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template<typename BasicJsonType, typename TargetType = ValueType>
static auto to_json(BasicJsonType& j, TargetType && val) noexcept(
    noexcept(::nlohmann::to_json(j, std::forward<TargetType>(val))))
-> decltype(::nlohmann::to_json(j, std::forward<TargetType>(val)), void())

This function is usually called by the constructors of the basic_json class.


j (out)
JSON value to write to
val (in)
value to read from



The example below shows how a to_json function can be implemented for a user-defined type. This function is called by the adl_serializer when the constructor basic_json(ns::person) is called.

#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

namespace ns
// a simple struct to model a person
struct person
    std::string name;
    std::string address;
    int age;
} // namespace ns

namespace ns
void to_json(json& j, const person& p)
    j = json{ {"name",}, {"address", p.address}, {"age", p.age} };
} // namespace ns

int main()
    ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};

    json j = p;

    std::cout << j << std::endl;


{"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

See also

Version history

  • Added in version 2.1.0.

Last update: November 1, 2023