Replace default_callback function with nullptr and check for null…

[aburgh]

… callback function before calling it. This is cleaner and better performing.

This is in response to your question about performance and the temporary object for the callback. This does not eliminate the temporary object, but it replaces the default callback with a check for null on the callback member variable, which is faster and avoids the temp object in the default case. In the case where the caller has supplied a callback function, presumably the overhead of a temp object is negligible compared to the processing that will be performed in the callback.

If you would like to optimize the callback to avoid the temp object, the only solution I can think of at the moment is to change the basic_json reference parameter of the callback function to a void * and require the callback function to cast it to a string pointer or basic_json pointer based on the event type. This solution sounds really ugly to me, so my vote is in favor of the temp object.

[coveralls]

Coverage remained the same at 99.87% when pulling ef54ace on aburgh:null-callback into 76d3433 on nlohmann:master.

[nlohmann]

Thanks a lot! That's a few percent performance improvement!