Could you explain what is meant by a functor in programming?

In programming, especially in C++, a functor is an object that can be called as a function. Usually, it is a class or a structure with an overloaded operator(). This allows objects to be used as functions, which is convenient for passing behavior into algorithms.

Example of a functor in C++:

#include <iostream>

struct MultiplyBy {
    int factor;
    MultiplyBy(int f) : factor(f) {}
    int operator()(int x) const {
        return x * factor;
    }
};

int main() {
    MultiplyBy multiplyBy3(3);
    std::cout << multiplyBy3(10); // Outputs 30
    return 0;
}

Functors are often used in the standard library to customize algorithm behavior.