For some reason I get conversion errors with this code:
template<typename T>
class MyType {
T data;
};
template<typename T>
void from_json(MyType<T>& t, const nlohmann::json& j) {}
template<typename T>
void to_json(const MyType<T>& t, nlohmann::json& j) {}
int main() {
MyType<std::string> f;
nlohmann::json j = f; //error
f = j.template get<MyType<std::string>>(); //error
}
I'm writing a library type that uses nlohmann::json and I would not like the user to have to write boilerplate serialization for every template instantiation. Is there a way to do this? If not, why exactly is the compiler complaining?
You have got the parameters of from_json and to_json inverted. The signatures of from_json and to_json are:
template<typename ValueType>
struct adl_serializer {
template<typename BasicJsonType>
static void to_json(BasicJsonType& j, const T& value) {
// calls the "to_json" method in T's namespace
}
template<typename BasicJsonType>
static void from_json(const BasicJsonType& j, T& value) {
// same thing, but with the "from_json" method
}
};
Therefore, you should list Json as the first parameter. Once you do that, the code compiles successfully.
Fixed code:
template<typename T>
class MyType {
T data;
};
template<typename T>
void from_json(const nlohmann::json& j, MyType<T>& t) {}
template<typename T>
void to_json(nlohmann::json& j, const MyType<T>& t) {}
int main() {
MyType<std::string> f;
nlohmann::json j = f;
f = j.template get<MyType<std::string>>();
}