emplace throws exception if key already exists

[hoytech]

If I have a tao::json::value and I call emplace on it like this:

auto pair = output.emplace(key, tao::json::empty_object);

If the key already exists I see this error: Fatal error: duplicate JSON object key "..."

Is this expected behaviour? With std::map I believe it will succeed and return a pair with an iterator to the existing element and false to indicate no insertion occurred.

Thanks!

[d-frey]

Yes, this is the intended behavior.

Our rationale is: A JSON value will always be a hybrid between different things as it is a union type. It has a (partial) map-like interface, a (partial) vector-like interface and a bunch of other methods.

If you use the direct interface of value, you'll get some "support" for what we consider reasonable behavior for a JSON value. In this case, you will get an exception if you use emplace with a duplicate key. We thought about this in the past and decided that it is the most commonly useful behavior for that method.

If you want the original map interface, you can always access the embedded map (given that the value currently stores a JSON object) and use the std::map's methods directly:

auto result = v.get_object().emplace(...);

(Note that get_object() will throw an exception if v does not contain an object)

As we haven't published a 1.0.0 version, please let us know if you have different ideas/reasoning about the interface, we are still open to discussions and improvements.

[hoytech]

Thanks for the explanation, it sounds reasonable. The get_object() approach worked fine for me.

I don't really have any strong preference either way. My only comment is that this wasn't clear to me from the documentation (or maybe I wasn't looking in the right spots?). Since this behaviour is different from how emplace usually works, I think it would be helpful to have this described more prominently in the docs.

I appreciate the prompt response, and the library in general!

[d-frey]

Thank you, and you are right that the documentation needs some improvement. We hope to get there once we will approach 1.0.0.