Map flat struct to nested object

[azais-corentin]

Hello,
I'm trying to map a flat c++ struct into a nested json object

Flat c++ struct:

struct Test
{
    int64_t id;
    double latitude;
    double longitude;
};

Nested json object:

{
  "id": 12,
  "coordinates": {
    "latitude": 1.23456789,
    "longitude": 9.87654321
  }
}

I have been playing with object mapping and object types but I was not able to achieve the desired result.
How can I do that?

[stephenberry]

This will be simpler when reading is added for glz::obj, but for now here is a solution:

• I've included my test case I added to json_tests. I had to accept rvalue objects to make this work, so I just merged rvalue inclusion for object reading #420.

struct test_mapping_t
{
    int64_t id;
    double latitude;
    double longitude;
};

struct coordinates_t
{
   double* latitude;
   double* longitude;
};

template <>
struct glz::meta<coordinates_t>
{
   using T = coordinates_t;
   static constexpr auto value = object("latitude", &T::latitude, "longitude", &T::longitude);
};

template <>
struct glz::meta<test_mapping_t>
{
   using T = test_mapping_t;
   static constexpr auto value = object("id", &T::id, "coordinates", [](auto& self) { return coordinates_t{&self.latitude, &self.longitude}; });
};

suite mapping_struct = [] {
   "mapping_struct"_test = [] {

      test_mapping_t obj{};
      std::string s = R"({
  "id": 12,
  "coordinates": {
    "latitude": 1.23456789,
    "longitude": 9.87654321
  }
})";
      expect(!glz::read_json(obj, s));
      expect(obj.id == 12);
      expect(obj.latitude == 1.23456789);
      expect(obj.longitude == 9.87654321);
   };
};

This will be easier in the future, but for now you have to build intermediate structures.

[azais-corentin]

Thank you for your quick answer, this is an acceptable workaround!

I hoped to able to do something similar to this in terms of syntax:

template <>
struct glz::meta<test_mapping_t>
{
    using T = test_mapping_t;
    static constexpr auto value = object( //
        "id", &T::id, "coordinates",
        object("latitude", &T::latitude, //
               "longitude", &T::longitude))
};

Do you believe this could be achievable in the future?

[stephenberry]

Yeah, in the future you'll be able to do:

template <>
struct glz::meta<test_mapping_t>
{
    using T = test_mapping_t;
    static constexpr auto value = object( //
        "id", &T::id, "coordinates",
        obj("latitude", &T::latitude, //
               "longitude", &T::longitude));
};

Just haven't added reading to glz::obj yet, but this should work for writing right now.

Getting glz::object to work in the same way should also be possible, but will probably be further down the road.