Iterate over key and value fields of json objects.

[GJanos]

I checked and have not found a way to iterate over the key and value fields of json objects implemented inside glz yet.

One good use case would be, when you have a string containing a serialized json, (contents of the json are not known), and would like to convert it to a glz::obj or glz::json_t (depending on the use case) instance. Then you would have to make operations (save / send / modify) on the individual fields.
Something simmilar to nlohmann's approach could be cool.

        nlohmann::json json_obj = nlohmann::json::parse(json_str);

        for (nlohmann::json::iterator it = json_obj.begin(); it != json_obj.end(); ++it) {
            map_storing_json_fields[it.key()] = it.value().dump();
        }

But for this first glz would need a way to directly serialoze strings to glz::obj, just like how it currently does with glz::json_t.

Thank you for your time! If this feature is already supported and I was careless, please let me know.

[stephenberry]

@GJanos
Both glz::json_t and glz::obj can be iterated in this way to build a map like your example. The glz::obj approach is a bit complex and should probably be cleaned up in the future.

I've written these in the form of unit tests:

For glz::json_t:

std::map<std::string, glz::raw_json> map;
glz::json_t obj{{ "arr", { 1, 2, 3 }}, { "hello", "world" }};
auto& o = obj.get<glz::json_t::object_t>();
for (auto&[key, value] : o) {
   map[key] = glz::write_json(value);
}

auto s = glz::write_json(map);
expect(s == R"({"arr":[1,2,3],"hello":"world"})") << s;

For glz::obj:

std::map<std::string, glz::raw_json> map;
glz::obj obj{ "arr", glz::arr{ 1, 2, 3 }, "hello", "world" };
using T = std::decay_t<decltype(obj.value)>;
glz::for_each<std::tuple_size_v<T>>([&](auto I) {
   if constexpr (I % 2 == 0) {
      map[std::string(glz::tuplet::get<I>(obj.value))] = glz::write_json(glz::tuplet::get<I + 1>(obj.value));
   }
});

auto s = glz::write_json(map);
expect(s == R"({"arr":[1,2,3],"hello":"world"})") << s;

[GJanos]

Thank you very much for letting me know. Indeed the glz::obj example would be better if simplified in the future.
Why is the if constexpr (I % 2 == 0) { ...} part needed?

Are you planning on adding the feature of directly reading string into glz::obj?

[stephenberry]

Yes, I do plan to add parsing directly into glz::obj, just not sure when I'll get to it.

The if constexpr (I % 2 == 0) { ...} is needed because glz::obj is essentially a tuple, where every two items first refers to the key and then refers to the value. This is checking that we are accessing each pair of items instead of handling the key and value separately. even number value of I refer to the start of the pair. Odd values of I would incorrectly grab the value as the key.