9.5 KiB
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:
namespace ns {
// a simple struct to model a person
struct person {
std::string name;
std::string address;
int age;
};
}
ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
// convert to JSON: copy each value into the JSON object
json j;
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;
// ...
// convert from JSON: copy each value from the JSON object
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... Fortunately, there's a better way:
// create a person
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};
// conversion: person -> json
json j = p;
std::cout << j << std::endl;
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}
// conversion: json -> person
auto p2 = j.get<ns::person>();
// that's it
assert(p == p2);
Basic usage
To make this work with one of your types, you only need to provide two functions:
using json = nlohmann::json;
namespace ns {
void to_json(json& j, const person& p) {
j = json{ {"name", p.name}, {"address", p.address}, {"age", p.age} };
}
void from_json(const json& j, person& p) {
j.at("name").get_to(p.name);
j.at("address").get_to(p.address);
j.at("age").get_to(p.age);
}
} // 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>()
or get_to(your_type&)
, the from_json
method will be called.
Some important things:
- Those methods MUST be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace
ns
, whereperson
is defined). - Those methods MUST be available (e.g., proper headers must be included) everywhere you use these conversions. Look at issue 1108 for errors that may occur otherwise.
- When using
get<your_type>()
,your_type
MUST be DefaultConstructible. (There is a way to bypass this requirement described later.) - In function
from_json
, use functionat()
to access the object values rather thanoperator[]
. In case a key does not exist,at
throws an exception that you can handle, whereasoperator[]
exhibits undefined behavior. - You do not need to add serializers or deserializers for STL types like
std::vector
: the library already implements these.
Simplify your life with macros
If you just want to serialize/deserialize some structs, the to_json
/from_json
functions can be a lot of boilerplate.
There are two macros to make your life easier as long as you (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object:
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, ...)
is to be defined inside of the namespace of the class/struct to create code for.NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, ...)
is to be defined inside of the class/struct to create code for. This macro can also access private members.
In both macros, the first parameter is the name of the class/struct, and all remaining parameters name the members.
!!! note
At most 64 member variables can be passed to `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE` or `NLOHMANN_DEFINE_TYPE_INTRUSIVE`.
??? example
The `to_json`/`from_json` functions for the `person` struct above can be created with:
```cpp
namespace ns {
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
}
```
Here is an example with private members, where `NLOHMANN_DEFINE_TYPE_INTRUSIVE` is needed:
```cpp
namespace ns {
class address {
private:
std::string street;
int housenumber;
int postcode;
public:
NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)
};
}
```
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).
It is implemented like this (simplified):
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:
// 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 = boost::none;
} else {
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:
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 thannlohmann::json
(the last template parameter ofbasic_json
is theJSONSerializer
) - use your
basic_json
alias (or a template parameter) in all yourto_json
/from_json
methods - use
nlohmann::to_json
andnlohmann::from_json
when you need ADL
Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.
// 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 {
template <typename BasicJsonType>
static void to_json(BasicJsonType& 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 BasicJsonType>
static void from_json(const BasicJsonType& 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:
template <typename T, void>
struct bad_serializer
{
template <typename BasicJsonType>
static void to_json(BasicJsonType& j, const T& value) {
// this calls BasicJsonType::json_serializer<T>::to_json(j, value);
// if BasicJsonType::json_serializer == bad_serializer ... oops!
j = value;
}
template <typename BasicJsonType>
static void to_json(const BasicJsonType& j, T& value) {
// this calls BasicJsonType::json_serializer<T>::from_json(j, value);
// if BasicJsonType::json_serializer == bad_serializer ... oops!
value = j.template get<T>(); // oops!
}
};