6.1 Basics of Structures 结构的基本知识

Let us create a few structures suitable for graphics. The basic object is a point, which we will assume has an x coordinate and a y coordinate, both integers.

我们首先来建立一些适用于图形领域的结构。点是最基本的对象，假定用x与y坐标表示它，且x、y的坐标值都为整数(参见图6-1)。

The two components can be placed in a structure declared like this:

   1 struct point {
   2     int x;
   3     int y;
   4 };

我们可以采用结构存放这两个坐标，其声明如下：

   1 struct point {
   2     int x;
   3     int y;
   4 };

The keyword struct introduces a structure declaration, which is a list of declarations enclosed in braces. An optional name called a structure tag may follow the word struct (as with point here). The tag names this kind of structure, and can be used subsequently as a shorthand for the part of the declaration in braces.

关键字struct引入结构声明。结构声明由包含在花括号内的一系列声明组成。关键字struct后面的名字是可选的，称为结构标记(这里是point)。结构标记用于为结构命名，在定义之后，结构标记就代表花括号内的声明，可以用它作为该声明的简写形式。

The variables named in a structure are called members. A structure member or tag and an ordinary (i.e., non-member) variable can have the same name without conflict, since they can always be distinguished by context. Furthermore, the same member names may occur in different structures, although as a matter of style one would normally use the same names only for closely related objects.

结构中定义的变量称为成员。结构成员、结构标记和普通变量(即非成员)可以采用相同的名字，它们之间不会冲突，因为通过上下文分析总可以对它们进行区分。另外，不同结构中的成员可以使用相同的名字，但是，从编程风格方面来说，通常只有密切相关的对象才会使用相同的名字。

A struct declaration defines a type. The right brace that terminates the list of members may be followed by a list of variables, just as for any basic type. That is,

struct { ... } x, y, z;

is syntactically analogous to

int x, y, z;

in the sense that each statement declares x, y and z to be variables of the named type and causes space to be set aside for them.

struct声明定义了一种数据类型。在标志结构成员表结束的右花括号之后可以跟一个变量表，这与其他基本类型的变量声明是相同的。例如：

struct { ... } x, y, z;

从语法角度来说，这种方式的声明与声明

int x, y, z;

具有类似的意义。这两个声明都将x、y与z声明为指定类型的变量，并且为它们分配存储空间。

A structure declaration that is not followed by a list of variables reserves no storage; it merely describes a template or shape of a structure. If the declaration is tagged, however, the tag can be used later in definitions of instances of the structure. For example, given the declaration of point above,

struct point pt;

defines a variable pt which is a structure of type struct point. A structure can be initialized by following its definition with a list of initializers, each a constant expression, for the members:

struct point maxpt = { 320, 200 };

An automatic structure may also be initialized by assignment or by calling a function that returns a structure of the right type.

如果结构声明的后面不带变量表，则不需要为它分配存储空间，它仅仅描述了一个结构的模板或轮廓。但是，如果结构声明中带有标记，那么在以后定义结构实例时便可以使用该标记定义。例如，对于上面给出的结构声明point，语句

struct point pt;

定义了一个struct point类型的变员pt。结构的初始化可以在定义的后面使用初值表进行。初值表中同每个成员对应的初值必须是常量表达式，例如：

struct point maxpt = { 320, 200 };

自动结构也可以通过赋值初始化，还可以通过调用返回相应类型结构的函数进行初始化。

A member of a particular structure is referred to in an expression by a construction of the form

structure-name.member

The structure member operator "." connects the structure name and the member name. To print the coordinates of the point pt, for instance,

printf("%d,%d", pt.x, pt.y);

or to compute the distance from the origin (0,0) to pt,

double dist, sqrt(double);

dist = sqrt((double)pt.x * pt.x + (double)pt.y * pt.y);

在表达式中，可以通过下列形式引用某个特定结构中的成员：

structure-name.member

其中的结构成员运算符“.”将结构名与成员名连接起来。例如，可用下列语句打印点pt的坐标：

printf("%d,%d", pt.x, pt.y);

或者通过下列代码计算原点(0,0)到点pt的距离：

double dist, sqrt(double);

dist = sqrt((double)pt.x * pt.x + (double)pt.y * pt.y);

Structures can be nested. One representation of a rectangle is a pair of points that denote the diagonally opposite corners:

   1 struct rect {
   2     struct point pt1;
   3     struct point pt2;
   4 };

The rect structure contains two point structures. If we declare screen as

struct rect screen;

then

screen.pt1.x

refers to the x coordinate of the pt1 member of screen.

结构可以嵌套。我们可以用对角线上的两个点来定义矩形〔参见图6-2)，相应的结构定义如下：

   1 struct rect {
   2     struct point pt1;
   3     struct point pt2;
   4 };

结构rect包含两个point类型的成员。如果按照下列方式声明screen变量

struct rect screen;

则可以用语句

screen.pt1.x

引用screen的成员pt1的x坐标。