Describe for third party's type

Question

I use boost::json library to generate json string, but with 3rd party type, BOOST_DESCRIBE_CLASS can't understand. I can use friend function for classes that use 3rd party styles but it is too much. any idea to handle 1 time only and use everywhere? Example:

class CTest
{
public:
    Imath::V3d v;

    BOOST_DESCRIBE_CLASS(CTest, (), (v), (), ())
};

Answer 1

I had to guess, so I initially set out to work on a generic transformation function.

You would indeed use tag_invoke:

namespace bd = boost::describe;

template <class T, //
          class Bd = bd::describe_bases<T, bd::mod_any_access>,
          class Md = bd::describe_members<T, bd::mod_any_access>,
          class En = std::enable_if_t<!std::is_union<T>::value>>
void tag_invoke(json::value_from_tag, json::value& v, T const& obj) {
    json::object bases, members;

    boost::mp11::mp_for_each<Bd>([&, first = true](auto const& D) mutable {
        using B = typename std::decay_t<decltype(D)>::type;
        bases.emplace(boost::core::demangle(typeid(B).name()),
                      json::value_from(static_cast<B const&>(obj)));
    });

    boost::mp11::mp_for_each<Md>([&](auto D) {
        members.emplace(D.name, json::value_from(obj.*D.pointer));
    });

    json::object result;
    if (bases.size())
        result.emplace("bases", std::move(bases));
    if (members.size())
        result.emplace("members", std::move(members));

    v = std::move(result);
}

Here it is in demonstration:

Live On Coliru

class CTest
{
public:
    int v = 42;
    BOOST_DESCRIBE_CLASS(CTest, (), (v), (), ())
};

struct X { int m1 = 1; };
struct Y { int m2 = 2; };
BOOST_DESCRIBE_STRUCT(X, (), (m1))
BOOST_DESCRIBE_STRUCT(Y, (), (m2))

class Z : public X , public Y {
    int m1 = 3, m2 = 4;
    BOOST_DESCRIBE_CLASS(Z, (X, Y), (), (), (m1, m2))
};

int main() 
{
    std::cout << json::value_from(CTest{}) << "\n";
    std::cout << json::value_from(Z{}) << "\n";
}

Prints

{"members":{"v":42}}
{"bases":{"X":{"members":{"m1":1}},"Y":{"members":{"m2":2}}},"members":{"m1":3,"m2":4}}

Third Party Types

Third-party types can be transformed in just the same way. Let's imagine a third-party header contains stuff like:

namespace Imath {
    struct V3d {
        std::tuple<float, float, float> coords;
        std::string meta;
    };
};

You can't edit the header. But you can add this in your own file after including that:

namespace boost::json {
    void tag_invoke(json::value_from_tag, json::value& v,
                    Imath::V3d const& v3d) {
        v = json::object{
            {"x", std::get<0>(v3d.coords)},
            {"y", std::get<1>(v3d.coords)},
            {"z", std::get<2>(v3d.coords)},
            {"meta", v3d.meta},
        };
    }
} // namespace json

For ADL it's also fine to put the tag_invoke overload into the Imath namespace (or rather, the namespace declaring V3d), but you might prefer the boost::json namespace as it specifically expects overloads of this customization point.

Now the output becomes:

Live On Coliru

class CTest
{
  public:
    Imath::V3d v{{11, 22, 33}, "This is very nifty"};
    BOOST_DESCRIBE_CLASS(CTest, (), (v), (), ())
};

struct X { int m1 = 1; };
struct Y { int m2 = 2; };
BOOST_DESCRIBE_STRUCT(X, (), (m1))
BOOST_DESCRIBE_STRUCT(Y, (), (m2))

class Z : public X , public Y {
    int m1 = 3, m2 = 4;
    BOOST_DESCRIBE_CLASS(Z, (X, Y), (), (), (m1, m2))
};

int main() 
{
    std::cout << json::value_from(CTest{}) << "\n";
    std::cout << json::value_from(Z{}) << "\n";
}

Printing:

{"members":{"v":{"x":1.1E1,"y":2.2E1,"z":3.3E1,"meta":"This is very nifty"}}}
{"bases":{"X":{"members":{"m1":1}},"Y":{"members":{"m2":2}}},"members":{"m1":3,"m2":4}}