No, sum_f is a functor, only its operator() is overloaded to receive 2 parameters, the constructor of the struct is a default constructor that takes no argument

No, sum_f is a functor, only its operator() is overloaded to receive 2 parameters, the constructor of the struct is a default constructor that takes no argument. http://www.cprogramming.com/tutorial/functors-function-objects-in-c++.html http://www.cprogramming.com/tutorial/functors-function-objec...