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📝 overworked README
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Niels Lohmann
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README.md
227
README.md
@ -442,15 +442,20 @@ int vi = jn.get<int>();
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// etc.
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```
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### Arbitrary types conversions
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Every type can be serialized in JSON, not just STL-containers and scalar types.
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Usually, you would do something along those lines:
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Every type can be serialized in JSON, not just STL-containers and scalar types. Usually, you would do something along those lines:
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```cpp
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namespace ns {
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struct person { std::string name; std::string address; int age; };
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struct person {
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std::string name;
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std::string address;
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int age;
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};
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}
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// convert to JSON
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json j;
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ns::person p = createSomeone();
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@ -461,10 +466,14 @@ j["age"] = p.age;
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// ...
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// convert from JSON
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ns::person p {j["name"].get<std::string>(), j["address"].get<std::string>(), j["age"].get<int>()};
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ns::person p {
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j["name"].get<std::string>(),
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j["address"].get<std::string>(),
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j["age"].get<int>()
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};
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```
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It works, but that's quite a lot of boilerplate.. Hopefully, there's a better way:
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It works, but that's quite a lot of boilerplate... Hopefully, there's a better way:
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```cpp
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ns::person p = createPerson();
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@ -482,27 +491,25 @@ To make this work with one of your types, you only need to provide two methods:
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using nlohmann::json;
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namespace ns {
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void to_json(json& j, person const& p)
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{
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j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
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}
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void to_json(json& j, person const& p) {
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j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
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}
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void from_json(json const& j, person& p)
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{
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p.name = j["name"].get<std::string>();
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p.address = j["address"].get<std::string>();
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p.age = j["age"].get<int>();
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}
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void from_json(json const& j, person& p) {
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p.name = j["name"].get<std::string>();
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p.address = j["address"].get<std::string>();
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p.age = j["age"].get<int>();
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}
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} // namespace ns
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```
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That's all. When calling the json constructor with your type, your custom `to_json` method will be automatically called.
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That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
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Likewise, when calling `get<your_type>()`, the `from_json` method will be called.
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Some important things:
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* Those methods **MUST** be in your type's namespace, or the library will not be able to locate them (in this example, they are in namespace `ns`, where `person` is defined).
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* When using `get<your_type>()`, `your_type` **MUST** be DefaultConstructible and CopyConstructible (There is a way to bypass those requirements described later)
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* When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible) and [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible). (There is a way to bypass those requirements described later.)
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#### How do I convert third-party types?
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@ -510,124 +517,117 @@ This requires a bit more advanced technique.
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But first, let's see how this conversion mechanism works:
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The library uses **JSON Serializers** to convert types to json.
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The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](http://en.cppreference.com/w/cpp/language/adl))
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The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](http://en.cppreference.com/w/cpp/language/adl)).
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It is implemented like this (simplified):
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```cpp
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template <typename T>
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struct adl_serializer
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{
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static void to_json(json& j, const T& value)
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{
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// calls the "to_json" method in T's namespace
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}
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static void from_json(const json& j, T& value)
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{
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// same thing, but with the "from_json" method
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}
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struct adl_serializer {
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static void to_json(json& j, const T& value) {
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// calls the "to_json" method in T's namespace
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}
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static void from_json(const json& j, T& value) {
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// same thing, but with the "from_json" method
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}
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};
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```
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This serializer works fine when you have control over the type's namespace.
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However, what about `boost::optional`, or `std::filesystem::path` (C++17)?
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Hijacking the `boost` namespace is pretty bad, and it's illegal to add something other than template specializations to `std`...
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This serializer works fine when you have control over the type's namespace. However, what about `boost::optional`, or `std::filesystem::path` (C++17)? Hijacking the `boost` namespace is pretty bad, and it's illegal to add something other than template specializations to `std`...
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To solve this, you need to add a specialization of `adl_serializer` to the `nlohmann` namespace, here's an example:
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```cpp
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// partial specialization (full specialization works too)
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namespace nlohmann {
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template <typename T>
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struct adl_serializer<boost::optional<T>>
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{
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static void to_json(json& j, const boost::optional<T>& opt)
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{
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if (opt == boost::none)
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j = nullptr;
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else
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j = *opt; // this will call adl_serializer<T>::to_json, which will find the free function to_json in T's namespace!
