A functor is a class that implements the () operator. This means that you can call an object using the same syntax as a function. Consider this:
class factor { double f = 1.0; public: factor(double d) : f(d) {} double operator()(double x) const { return f * x; } };
This code can be called like this:
factor threeTimes(3); // create the functor object double ten = 10.0; double d1 = threeTimes(ten); // calls operator(double) double d2 = threeTimes(d1); // calls operator(double)
This code shows that the functor object not only provides some behavior (in this case, performing an action on the parameter) but it also can have a state. The preceding two lines are called through the operator() method on an object: ...
