alias object_t::key_type in basic_json

[FrancoisChabot]

• Describe the feature in as much detail as possible.

I propose simply adding using key_type = typename object_t::key_type; to basic_json<>

Considering at() and operator[] already make use of it in the public interface, it doesn't seem unreasonable.

N.B. mapped_type might as well be aliased too for completeness.

Caveat: I understand that find() does not return an iterator to a std::pair<key_type, mapped_type> so basic_json<> can never be a true map-like as it stands, but unlike the key_type issue I illustrate lower down, this is something that can be SFINAE'd around.

• Include sample usage where appropriate.

This would be useful when duck-typing basic_json<> as a map-like type. Specifically, the fact that std::string_view is not implicitly convertible to std::string is currently making this difficult.

Example:

template<typename MapLike_t>
void do_something(std::string_view key, MapLike_t& obj) {
  using key_type = typename MapLike_t::key_type;

  foo(obj[key_type(key)]);
}

[nlohmann]

Any opinion on this?

[FrancoisChabot]

It's worth noting that this could potentially cause existing users of the library to start detecting basic_json<> as a map-type where they didn't use to if they enable_if<> against the presence of key_type and/or mapped_type.

From my perspective, that's part of the point, but there is potential for this change to be subtly disruptive to upstream users.

[stale]

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