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}
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static void from_json(const json& j, boost::optional<T>& opt)
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{
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if (!j.is_null())
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opt = j.get<T>(); // same as above, but with adl_serializer<T>::from_json
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}
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};
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template <typename T>
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struct adl_serializer<boost::optional<T>> {
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static void to_json(json& j, const boost::optional<T>& opt) {
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if (opt == boost::none) {
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j = nullptr;
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} else {
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j = *opt; // this will call adl_serializer<T>::to_json
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// which will find the free function to_json
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// in T's namespace!
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}
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}
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static void from_json(const json& j, boost::optional<T>& opt) {
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if (!j.is_null()) {
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opt = j.get<T>(); // same as above, but with
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// adl_serializer<T>::from_json
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}
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}
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};
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}
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```
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#### How can I use `get()` for non-default constructible/non-copyable types?
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There is a way, if your type is **MoveConstructible**.
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You will need to specialize the `adl_serializer` as well, but with a special `from_json` overload:
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There is a way, if your type is [MoveConstructible](http://en.cppreference.com/w/cpp/concept/MoveConstructible). You will need to specialize the `adl_serializer` as well, but with a special `from_json` overload:
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```cpp
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struct move_only_type {
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move_only_type() = delete;
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move_only_type(int ii): i(ii) {}
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move_only_type(const move_only_type&) = delete;
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move_only_type(move_only_type&&) = default;
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:
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int i;
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move_only_type() = delete;
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move_only_type(int ii): i(ii) {}
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move_only_type(const move_only_type&) = delete;
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move_only_type(move_only_type&&) = default;
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private:
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int i;
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};
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namespace nlohmann {
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template <>
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struct adl_serializer<move_only_type>
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{
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// note: the return type is no longer 'void', and the method only takes one argument
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static move_only_type from_json(const json& j)
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{
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return {j.get<int>()};
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}
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// Here's the catch! You must provide a to_json method!
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// Otherwise you will not be able to convert move_only_type to json,
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// since you fully specialized adl_serializer on that type
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static void to_json(json& j, move_only_type t)
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{
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j = t.i;
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}
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};
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template <>
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struct adl_serializer<move_only_type> {
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// note: the return type is no longer 'void',
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// and the method only takes one argument
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static move_only_type from_json(const json& j) {
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return {j.get<int>()};
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}
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// Here's the catch! You must provide a to_json method!
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// Otherwise you will not be able to convert move_only_type to json,
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// since you fully specialized adl_serializer on that type
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static void to_json(json& j, move_only_type t) {
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j = t.i;
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}
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};
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}
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```
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#### Can I write my own serializer? (Advanced use)
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Yes. You might want to take a look at `unit-udt.cpp` in the test suite, to see a few examples.
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Yes. You might want to take a look at `[unit-udt.cpp](https://github.com/nlohmann/json/blob/develop/test/src/unit-udt.cpp)` in the test suite, to see a few examples.
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If you write your own serializer, you'll need to do a few things:
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* use a different `basic_json` alias than nlohmann::json (the last template parameter of basic_json is the JSONSerializer)
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* use a different `basic_json` alias than `nlohmann::json` (the last template parameter of `basic_json` is the `JSONSerializer`)
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* use your `basic_json` alias (or a template parameter) in all your `to_json`/`from_json` methods
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* use `nlohmann::to_json` and `nlohmann::from_json` when you need ADL
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Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.
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```cpp
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// You should use void as a second template argument if you don't need compile-time checks on T
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template <typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
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struct less_than_32_serializer // if someone tries to use a type bigger than 32, the compiler will complain
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{
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template <typename Json>
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static void to_json(Json& j, T value)
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{
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// we want to use ADL, and call the correct to_json overload
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using nlohmann::to_json; // this method is called by adl_serializer, this is where the magic happens
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to_json(j, value);
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}
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template <typename Json>
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static void from_json(const Json& j, T& value)
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{
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// same thing here
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using nlohmann::from_json;
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from_json(j, value);
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}
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// You should use void as a second template argument
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// if you don't need compile-time checks on T
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template<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
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struct less_than_32_serializer {
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template <typename BasicJsonType>
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static void to_json(BasicJsonType& j, T value) {
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// we want to use ADL, and call the correct to_json overload
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using nlohmann::to_json; // this method is called by adl_serializer,
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// this is where the magic happens
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to_json(j, value);
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}
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template <typename BasicJsonType>
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static void from_json(const BasicJsonType& j, T& value) {
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// same thing here
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using nlohmann::from_json;
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from_json(j, value);
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}
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};
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```
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@ -637,21 +637,19 @@ Be **very** careful when reimplementing your serializer, you can stack overflow
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template <typename T, void>
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struct bad_serializer
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{
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template <typename Json>
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static void to_json(Json& j, const T& value)
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{
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// this calls Json::json_serializer<T>::to_json(j, value);
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// if Json::json_serializer == bad_serializer ... oops!
