Andrey Tcherepanov wrote:
bad_function_call will be thrown right now
I was unaware of that, but that's not much better IMO. If I can use the type system to prevent exceptional erroneous situations at runtime, that's better.
Hmm... and what will happen if I call this when it is not initialized yet?
That is supposing function<sig> cannot be empty. Using optional<function<sig> >, you get the same thing as the old behaviour. optional is made so that you can make any type optional, so why not use it when you need to achieve that need?
So (if I understand you correctly), you want not function-pointer behaviour, but something more like reference - have to be initialized, cannot be null?
Well yeah, preventing an "empty" state means that you have to initialize the thing, it cannot be null. That's what the whole thread is about.
It is usable, indeed, but instead of modifying existing Function (and fix all the user code), may be it is easier to create something new, something like FunctionRef library?...
Giving it another name or putting it in another namespace is of course required to achieve compability. The issue is that function is presented as being the right tool to use, but better is possible. Of course that is not function-specific. As I said, quite a few constructs do not force the never-empty guarantee, and it would be better they would (or alternative similar tools, making the current ones deprecated) in my opinion, since that's a safer programming paradigm. That reminds me of a discussion I read recently about iterators, which have to be default constructible. I believe it is a bad idea to require that, but Stepanov thought default-constructibility was an essential propriety of regular types.
BTW, shall it be not reassignable too?
Yes, there is no reason for that to change. If you don't want it to be reassignable, simply make it const, I guess.
If not, assignment should work like atomic op, and I am not sure if this is possible to implement for all user specified arguments simultaneously.
It should work like the current one but use techniques to make sure that the object is not empty even if an exception is raised during its execution.