C++ Primer Plus

The main reason for keeping the list specification simple is to simplify this programming exercise. You can implement the list as an array or, if you’re familiar with the data type, as a linked list. But the public interface should not depend on your choice. That is, the public interface should not have array indices, pointers to nodes, and so on. It should be expressed in the general concepts of creating a list, adding an item to the list, and so on. The usual way to handle visiting each item and performing an action is to use a function that takes a function pointer as an argument:

void visit(void (*pf)(Item &));

Here pf points to a function (not a member function) that takes a reference to Item argument, where Item is the type for items in the list. The visit() function applies this function to each item in the list. You can use the Stack class as a general guide.

11. Working with Classes

In this chapter you’ll learn about the following:

• Operator overloading

• Friend functions

• Overloading the << operator for output

• State members

• Using rand() to generate random values

• Automatic conversions and type casts for classes

• Class conversion functions

C++ classes are feature-rich, complex, and powerful. In Chapter 10, “Objects and Classes,” you began a journey toward object-oriented programming by learning to define and use a simple class. You saw how a class defines a data type by defining the type of data to be used to represent an object and by also defining, through member functions, the operations that can be performed with that data. And you learned about two special member functions, the constructor and the destructor, that manage creating and discarding objects made to a class specification. This chapter takes you a few steps further in the exploration of class properties, concentrating on class design techniques rather than on general principles. You’ll probably find some of the features covered here straightforward and some a bit more subtle. To best understand these new features, you should try the examples and experiment with them: What happens if you use a regular argument instead of a reference argument for this function? What happens if you leave something out of a destructor? Don’t be afraid to make mistakes; usually you can learn more from unraveling an error than by doing something correctly but by rote. (However, don’t assume that a maelstrom of mistakes inevitably leads to incredible insight.) In the end, you’ll be rewarded with a fuller understanding of how C++ works and of what C++ can do for you.

This chapter starts with operator overloading, which lets you use standard C++ operators such as = and + with class objects. Then it examines friends, the C++ mechanism for letting nonmember functions access private data. Finally, it looks at how you can instruct C++ to perform automatic type conversions with classes. As you go through this chapter and Chapter 12, “Classes and Dynamic Memory Allocation,” you’ll gain a greater appreciation of the roles class constructors and class destructors play. Also you’ll see some of the stages you may go through as you develop and improve a class design.

One difficulty with learning C++, at least by the time you’ve gotten this far into the subject, is that there is an awful lot to remember. And it’s unreasonable to expect to remember it all until you’ve logged enough experience on which to hang your memories. Learning C++, in this respect, is like learning a feature-laden word processor or spreadsheet program. No one feature is that daunting, but, in practice, most people really know well only those features they use regularly, such as searching for text or italicizing. You may recall having read somewhere how to generate alternative characters or create a table of contents, but those skills probably won’t be part of your daily repertoire until you face a situation in which you need them frequently. Probably the best approach to absorbing the wealth of material in this chapter is to begin incorporating just some of these new features into your own C++ programming. As your experiences enhance your understanding and appreciation of these features, you can begin adding other C++ features. As Bjarne Stroustrup, the creator of C++, suggested at a C++ conference for professional programmers: “Ease yourself into the language. Don’t feel you have to use all of the features, and don’t try to use them all on the first day.”

Operator Overloading