read_to & rfl::SkipIfMissing processor

[golxzn]

Overview

Sometimes it's necessary to update the existing structure using JSON, which doesn't contain all the fields, but modified:

struct player_info {
	std::string id{};
	std::string name{};
	uint32_t steam_id{};
// and more fields...
};

// somewhere
request(info_update_request).then([] (auto patch) {
	// patch: {"name":"i've changed nickname!"}
	auto &info{ services<player_info>() };
	rfl::json::read_to<player_info, rlf::SkipIfMissing>(info, patch);
});

I hope the idea is clear.

Example

We can try to make our own read and write methods like that:

auto original{ write_doc(patching_struct) }; // aka rfl::json::write, but stealing yyjson_doc
auto patch{ yyjson_read(std::data(json), std::size(json), YYJSON_READ_NOFLAG) };
auto patched{ yyjson_merge_patch(original,
	yyjson_doc_get_root(original),
	yyjson_doc_get_root(patch)
) };
// yyjson_mut_doc_get_root(patched)
const auto root{ yyjson_doc_get_root(patched) };
const auto value{ rfl::json::read<T, rfl::DefaultIfMissing>(
	rfl::json::Reader::InputVarType{ root }
) };

But there are 2 problems:

1. The parser could use yyjson_val, but yyjson_merge_patch returns yyjson_mut_val;
2. It's inefficient to parse the structure to the yyjson_doc object just for filling missing fields.

The problem could be solved by ourselves by implementing the whole writer and reader, but the author could do it much more elegant. The first issue could be solved by yyjson_mut_doc_imut_copy as well, but again, such simple stuff like changing field inside an existing object shouldn't be such complicated and slow.

[liuzicheng1987]

@golxzn , let me think about this...I think this is a very interesting problem.