What are C++ functors and their uses?

Question

I keep hearing a lot about functors in C++. Can someone give me an overview as to what they are and in what cases they would be useful?

Answer 1

Here's an actual situation where I was forced to use a Functor to solve my problem:

I have a set of functions (say, 20 of them), and they are all identical, except each calls a different specific function in 3 specific spots.

This is incredible waste, and code duplication. Normally I would just pass in a function pointer, and just call that in the 3 spots. (So the code only needs to appear once, instead of twenty times.)

But then I realized, in each case, the specific function required a completely different parameter profile! Sometimes 2 parameters, sometimes 5 parameters, etc.

Another solution would be to have a base class, where the specific function is an overridden method in a derived class. But do I really want to build all of this INHERITANCE, just so I can pass a function pointer????

SOLUTION: So what I did was, I made a wrapper class (a "Functor") which is able to call any of the functions I needed called. I set it up in advance (with its parameters, etc) and then I pass it in instead of a function pointer. Now the called code can trigger the Functor, without knowing what is happening on the inside. It can even call it multiple times (I needed it to call 3 times.)

---

That's it -- a practical example where a Functor turned out to be the obvious and easy solution, which allowed me to reduce code duplication from 20 functions to 1.