
                if (yych != 'e')
                {
                    goto basic_json_parser_32;
                }
                ++m_cursor;
                {
                    return token_type::literal_true;
                }
basic_json_parser_59:
                yych = *++m_cursor;
                if (yych != 'l')
                {
                    goto basic_json_parser_32;
                }
                yych = *++m_cursor;
                if (yych != 'l')
                {
                    goto basic_json_parser_32;
                }
                ++m_cursor;
                {
                    return token_type::literal_null;
                }
            }


        }

        /// append data from the stream to the internal buffer
        void yyfill() noexcept
        {
            if (not m_stream or not * m_stream)
            {
                return;
            }

            const ssize_t offset_start = m_start - m_content;
            const ssize_t offset_marker = m_marker - m_start;
            const ssize_t offset_cursor = m_cursor - m_start;

            m_buffer.erase(0, static_cast<size_t>(offset_start));
            std::string line;
            std::getline(*m_stream, line);
            m_buffer += "\n" + line; // add line with newline symbol

            m_content = reinterpret_cast<const lexer_char_t*>(m_buffer.c_str());
            m_start  = m_content;
            m_marker = m_start + offset_marker;
            m_cursor = m_start + offset_cursor;
            m_limit  = m_start + m_buffer.size() - 1;
        }

        /// return string representation of last read token
        string_t get_token() const noexcept
        {
            return string_t(reinterpret_cast<typename string_t::const_pointer>(m_start),
                            static_cast<size_t>(m_cursor - m_start));
        }

        /*!
        @brief return string value for string tokens

        The function iterates the characters between the opening and closing
        quotes of the string value. The complete string is the range
        [m_start,m_cursor). Consequently, we iterate from m_start+1 to
        m_cursor-1.

        We differentiate two cases:

        1. Escaped characters. In this case, a new character is constructed
           according to the nature of the escape. Some escapes create new
           characters (e.g., @c "\\n" is replaced by @c "\n"), some are copied
           as is (e.g., @c "\\\\"). Furthermore, Unicode escapes of the shape
           @c "\\uxxxx" need special care. In this case, to_unicode takes care
           of the construction of the values.
        2. Unescaped characters are copied as is.

        @return string value of current token without opening and closing quotes
        @throw std::out_of_range if to_unicode fails
        */
        string_t get_string() const
        {
            string_t result;
            result.reserve(static_cast<size_t>(m_cursor - m_start - 2));

            // iterate the result between the quotes
            for (const lexer_char_t* i = m_start + 1; i < m_cursor - 1; ++i)
            {
                // process escaped characters
                if (*i == '\\')
                {
                    // read next character
                    ++i;

                    switch (*i)
                    {
                        // the default escapes
                        case 't':
                        {
                            result += "\t";
                            break;
                        }
                        case 'b':
                        {
                            result += "\b";
                            break;
                        }
                        case 'f':
                        {
                            result += "\f";
                            break;
                        }
                        case 'n':
                        {
                            result += "\n";
                            break;
                        }
                        case 'r':
                        {
                            result += "\r";
                            break;
                        }
                        case '\\':
                        {
                            result += "\\";
                            break;
                        }
                        case '/':
                        {
                            result += "/";
                            break;
                        }
                        case '"':
                        {
                            result += "\"";
                            break;
                        }

                        // unicode
                        case 'u':
                        {
                            // get code xxxx from uxxxx
                            auto codepoint = std::strtoul(std::string(reinterpret_cast<typename string_t::const_pointer>(i + 1),
                                                          4).c_str(), nullptr, 16);

                            // check if codepoint is a high surrogate
                            if (codepoint >= 0xD800 and codepoint <= 0xDBFF)
                            {
                                // make sure there is a subsequent unicode
                                if ((i + 6 >= m_limit) or * (i + 5) != '\\' or * (i + 6) != 'u')
                                {
                                    throw std::invalid_argument("missing low surrogate");
                                }

                                // get code yyyy from uxxxx\uyyyy
                                auto codepoint2 = std::strtoul(std::string(reinterpret_cast<typename string_t::const_pointer>
                                                               (i + 7), 4).c_str(), nullptr, 16);
                                result += to_unicode(codepoint, codepoint2);
                                // skip the next 11 characters (xxxx\uyyyy)
                                i += 11;
                            }
                            else
                            {
                                // add unicode character(s)
                                result += to_unicode(codepoint);
                                // skip the next four characters (xxxx)
                                i += 4;
                            }
                            break;
                        }
                    }
                }
                else
                {
                    // all other characters are just copied to the end of the
                    // string
                    result.append(1, static_cast<typename string_t::value_type>(*i));
                }
            }

            return result;
        }

        /*!
        @brief return number value for number tokens

        This function translates the last token into a floating point number.
        The pointer m_begin points to the beginning of the parsed number. We
        pass this pointer to std::strtod which sets endptr to the first
        character past the converted number. If this pointer is not the same as
        m_cursor, then either more or less characters have been used during the
        comparison. This can happen for inputs like "01" which will be treated
        like number 0 followed by number 1.

