As this library already requires C++ 14 , using std::function (https://en.cppreference.com/w/cpp/utility/functional/function) for the notifications should improve the usability a lot (e.g. bind to class members, use custom handler types, ...). As a function pointer can be bound to an std::function, too, this would not break existing code.

This would require changes in these places:

PS: I'm aware that this is a feature request, but I just want to get a general opinion about this, before I potentially work on this.