它是未定义的行为＃define /＃undef一个具有特殊意义的标识符？ [英] Is it undefined behavior to #define/#undef an identifier with special meaning?

问题描述

禁用在gcc中覆盖的检查的问题的答案建议使用 -Doverride = ，以禁用错误使用覆盖的错误>，它实际上与添加相同：

  #define override 
  pre> 
 
 
到源文件。
 
 
 
我的初步反应是，这看起来像未定义的行为，因为我们重新定义关键字，但是查看草稿C ++ 11标准部分 2.12  关键字[lex.key] 我惊讶的是， / em>或 final 都是关键字。它们在上一节 2.11   [lex.name] 中有说明，它们是具有特殊含义的标识符：
 
 ... 
 
 
 
表3中的标识符在出现在
时有特殊的含义[...] 
 
 
和表3 标记为具有特殊含义的标识符，包括覆盖和< 
 
 
 
问题是，它是未定义的行为，重新定义（使用#define ） / em>？ 
解决方案
如果您使用的是C ++标准库是未定义的行为以重新定义具有特殊含义的标识符，这也适用于关键字。根据 17.6.4   [constraints] 下的C ++ 11标准草案，我们有 17.6.4.1    [constraints.overview] 这说明：
 
 
使用
设施的C ++标准库[...] 
 
 
和 17.6。 4 我们有 17.6.4.3.1   [macro.names] 部分：
 
 
翻译单元不应将#define或#undef命名为
与关键字相同，对于表3中列出的标识符或$ b $表3列出了具有特殊含义的标识符    / em>。我们可以看到此段落还包括关键字，并以相同的方式对待。
 
An answer to the question Disable check for override in gcc suggested using -Doverride= on the command line to disable errors for erroneous use of override, which is effectively the same as adding:
#define override
to the source file.

My initial reaction was that this seems like undefined behavior since we are redefining a keyword but looking at the draft C++11 standard section 2.12 Keywords [lex.key] I was surprised that neither override nor final are keywords. They are covered in the previous section 2.11 [lex.name] which says they are identifiers with special meaning:

  
The identifiers in Table 3 have a special meaning when appearing in a
  certain context[...]
and Table 3 is labelled Identifiers with special meaning and includes both override and final.

The question is, is it undefined behavior to redefine(using #define) identifiers with special meaning? Are they treated any differently than keywords in this respect?
解决方案
If you are using the C++ standard library it is undefined behavior to redefine identifiers with special meaning, this also applies to keywords. From the draft C++11 standard under section 17.6.4 [constraints] we have section 17.6.4.1 [constraints.overview] which says:

  
This section describes restrictions on C++ programs that use the
  facilities of the C++ standard library [...]
and under 17.6.4 we have section 17.6.4.3.1 [macro.names] which says:

  
A translation unit shall not #define or #undef names lexically
  identical to keywords, to the identifiers listed in Table 3, or to the
  attribute-tokens described in 7.6.
Table 3 list the Identifiers with special meaning. We can see this paragraph also covers keywords and they are treated in the same manner.

                        
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