How can it be useful to overload the "function call" operator?

Question

I recently discovered that in C++ you can overload the "function call" operator, in a strange way in which you have to write two pair of parenthesis to do so:

class A {    int n; public:    void operator ()() const;  }; 

And then use it this way:

A a; a(); 

When is this useful?

Answer 1

This can be used to create "functors", objects that act like functions:

class Multiplier { public:     Multiplier(int m): multiplier(m) {}     int operator()(int x) { return multiplier * x; } private:     int multiplier; };  Multiplier m(5); cout << m(4) << endl; 

The above prints 20. The Wikipedia article linked above gives more substantial examples.