Add nlohmann::json::key_type

[esraels]

• Describe the feature in as much detail as possible.
  I'm planning to make a template where it accepts any kind of maps. It need to have key_type and value_type just like the map containers in STL. I want also the nlohmann::json to be supported on that template helper functions. json::value_type is already there but the key_type is not yet. I checked the latest version 3.7.3 and it is still not yet there.

Here is one of my template functions:

template<typename M> inline bool HasKey(M const& p_map, typename M::key_type const& p_key) {
	return p_map.find(p_key) != p_map.end();
}

• Include sample usage where appropriate.

using json = nlohmann::json;

std::map mData = getSomeMapData();
if(MyLib::HasKey(mData , "keyX")){
      //found
}

json& jA = getSomeJSONData();
if(MyLib::HasKey(jA, "keyY")){
      //found
}

[FrancoisChabot]

I would still very much appreciate this change. See #1442.

[nlohmann]

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.

#1442 (comment)

This makes me hesitant.

[stale]

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