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C++ How to Program, 5/e

ISBN:
0-13-185757-6
© 2005
pages: 1500
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C++ allows programmers to specify how operators work with objects of new class types--a concept known as operator overloading. One example of an overloaded operator built into C++ is <<, which is used both as the stream insertion operator and as the bitwise left-shift operator. Similarly, >> is used as both the stream extraction operator and as the bitwise right-shift operator.

This tutorial discusses an Array class that overloads several operators. Our Array class provides enhanced functionality over traditional C++ arrays, such as assigning and comparing Array objects, and checking array indices to ensure that we do not access elements outside the bounds of the underlying C++ array. In addition, this tutorial introduces a copy constructor for initializing a new Array object with the contents of an existing Array object. This tutorial is intended for students and professionals who are familiar with basic array, pointer and class concepts in C++.

Download the code examples for this tutorial.

[Note: This tutorial is an excerpt (Section 11.8) of Chapter 11, Operator Overloading, from our textbook C++ How to Program, 5/e. These tutorials may refer to other chapters or sections of the book that are not included here. Permission Information: Deitel, Harvey M. and Paul J., C++ HOW TO PROGRAM, ©2005, pp.582-593. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.]

11.8 Case Study: Array Class (Continued)

Array Class Definition

Now that we have seen how this program operates, let us walk through the class header (Fig. 11.6). As we refer to each member function in the header, we discuss that function’s implementation in Fig. 11.7. In Fig. 11.6, lines 35–36 represent the private data members of class Array. Each Array object consists of a size member indicating the number of elements in the Array and an int pointer—ptr—that points to the dynamically allocated pointer-based array of integers managed by the Array object.

Overloading the Stream Insertion and Stream Extraction Operators as friends

Lines 12–13 of Fig. 11.6 declare the overloaded stream insertion operator and the overloaded stream extraction operator to be friends of class Array. When the compiler sees an expression like cout << arrayObject, it invokes global function operator<< with the call

operator<<( cout, arrayObject )

When the compiler sees an expression like cin >> arrayObject, it invokes global function operator>> with the call

operator>>( cin, arrayObject )

We note again that these stream insertion and stream extraction operator functions cannot be members of class Array, because the Array object is always mentioned on the right side of the stream insertion operator and the stream extraction operator. If these operator functions were to be members of class Array, the following awkward statements would have to be used to output and input an Array:

arrayObject << cout;
arrayObject >> cin;

Such statements would be confusing to most C++ programmers, who are familiar with cout and cin appearing as the left operands of << and >>, respectively.

Function operator<< (defined in Fig. 11.7, lines 127—144) prints the number of elements indicated by size from the integer array to which ptr points. Function operator>> (defined in Fig. 11.7, lines 118–124) inputs directly into the array to which ptr points. Each of these operator functions returns an appropriate reference to enable cascaded output or input statements, respectively. Note that each of these functions has access to an Array’s private data because these functions are declared as friends of class Array. Also, note that class Array’s getSize and operator[] functions could be used by operator<< and operator>>, in which case these operator functions would not need to be friends of class Array. However, the additional function calls might increase execution-time overhead.

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