        @return the result of the number conversion or NAN if the conversion
        read past the current token. The latter case needs to be treated by the
        caller function.

        @throw std::range_error if passed value is out of range
        */
        long double get_number() const
        {
            // conversion
            typename string_t::value_type* endptr;
            const auto float_val = std::strtold(reinterpret_cast<typename string_t::const_pointer>(m_start),
                                                &endptr);

            // return float_val if the whole number was translated and NAN
            // otherwise
            return (reinterpret_cast<lexer_char_t*>(endptr) == m_cursor) ? float_val : NAN;
        }

      private:
        /// optional input stream
        std::istream* m_stream;
        /// the buffer
        string_t m_buffer;
        /// the buffer pointer
        const lexer_char_t* m_content = nullptr;
        /// pointer to the beginning of the current symbol
        const lexer_char_t* m_start = nullptr;
        /// pointer for backtracking information
        const lexer_char_t* m_marker = nullptr;
        /// pointer to the current symbol
        const lexer_char_t* m_cursor = nullptr;
        /// pointer to the end of the buffer
        const lexer_char_t* m_limit = nullptr;
    };

    /*!
    @brief syntax analysis
    */
    class parser
    {
      public:
        /// constructor for strings
        parser(const string_t& s, parser_callback_t cb = nullptr) : callback(cb), m_lexer(s)
        {
            // read first token
            get_token();
        }

        /// a parser reading from an input stream
        parser(std::istream& _is, parser_callback_t cb = nullptr) : callback(cb),
            m_lexer(&_is)
        {
            // read first token
            get_token();
        }

        /// public parser interface
        basic_json parse()
        {
            basic_json result = parse_internal(true);

            expect(lexer::token_type::end_of_input);

            // return parser result and replace it with null in case the
            // top-level value was discarded by the callback function
            return result.is_discarded() ? basic_json() : result;
        }

      private:
        /// the actual parser
        basic_json parse_internal(bool keep)
        {
            auto result = basic_json(value_t::discarded);

            switch (last_token)
            {
                case (lexer::token_type::begin_object):
                {
                    if (keep and (not callback or (keep = callback(depth++, parse_event_t::object_start, result))))
                    {
                        // explicitly set result to object to cope with {}
                        result.m_type = value_t::object;
                        result.m_value = json_value(value_t::object);
                    }

                    // read next token
                    get_token();

                    // closing } -> we are done
                    if (last_token == lexer::token_type::end_object)
                    {
                        get_token();
                        if (keep and callback and not callback(--depth, parse_event_t::object_end, result))
                        {
                            result = basic_json(value_t::discarded);
                        }
                        return result;
                    }

                    // no comma is expected here
                    unexpect(lexer::token_type::value_separator);

                    // otherwise: parse key-value pairs
                    do
                    {
                        // ugly, but could be fixed with loop reorganization
                        if (last_token == lexer::token_type::value_separator)
                        {
                            get_token();
                        }

                        // store key
                        expect(lexer::token_type::value_string);
                        const auto key = m_lexer.get_string();

                        bool keep_tag = false;
                        if (keep)
                        {
                            if (callback)
                            {
                                basic_json k(key);
                                keep_tag = callback(depth, parse_event_t::key, k);
                            }
                            else
                            {
                                keep_tag = true;
                            }
                        }

                        // parse separator (:)
                        get_token();
                        expect(lexer::token_type::name_separator);

                        // parse and add value
                        get_token();
                        auto value = parse_internal(keep);
                        if (keep and keep_tag and not value.is_discarded())
                        {
                            result[key] = std::move(value);
                        }
                    }
                    while (last_token == lexer::token_type::value_separator);

                    // closing }
                    expect(lexer::token_type::end_object);
                    get_token();
                    if (keep and callback and not callback(--depth, parse_event_t::object_end, result))
                    {
                        result = basic_json(value_t::discarded);
                    }

                    return result;
                }

                case (lexer::token_type::begin_array):
                {
                    if (keep and (not callback or (keep = callback(depth++, parse_event_t::array_start, result))))
                    {
                        // explicitly set result to object to cope with []
                        result.m_type = value_t::array;
                        result.m_value = json_value(value_t::array);
                    }

