How to not serialize the field if it is std::optional and its value is std::nullopt

[TrueWodzu]

Hello,

I would like to serialize a field that is std::optional in a way that if its value is std::nullopt then the json for this field will not appear at all.

Here is the example:

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

using json = nlohmann::json;
using namespace nlohmann::literals;

NLOHMANN_JSON_NAMESPACE_BEGIN
    template <typename T>
    struct adl_serializer<std::optional<T>> {
    static void to_json(json& j, const std::optional<T>& opt) {
        if (opt == std::nullopt) {
            j = nullptr; // This will produce "age": null, but I would like to not to serialize that object at all.
        } 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, std::optional<T>& opt) {
        if (j.is_null()) {
            opt = std::nullopt;
        } else {
            opt = j.template get<T>(); // same as above, but with
            // adl_serializer<T>::from_json
        }
    }
};
NLOHMANN_JSON_NAMESPACE_END

    class person
    {
    private:
        std::string nameOfPerson = "John Doe";
        std::string address = "123 Fake St";
        std::optional<int> age = -1;

    public:
        person() = default;
        person(std::string name_, std::string address_, std::optional<int> age_)
                : nameOfPerson(std::move(name_)), address(std::move(address_)), age(age_)
        {}

        NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, nameOfPerson, address, age)
    };

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

    // serialization: person -> json
    json j = p;
    std::cout << "serialization: " << j << std::endl;
}

This will produce text:
serialization: {"address":"744 Evergreen Terrace","age":null,"nameOfPerson":"Ned Flanders"}

but I would like to have:

serialization: {"address":"744 Evergreen Terrace","nameOfPerson":"Ned Flanders"}

Is this possible to do in automatic way or do I have to write something like:

void to_json(json& j, const person& p) {
        j = json{{"nameOfPerson", p.nameOfPerson}, {"address", p.address}};
        if (p.age) {
            j["age"] = *p.age;
        }
    }

Thank you for any help.

[lingxd]

我也想要这个写法 因为之前在boost::optional是可以做这个操作的，但是在std::optional就没有作用了