C++ Academy · Lesson

Function Objects Functors

Write classes with operator() to create stateful callable objects.

What is a Functor?

A functor (function object) is a class that overloads operator(). Instances can be called like functions.

struct Square {
    int operator()(int x) const { return x * x; }
};

Square sq;
std::cout << sq(5);   // 25

Functors carry state. A plain function does not. State enables configuration, counting, and stateful callbacks.

struct Counter {
    int count = 0;
    int operator()(int x) { ++count; return x; }
};

Counter c;
c(10); c(20); c(30);
std::cout << c.count;   // 3