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4.8 Block Structure 程序块结构

C is not a block-structured language in the sense of Pascal or similar languages, because functions may not be defined within other functions. On the other hand, variables can be defined in a block-structured fashion within a function. Declarations of variables (including initializations) may follow the left brace that introduces any compound statement, not just the one that begins a function. Variables declared in this way hide any identically named variables in outer blocks, and remain in existence until the matching right brace. For example, in

   if (n > 0) {
       int i;  /* declare a new i */

       for (i = 0; i < n; i++)
           ...
   }

the scope of the variable i is the "true" branch of the if; this i is unrelated to any i outside the block. An automatic variable declared and initialized in a block is initialized each time the block is entered.

C语言并不是Pascal等语言意义上的程序块结构的语言，它不允许在函数中定义函数。但是，在函数中可以以程序块结构的形式定义变量。变量的声明(包括初始化)除了可以紧跟在函数开始的花括号之后，还可以紧跟在任何其他标识复合语句开始的左花括号之后。以这种方式声明的变量可以隐蔽程序块外与之同名的变量，它们之间没有任何关系，并在与左花括号匹配的右花括号出现之前一直存在。例如，在下面的程序段中：

   if (n > 0) {
       int i;  /* declare a new i */

       for (i = 0; i < n; i++)
           ...
   }

变量i的作用域是if语句的“真”分支，这个i与该程序块外声明的i无关。每次进入程序块时，在程序块内声明以及初始化的自动变量都将被初始化。静态变量只在第一次进入程序块时被初始化一次。

Automatic variables, including formal parameters, also hide external variables and functions of the same name. Given the declarations

   int x;
   int y;

   f(double x)
   {
       double y;
   }

then within the function f, occurrences of x refer to the parameter, which is a double; outside f, they refer to the external int. The same is true of the variable y.

自动变量(包括形式参数)也可以隐藏同名的外部变量与函数。在下面的声明中

   int x;
   int y;

   f(double x)
   {
       double y;
   }

函数f内的变量x引用的是函数的参数，类型为double；而在函数f外，x是int类型的外部变量。这段代码中的变量y也是如此。

As a matter of style, it's best to avoid variable names that conceal names in an outer scope; the potential for confusion and error is too great.

在一个好的程序设计风格中，应该避免出现变量名隐藏外部作用域中相同名字的情况，否则，很可能引起混乱和错误。