在哪里可以在C99中合法声明变量？ [英] Where can I legally declare a variable in C99?

问题描述

当我初次接触C时，被告知始终在函数顶部声明变量。现在，我已经对语言有了很好的了解，现在我将精力集中在编码风格上，尤其是限制变量的范围。我已经读到了限制范围的好处，并且遇到了一个有趣的例子。显然，C99允许您执行此操作...

When I was first introduced to C I was told to always declare my variables at the top of the function. Now that I have a strong grasp of the language I am focusing my efforts on coding style, particularly limiting the scope of my variables. I have read about the benefits to limiting the scope and I came across an interesting example. Apparently, C99 allows you to do this...

for (int i = 0; i < 10; i++)
{
   puts("hello");
}

我曾经认为变量范围受最内层环绕大括号 {} ，但在上面的示例中， int i 的范围似乎受到限制

I had thought that a variables scope was limited by the inner-most surrounding curly braces { }, but in the above example int i appears to be limited in scope by the curly braces of the for-loop even though it is declared outside of them.

我尝试通过 fgets（）扩展上面的示例。 做我认为类似的事情，但是两者都给了我一个语法错误。

I tried to extend the above example with fgets() to do what I thought was something similar but both of these gave me a syntax error.

fgets（char fpath [80]，80，stdin）； *请参见注释**

fgets(char fpath[80], 80, stdin); *See Note**

fgets（char * fpath = malloc（ 80），80，stdin）;

那么，在C99中声明变量到底在哪里合法？ for循环示例是否是规则的例外？这是否适用于 while 和 do while 循环？

So, just where exactly is it legal to declare variables in C99? Was the for-loop example an exception to the rule? Does this apply to while and do while loops as well?

*注意**：即使我可以在此处声明char数组，我也不确定这在语法上是否正确，因为 fgets（）正在寻找 char的指针，而不是 char的数组80的指针。这就是为什么我尝试了 malloc（）版本的原因。

*Note**: I'm not even sure this would be syntactically correct even if I could declare the char array there since fgets() is looking for pointer to char not pointer to array 80 of char. This is why I tried the malloc() version.

推荐答案

在C99，您可以在需要它们的地方声明变量，就像C ++允许您这样做一样。

In C99, you can declare your variables where you need them, just like C++ allows you to do that.

void somefunc(char *arg)
{
    char *ptr = "xyz";
    if (strcmp(arg, ptr) == 0)
    {
        int abc = 0;    /* Always could declare variables at a block start */

        somefunc(arg, &ptr, &abc);

        int def = another_func(abc, arg);   /* New in C99 */
        ...other code using def, presumably...
    }
}

• 您可以在 for循环的控制部分中声明一个变量：

  • You can declare a variable in the control part of a 'for' loop:

      for (int x = 0; x < 10; x++)    /* New in C99 */
    

    
    

  
  

  • 您不能在'while'循环或'if'语句的控件部分中声明变量。

  • You cannot declare a variable in the control part of a 'while' loop or an 'if' statement.

    您不能在函数调用中声明变量。

    You cannot declare a variable in a function call.

    很显然，您可以（并且始终可以）在函数调用中声明变量。

    Obviously, you can (and always could) declare variables in the block after any loop or an 'if' statement.

    C99标准说：

    
    

    6.8.5.3 for语句

    6.8.5.3 The for statement

    语句

    
    

    for ( clause-1 ; expression-2 ; expression-3 ) statement
    

    
    

    
    

    的行为如下：表达式expression-2是
    之前求值的控制表达式循环体的每次执行。表达式expression-3是
    ，在每次循环体执行后被评估为void表达式。如果子句1是
    声明，则它声明的任何变量的范围是声明的其余部分，而
    是整个循环（包括其他两个表达式）的范围；在控制表达式的第一次求值之前，它以执行顺序
    到达。如果子句1是一个表达式，则在对控制表达式进行第一次评估之前，将
    评估为void表达式。

    behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is a declaration, the scope of any variables it declares is the remainder of the declaration and the entire loop, including the other two expressions; it is reached in the order of execution before the first evaluation of the controlling expression. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.

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