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:

Answer 1

There's little more than a syntactic gain in using operator() until you start using templates. But when using templates you can treat real functions and functors (classes acting as functions) the same way.

class scaled_sine
{
    explicit scaled_sine( float _m ) : m(_m) {}
    float operator()(float x) const { return sin(m*x); }
    float m;
};

template
float evaluate_at( float x, const T& fn )
{
   return fn(x);
}

evaluate_at( 1.0, cos );
evaluate_at( 1.0, scaled_sine(3.0) );