Function Call Operator
	• The function call operator must be a member as the first operand is always an object of the class. • The data type, whether or not operands are modified, whether or not it is a const member, and the data type of the residual value all depend upon its application. Again, it is the only operator that has this type of wildcard flexibility! • return_type  class_type::operator () (argument list); • For a matrix of floats: float & matrix::operator () (int row, int col) const;