Function call operator

Possible Duplicates:
C++ Functors - and their uses.
Why override operator() ?

I've seen the use of operator() on STL containers but what is it and when do you use it?

That operator turns your object into functor. Here is nice example of how it is done.

Next example demonstrates how to implement a class to use it as a functor :

#include <iostream>

struct Multiply
{
    double operator()( const double v1, const double v2 ) const
    {
        return v1 * v2;
    }
};

int main ()
{
    const double v1 = 3.3;
    const double v2 = 2.0;

    Multiply m;

    std::cout << v1 << " * " << v2 << " = "
              << m( v1, v2 )
              << std::endl;
}