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

Name "functor" has been traditionaly used in category theory long before C++ appeared on the scene. This has nothing to do with C++ concept of functor. It's better to use name function object instead of what we call "functor" in C++. This is how other programming languages call similar constructs.

Used instead of plain function:

Features:

• Function object may have state
• Function object fits into OOP (it behaves like every other object).

Cons:

• Brings more complexity to the program.

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Used instead of function pointer:

Features:

• Function object often may be inlined

Cons:

• Function object can not be swapped with other function object type during runtime (at least unless it extends some base class, which therefore gives some overhead)

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Used instead of virtual function:

Features:

• Function object (non-virtual) doesn't require vtable and runtime dispatching, thus it is more efficient in most cases

Cons:

• Function object can not be swapped with other function object type during runtime (at least unless it extends some base class, which therefore gives some overhead)