C++ How to Program, 10/e

10.14 Overloading the Function Call Operator ()

Overloading the function call operator () is powerful, because functions can take an arbitrary number of comma-separated parameters. In a customized String class, for example, you could overload this operator to select a substring from a String—the operator’s two integer parameters could specify the start location and the length of the substring to be selected. The operator() function could check for such errors as a start location out of range or a negative substring length.

The overloaded function call operator must be a non-static member function and could be defined with the first line:

String String::operator()(size_t index, size_t length) const

In this case, it should be a const member ...