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:

In this case, it should be a const member function because obtaining a substring should not modify the original String object.

Suppose string1 is a String object containing the string "AEIOU". When the compiler encounters the expression string1(2, 3), it generates the member-function call

which returns a String containing "IOU".

Another possible use of the function call operator is to enable an alternate Array subscripting notation. Instead of using C++’s double-square-bracket notation, such as in chessBoard[row][column], you might prefer to overload the function call operator to enable the notation chessBoard(row, column), where chessBoard is an object of a modified two-dimensional Array class. The primary use of the function call operator is to define function objects, which we discuss in Chapter 16.