Request: 'emplace_back'

[jaredgrubb]

Out of habit, I tend to use 'emplace_back' instead of 'push_back' and I was surprised basic_json didn't support this. Can we add it?

[nlohmann]

I have to check whether this makes sense, but you're right that not having it, but having push_back seems odd.

[jaredgrubb]

I think it's considered part of the various "container" concepts (http://en.cppreference.com/w/cpp/concept/SequenceContainer). Having it missing isn't that big a deal, but would be nice.

[nlohmann]

Commit 1be73b9 contains a proposal for an emplace_back function (for arrays) and an emplace function (for objects). These functions more or less mimic the behavior of push_back (which is overloaded to add to an array if a basic_json value is passed and to an object if an basic_json::object_t is passed).

It would be nice to discuss this proposal:

• Is the implementation as expected?
• Is it OK to have two functions?

[jaredgrubb]

The implementation looks correct. Although on the one hand I think it's odd for the json object to have both methods, it matches the convention that json has object and array methods and it's up to you to call the right ones. So it looks good to me!

[nlohmann]

Thanks. Yes, it is a weird situation to have a combination of a map and a vector, and I am not sure whether users have the mindset "I am using a general container" or rather "I am using an object" / "I am using an array".

Another aspect: If there was a single emplace function, it could forward the arguments to to the underlying vector or map depending whether the type of the JSON value is and array or object. This would make the automatic translation of null into array or object as it is done in push_back impossible.

[nlohmann]

Any opinions on this?

[jaredgrubb]

There are times when I wish I had a stronger type that represented what I know it is but I can see the utility of the do-everything type. I dont see a strong reason to change it myself! So The emplace_back would be awesome :)

[TurpentineDistillery]

Note that std::map::emplace has a return type that is not void. Was it a conscious decision not to do the same in basic_json-object ?