Читаем C++ Primer Plus полностью

C++ Primer Plus

The answer to your desire (the one about storing information about a basketball player) is the C++ structure. A structure is a more versatile data form than an array because a single structure can hold items of more than one data type. This enables you to unify your data representation by storing all the related basketball information in a single structure variable. If you want to keep track of a whole team, you can use an array of structures. The structure type is also a stepping stone to that bulwark of C++ OOP, the class. Learning a little about structures now takes you that much closer to the OOP heart of C++.

A structure is a user-definable type, with a structure declaration serving to define the type’s data properties. After you define the type, you can create variables of that type. Thus, creating a structure is a two-part process. First, you define a structure description that describes and labels the different types of data that can be stored in a structure. Then you can create structure variables, or, more generally, structure data objects, that follow the description’s plan.

For example, suppose that Bloataire, Inc., wants to create a type to describe members of its product line of designer inflatables. In particular, the type should hold the name of the item, its volume in cubic feet, and its selling price. Here is a structure description that meets those needs:

struct inflatable // structure declaration

{

char name[20];

float volume;

double price;

};

The keyword struct indicates that the code defines the layout for a structure. The identifier inflatable is the name, or tag, for this form; this makes inflatable the name for the new type. Thus, you can now create variables of type inflatable just as you create variables of type char or int. Next, between braces are the list of data types to be held in the structure. Each list item is a declaration statement. You can use any of the C++ types here, including arrays and other structures. This example uses an array of char, which is suitable for storing a string, a float, and a double. Each individual item in the list is called a structure member, so the inflatable structure has three members (see Figure 4.6). In short, the structure definition defines the characteristics of a type—in this case, the inflatable type.

Figure 4.6. Parts of a structure description.

After you have defined the structure, you can create variables of that type:

inflatable hat; // hat is a structure variable of type inflatable

inflatable woopie_cushion; // type inflatable variable

inflatable mainframe; // type inflatable variable

If you’re familiar with C structures, you’ll notice (probably with pleasure) that C++ allows you to drop the keyword struct when you declare structure variables:

struct inflatable goose; // keyword struct required in C

inflatable vincent; // keyword struct not required in C++

In C++, the structure tag is used just like a fundamental type name. This change emphasizes that a structure declaration defines a new type. It also removes omitting struct from the list of curse-inducing errors.

Given that hat is type inflatable, you use the membership operator (.) to access individual members. For example, hat.volume refers to the volume member of the structure, and hat.price refers to the price member. Similarly, vincent.price is the price member of the vincent variable. In short, the member names enable you to access members of a structure much as indices enable you to access elements of an array. Because the price member is declared as type double, hat.price and vincent.price are both equivalent to type double variables and can be used in any manner an ordinary type double variable can be used. In short, hat is a structure, but hat.price is a double. By the way, the method used to access class member functions such as cin.getline() has its origins in the method used to access structure member variables such as vincent.price.

Using a Structure in a Program

Now that we’ve covered some of the main features of structures, it’s time to put the ideas together in a structure-using program. Listing 4.11 illustrates these points about a structure. Also it shows how to initialize one.

Listing 4.11. structur.cpp

// structur.cpp -- a simple structure

#include

struct inflatable // structure declaration

{

char name[20];

float volume;

double price;

};

int main()

{

using namespace std;