兰州理工大学,研究生VC++课件7

THE STRUCT IN C++
• This chapter is about creating your own data types to suit your particular problem. • It’s also about creating objects, the building blocks of object-oriented programming. • An object can seem a bit mysterious to the uninitiated, but as you will see in this chapter, an object can be just an instance of one of your own data types.
Defining a struct
• Suppose that you just want to include the title, author, publisher, and year of publication within your definition of a book. • You could declare a structure to accommodate this as follows:
What Is a struct?
• Almost all the variables have been able to store a single type of entity — a number of some kind, a character, or an array of elements of the same type. • The real world is a bit more complicated than that, and just about any physical object you can think of needs several items of data to describe it even minimally. • Think about the information that might be needed to describe something as simple as a book. • You might consider title, author, publisher, date of publication, number of pages, price, topic or classification, and ISBN number, just for starters, and you can probably come up with a few more without too much difficulty. • You could specify separate variables to contain each of the parameters that you need to describe a book, but ideally, you would want to have a single data type, BOOK, say, which embodied all of these parameters.
• Every object of type BOOK contains its own set of the members Title, Author, Publisher, and Year.
• You can create variables of type BOOK in exactly the same way as you create variables of any other type:
• You may think this is a limitation, but note that you could include a pointer of type BOOK.
• The elements Title, Author, Publisher, and Year enclosed between the braces in the preceding definition are referred to as members or fields of the BOOK structure. • An object of type BOOK is called an instance of the class type.
BOOK Novel; // Declare variable Novel of type BOOK
• This declares a variable with the name Novel that you can use to store information about a book. • All you need is to understand how you get data into the various members that make up a variable of type BOOK.
• With this wealth of information, you can write the declaration for Novel as:
BOOK Novel = { "Paneless Programming", // Initial value for Title "I.C. Fingers", // Initial value for Author "Gutter Press", // Initial value for Publisher 1981 // Initial value for Year };
• A structure is a user-defined type that you define using the struct keyword so it is often referred to as a struct. • The struct originated back in the C language, and C++ incorporates and expands on the C struct. • A struct in C++ is functionally replaceable by a class insofar as anything you can do with a struct, you can also achieve by using a class; however, because Windows was written in C before C++ became widely used, the struct appears pervasively in Windows programming. • It is also widely used today, so you really need to know something about structs.
Accessing the Members of a struct
• To access individual members of a struct, you use the member selection operator, which is a period; this is sometimes referred to as the member access operator. • To refer to a member, you write the struct variable name, followed by a period, followed by the name of the member. • To change the Year member of the Novel structure, you could write:
struct BOOK { char Title[80]; char Author[80]; char Publisher[80]; int Year; };
• This doesn’t define any variables, but it does define a new type for variables, and the name of the type is BOOK. • The struct keyword defines BOOK as such, and the elements making up an object of this type are defined within the braces.
7.Defining Your Own Data Type
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WHAT YOU WILL LEARN IN THIS CHAPTER:
• How structures are used • How classes are used • The basic components of a class and how you define class types • How to create, and use, objects of a class • How to control access to members of a class • How to create constructors • The default constructor • References in the context of classes • How to implement the copy constructor
Initializing a struct
• The first way to get data into the members of a struct object is to define initial values for them in the declaration of the object. • Suppose you wanted to initialize Novel to contain the data for one of your favorite books, Paneless Programming, published in 1981 by the Gutter Press. • This is a story of a guy performing heroic code development while living in an igloo, and, as you probably know, inspired the famous Hollywood box office success, Gone with the Window. • It was written by I.C. Fingers, who is also the author of that seminal three-volume work, The Connoisseur’s Guide to the Paper Clip.
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• Note that each line defining an element in the struct is terminated by a semicolon, and that a semicolon also appears after the closing brace. • The elements of a struct can be of any type, except the same type as the struct being defined. • You couldn’t have an element of type BOOK included in the structure definition for BOOK, for example.