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j = value;
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}
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template <typename Json>
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static void to_json(const Json& j, T& value)
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{
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// this calls Json::json_serializer<T>::from_json(j, value);
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// if Json::json_serializer == bad_serializer ... oops!
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value = j.template get<T>(); // oops!
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}
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template <typename BasicJsonType>
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static void to_json(BasicJsonType& j, const T& value) {
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// this calls BasicJsonType::json_serializer<T>::to_json(j, value);
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// if BasicJsonType::json_serializer == bad_serializer ... oops!
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j = value;
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}
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template <typename BasicJsonType>
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static void to_json(const BasicJsonType& j, T& value) {
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// this calls BasicJsonType::json_serializer<T>::from_json(j, value);
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// if BasicJsonType::json_serializer == bad_serializer ... oops!
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value = j.template get<T>(); // oops!
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}
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};
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```
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@ -759,7 +757,7 @@ I deeply appreciate the help of the following people.
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- [Eric Cornelius](https://github.com/EricMCornelius) pointed out a bug in the handling with NaN and infinity values. He also improved the performance of the string escaping.
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- [易思龙](https://github.com/likebeta) implemented a conversion from anonymous enums.
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- [kepkin](https://github.com/kepkin) patiently pushed forward the support for Microsoft Visual studio.
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- [gregmarr](https://github.com/gregmarr) simplified the implementation of reverse iterators and helped with numerous hints and improvements.
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- [gregmarr](https://github.com/gregmarr) simplified the implementation of reverse iterators and helped with numerous hints and improvements. In particular, he pushed forward the implementation of user-defined types.
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- [Caio Luppi](https://github.com/caiovlp) fixed a bug in the Unicode handling.
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- [dariomt](https://github.com/dariomt) fixed some typos in the examples.
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- [Daniel Frey](https://github.com/d-frey) cleaned up some pointers and implemented exception-safe memory allocation.
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@ -787,7 +785,7 @@ I deeply appreciate the help of the following people.
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- [duncanwerner](https://github.com/duncanwerner) found a really embarrassing performance regression in the 2.0.0 release.
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- [Damien](https://github.com/dtoma) fixed one of the last conversion warnings.
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- [Thomas Braun](https://github.com/t-b) fixed a warning in a test case.
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- [Théo DELRIEU](https://github.com/theodelrieu) patiently and constructively oversaw the long way toward [iterator-range parsing](https://github.com/nlohmann/json/issues/290).
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- [Théo DELRIEU](https://github.com/theodelrieu) patiently and constructively oversaw the long way toward [iterator-range parsing](https://github.com/nlohmann/json/issues/290). He also implemented the magic behind the serialization/deserialization of user-defined types.
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- [Stefan](https://github.com/5tefan) fixed a minor issue in the documentation.
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- [Vasil Dimov](https://github.com/vasild) fixed the documentation regarding conversions from `std::multiset`.
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- [ChristophJud](https://github.com/ChristophJud) overworked the CMake files to ease project inclusion.
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@ -801,6 +799,9 @@ I deeply appreciate the help of the following people.
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- [Bosswestfalen](https://github.com/Bosswestfalen) merged two iterator classes into a smaller one.
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- [Daniel599](https://github.com/Daniel599) helped to get Travis execute the tests with Clang's sanitizers.
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- [Jonathan Lee](https://github.com/vjon) fixed an example in the README file.
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- [gnzlbg](https://github.com/gnzlbg) supported the implementation of user-defined types.
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- [Alexej Harm](https://github.com/qis) helped to get the user-defined types working with Visual Studio.
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- [Jared Grubb](https://github.com/jaredgrubb) supported the implementation of user-defined types.
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Thanks a lot for helping out! Please [let me know](mailto:mail@nlohmann.me) if I forgot someone.
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