Overloading the Function Call Operator

Overloading the function call operator () allows objects of your custom type to be used as if they were functions. This is useful for creating function objects, also known as functors.

Here's an example of overloading the function call operator for a custom Multiplier class:

1#include <iostream>
2
3class Multiplier {
4private:
5  int factor;
6
7public:
8  Multiplier(int f) : factor(f) {}
9
10  int operator()(int x) const {
11    return x * factor;
12  }
13};
14
15int main() {
16  Multiplier doubler(2);
17  Multiplier tripler(3);
18
19  std::cout << doubler(5) << "\n";
20  std::cout << tripler(4) << "\n";
21}

110
212

In this example, the Multiplier class overloads the function call operator to take an integer parameter x and return the result of multiplying x by the factor stored in the Multiplier object.

This allows us to create Multiplier objects that behave like functions. We can invoke the function call operator on these objects just like we would call a regular function, passing arguments within parentheses.

Overloading the function call operator is particularly useful when working with algorithms and libraries that expect function objects, such as the STL algorithms that take predicate functions.