正在枚举值preprocess时间或在编译时解决？ [英] Are enum values resolved in preprocess time or in compile time?

问题描述

当枚举值解决？换句话说，就是下面的code片段符合标准的？

  {枚举
    一个，
    B，
    MAX
}＃如果MAX＆GT; 42
＃错误哇！MAX是很多！
＃万一
 

解决方案

在preprocessor没有什么关系枚举。但是，你的榜样编译没有错误，因此这是怎么回事与＃如果MAX＆GT; 42 指令？

只要preprocessor正在处理条件指令，也没有定义为宏的标识符为0。对待这样假设 MAX 未在其他地方定义作为一个宏，你的code的片段是等价于：

  {枚举
    一个，
    B，
    MAX
}＃如果0＆GT; 42
＃错误哇！MAX是很多！
＃万一
 

从C99 6.10.1 / 3有条件纳入

  

...一切因宏扩展和替换定义后，
  一元运算符已经执行了所有剩余的标识符是
  与PP-号0，然后将每个preprocessing令牌是取代
  转换成令牌。 ...

同样的措辞是在C89 / C90标准。

When are enum values resolved? In other words, is the following code snippet standard-compliant?

enum{
    A,
    B,
    MAX
}

#if MAX > 42
#    error "Woah! MAX is a lot!"
#endif

解决方案

The preprocessor doesn't have anything to do with enums. But your example compiles without error, so what's going on with the #if MAX > 42 directive?

Whenever the preprocessor is handling a conditional directive, any identifiers that are not defined as macros are treated as 0. So assuming that MAX isn't defined elsewhere as a macro, your snippet of code is equivalent to:

enum{
    A,
    B,
    MAX
}

#if 0 > 42
#    error "Woah! MAX is a lot!"
#endif

From C99 6.10.1/3 "Conditional inclusion":

... After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number 0, and then each preprocessing token is converted into a token. ...

The same wording is in the C89/C90 standard.

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