Question

A lot of C++ books and tutorials explain how to do this, but I haven’t seen one that gives a convincing reason to choose to do this.

I understand very well why function pointers were necessary in C (e.g., when using some POSIX facilities). However, AFAIK you can’t send them a member function because of the ‘this’ parameter. But if you’re already using classes and objects, why not just use an object oriented solution like functors?

Real world examples of where you had to use such function pointers would be appreciated.

Update: I appreciate everyone’s answers. I have to say, though, that none of these examples really convinces me that this is a valid mechanism from a pure-OO perspective…

Answer 1

Functors are not a priori object-oriented (in C++, the term “functor” usually means a struct defining an operator () with arbitrary arguments and return value that can be used as syntactical drop-in replacements to real functions or function pointers). However, their object-oriented problem has a lot of issues, first and foremost usability. It’s just a whole lot of complicated boilerplate code. In order for a decent signalling framework as in most dialog frameworks, a whole lot of inheritance mess becomes necessary.

Instance-bound function pointers would be very beneficial here (.NET demonstrates this amply with delegates).

However, C++ member function pointers satisfy another need still. Imagine, for example, that you’ve got a lot of values in a list of which you want to execute one method, say its print(). A function pointer to YourType::size helps here because it lets you write such code:

std::for_each(lst.begin(), lst.end(), std::mem_fun(&YourType::print))