                    // read next token
                    get_token();

                    // closing ] -> we are done
                    if (last_token == lexer::token_type::end_array)
                    {
                        get_token();
                        if (callback and not callback(--depth, parse_event_t::array_end, result))
                        {
                            result = basic_json(value_t::discarded);
                        }
                        return result;
                    }

                    // no comma is expected here
                    unexpect(lexer::token_type::value_separator);

                    // otherwise: parse values
                    do
                    {
                        // ugly, but could be fixed with loop reorganization
                        if (last_token == lexer::token_type::value_separator)
                        {
                            get_token();
                        }

                        // parse value
                        auto value = parse_internal(keep);
                        if (keep and not value.is_discarded())
                        {
                            result.push_back(std::move(value));
                        }
                    }
                    while (last_token == lexer::token_type::value_separator);

                    // closing ]
                    expect(lexer::token_type::end_array);
                    get_token();
                    if (keep and callback and not callback(--depth, parse_event_t::array_end, result))
                    {
                        result = basic_json(value_t::discarded);
                    }

                    return result;
                }

                case (lexer::token_type::literal_null):
                {
                    get_token();
                    result.m_type = value_t::null;
                    break;
                }

                case (lexer::token_type::value_string):
                {
                    const auto s = m_lexer.get_string();
                    get_token();
                    result = basic_json(s);
                    break;
                }

                case (lexer::token_type::literal_true):
                {
                    get_token();
                    result.m_type = value_t::boolean;
                    result.m_value = true;
                    break;
                }

                case (lexer::token_type::literal_false):
                {
                    get_token();
                    result.m_type = value_t::boolean;
                    result.m_value = false;
                    break;
                }

                case (lexer::token_type::value_number):
                {
                    auto float_val = m_lexer.get_number();

                    // NAN is returned if token could not be translated
                    // completely
                    if (std::isnan(float_val))
                    {
                        throw std::invalid_argument(std::string("parse error - ") +
                                                    m_lexer.get_token() + " is not a number");
                    }

                    get_token();

                    // check if conversion loses precision
                    const auto int_val = static_cast<number_integer_t>(float_val);
                    if (approx(float_val, static_cast<long double>(int_val)))
                    {
                        // we basic_json not lose precision -> return int
                        result.m_type = value_t::number_integer;
                        result.m_value = int_val;
                    }
                    else
                    {
                        // we would lose precision -> returnfloat
                        result.m_type = value_t::number_float;
                        result.m_value = static_cast<number_float_t>(float_val);
                    }
                    break;
                }

                default:
                {
                    // the last token was unexpected
                    unexpect(last_token);
                }
            }

            if (keep and callback and not callback(depth, parse_event_t::value, result))
            {
                result = basic_json(value_t::discarded);
            }
            return result;
        }

        /// get next token from lexer
        typename lexer::token_type get_token()
        {
            last_token = m_lexer.scan();
            return last_token;
        }

        void expect(typename lexer::token_type t) const
        {
            if (t != last_token)
            {
                std::string error_msg = "parse error - unexpected \'";
                error_msg += m_lexer.get_token();
                error_msg += "\' (" + lexer::token_type_name(last_token);
                error_msg += "); expected " + lexer::token_type_name(t);
                throw std::invalid_argument(error_msg);
            }
        }

        void unexpect(typename lexer::token_type t) const
        {
            if (t == last_token)
            {
                std::string error_msg = "parse error - unexpected \'";
                error_msg += m_lexer.get_token();
                error_msg += "\' (";
                error_msg += lexer::token_type_name(last_token) + ")";
                throw std::invalid_argument(error_msg);
            }
        }

      private:
        /// levels of recursion
        int depth = 0;
        /// callback function
        parser_callback_t callback;
        /// the type of the last read token
        typename lexer::token_type last_token = lexer::token_type::uninitialized;
        /// the lexer
        lexer m_lexer;
    };
};


/////////////
// presets //
/////////////

/*!
@brief default JSON class

This type is the default specialization of the @ref basic_json class which uses
the standard template types.
*/
using json = basic_json<>;
}


/////////////////////////
// nonmember functions //
/////////////////////////

// specialization of std::swap, and std::hash
namespace std
{
/*!
@brief exchanges the values of two JSON objects
@ingroup container
*/
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>
{
    /// return a hash value for a JSON object
    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());
    }
};
}

/*!
@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 \p "_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
@return a JSON object
*/
inline nlohmann::json operator "" _json(const char* s, std::size_t)
{
    return nlohmann::json::parse(reinterpret_cast<nlohmann::json::string_t::value_type*>
                                 (const_cast<char*>(s)));
}

#endif