在类声明中的全局限定class-head [英] Global qualification in a class declarations class-head

问题描述

我们发现了类似以下的东西（不要问...）：

 命名空间N {
 struct A {struct B; }; 
} 
 
 struct A {struct B; }; 
 
使用命名空间N; 
 
 struct :: A :: B {}; // < - 兴趣点
  

有趣的是，这与VS2005，icc 11.1和Comeau在线），但无法通过GCC：

类名称的全局限定在{'token

从C ++ 03，附件A，在我看来像GCC是正确的：

• class-head 可以包含嵌套名称说明符和标识符


• 嵌套名称说明符不能以全局限定符开头显然，标识符





...或我可以俯瞰某物吗？

解决方案

你是正确的：GCC在这种情况下实现标准的字母，而其他实施它不那么严格（看看 issue＃355 ）。





您可以执行下列操作来解决语法的限制

  struct identity< :: A> :: type :: B {}; 
  

或者使用显式名称 typedef / p>



  typedef :: A AHidden; 
 struct AHidden :: B {}; 
  

或者，当然，您可以使用命名空间交换的顺序和嵌套类定义。请注意，附件A仅供参考。规范性文本在 5.1 / 7 和 9 条款。




We found something similar to the following (don't ask ...):

namespace N {
    struct A { struct B; };
}

struct A { struct B; };

using namespace N;

struct ::A::B {}; // <- point of interest

Interestingly, this compiles fine with VS2005, icc 11.1 and Comeau (online), but fails with GCC:

global qualification of class name is invalid before '{' token

From C++03, Annex A, it seems to me like GCC is right:

• the class-head can consist of nested-name-specifier and identifier
• nested-name-specifier can't begin with a global qualification (::)
• obviously, neither can identifier

... or am i overlooking something?

解决方案

I think you are getting it right: GCC implements the standard to the letter in this case, while the others implement it less strict (have a look at issue #355).

You could do the following to work-around the limitation of the syntax

struct identity< ::A >::type::B {}; 

Or you use an explicit named typedef

typedef ::A AHidden;
struct AHidden::B { };

Or, of course, you exchange the order of using namespace and the nested class definition. Notice that Annex A is informative only. The normative text is at clauses 5.1/7 and 9.

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