Commit fbac056c authored by Théo DELRIEU's avatar Théo DELRIEU
Browse files

add readme

parent b56117b1
......@@ -21,6 +21,7 @@
- [Conversion from STL containers](#conversion-from-stl-containers)
- [JSON Pointer and JSON Patch](#json-pointer-and-json-patch)
- [Implicit conversions](#implicit-conversions)
- [Conversions to arbitrary types](#arbitrary-types-conversions)
- [Binary formats (CBOR and MessagePack)](#binary-formats-cbor-and-messagepack)
- [Supported compilers](#supported-compilers)
- [License](#license)
......@@ -441,6 +442,218 @@ int vi = jn.get<int>();
// etc.
```
### Arbitrary types conversions
Every type can be serialized in JSON, not just STL-containers and scalar types.
Usually, you would do something along those lines:
```cpp
namespace ns {
struct person { std::string name; std::string address; int age; };
}
// convert to JSON
json j;
ns::person p = createSomeone();
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;
// ...
// convert from JSON
ns::person p {j["name"].get<std::string>(), j["address"].get<std::string>(), j["age"].get<int>()};
```
It works, but that's quite a lot of boilerplate.. Hopefully, there's a better way:
```cpp
ns::person p = createPerson();
json j = p;
auto p2 = j.get<ns::person>();
assert(p == p2);
```
#### Basic usage
To make this work with one of your types, you only need to provide two methods:
```cpp
using nlohmann::json;
namespace ns {
void to_json(json& j, person const& p)
{
j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
}
void from_json(json const& j, person& p)
{
p.name = j["name"].get<std::string>();
p.address = j["address"].get<std::string>();
p.age = j["age"].get<int>();
}
} // namespace ns
```
That's all. When calling the json constructor with your type, your custom `to_json` method will be automatically called.
Likewise, when calling `get<your_type>()`, the `from_json` method will be called.
Some important things:
* 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).
* When using `get<your_type>()`, `your_type` **MUST** be DefaultConstructible and CopyConstructible (There is a way to bypass those requirements described later)
#### How do I convert third-party types?
This requires a bit more advanced technique.
But first, let's see how this conversion mechanism works:
The library uses **JSON Serializers** to convert types to json.
The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](http://en.cppreference.com/w/cpp/language/adl))
It is implemented like this (simplified):
```cpp
template <typename T>
struct adl_serializer
{
static void to_json(json& j, const T& value)
{
// calls the "to_json" method in T's namespace
}
static void from_json(const json& j, T& value)
{
// same thing, but with the "from_json" method
}
};
```
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`...
To solve this, you need to add a specialization of `adl_serializer` to the `nlohmann` namespace, here's an example:
```cpp
// partial specialization (full specialization works too)
namespace nlohmann {
template <typename T>
struct adl_serializer<boost::optional<T>>
{
static void to_json(json& j, const boost::optional<T>& opt)
{
if (opt == boost::none)
j = nullptr;
else
j = *opt; // this will call adl_serializer<T>::to_json, which will find the free function to_json in T's namespace!
}
static void from_json(const json& j, boost::optional<T>& opt)
{
if (!j.is_null())
opt = j.get<T>(); // same as above, but with adl_serializer<T>::from_json
}
};
}
```
#### How can I use `get()` for non-default constructible/non-copyable types?
There is a way, if your type is **MoveConstructible**.
You will need to specialize the `adl_serializer` as well, but with a special `from_json` overload:
```cpp
struct move_only_type {
move_only_type() = delete;
move_only_type(int ii): i(ii) {}
move_only_type(const move_only_type&) = delete;
move_only_type(move_only_type&&) = default;
:
int i;
};
namespace nlohmann {
template <>
struct adl_serializer<move_only_type>
{
// note: the return type is no longer 'void', and the method only takes one argument
static move_only_type from_json(const json& j)
{
return {j.get<int>()};
}
// Here's the catch! You must provide a to_json method!
// Otherwise you will not be able to convert move_only_type to json,
// since you fully specialized adl_serializer on that type
static void to_json(json& j, move_only_type t)
{
j = t.i;
}
};
}
```
#### Can I write my own serializer? (Advanced use)
Yes. You might want to take a look at `unit-udt.cpp` in the test suite, to see a few examples.
If you write your own serializer, you'll need to do a few things:
* use a different `basic_json` alias than nlohmann::json (the last template parameter of basic_json is the JSONSerializer)
* use your `basic_json` alias (or a template parameter) in all your `to_json`/`from_json` methods
* use `nlohmann::to_json` and `nlohmann::from_json` when you need ADL
Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.
```cpp
// You should use void as a second template argument if you don't need compile-time checks on T
template <typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
struct less_than_32_serializer // if someone tries to use a type bigger than 32, the compiler will complain
{
template <typename Json>
static void to_json(Json& j, T value)
{
// we want to use ADL, and call the correct to_json overload
using nlohmann::to_json; // this method is called by adl_serializer, this is where the magic happens
to_json(j, value);
}
template <typename Json>
static void from_json(const Json& j, T& value)
{
// same thing here
using nlohmann::from_json;
from_json(j, value);
}
};
```
Be **very** careful when reimplementing your serializer, you can stack overflow if you don't pay attention:
```cpp
template <typename T, void>
struct bad_serializer
{
template <typename Json>
static void to_json(Json& j, const T& value)
{
// this calls Json::json_serializer<T>::to_json(j, value);
// if Json::json_serializer == bad_serializer ... oops!
j = value;
}
template <typename Json>
static void to_json(const Json& j, T& value)
{
// this calls Json::json_serializer<T>::from_json(j, value);
// if Json::json_serializer == bad_serializer ... oops!
value = j.template get<T>(); // oops!
}
};
```
### Binary formats (CBOR and MessagePack)
......
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