C++ Primer Plus, Fourth Edition

Function Objects (aka Functors)

Many STL algorithms use function objects, also known as functors. A functor is any object that can be used with () in the manner of a function. This includes normal function names, pointers to functions, and class objects for which the () operator is overloaded, that is, classes for which the peculiar-looking function operator()() is defined. For example, you could define a class like this:

class Linear
{
private:
    double slope;
    double y0;
public:
    Linear(double _sl = 1, double _y = 0)
        : slope(_sl), y0(_y) {}
    double operator()(double x) {return y0 + slope * x; }
};

The overloaded () operator then allows you to use Linear objects like functions:

 Linear f1; Linear f2(2.5, 10.0); double y1 = f1(12.5); // rhs is f1.operator()(12.5) ...