Global qualification in a class declarations class-head

问题

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?

回答1:

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.

来源：https://stackoverflow.com/questions/2781339/global-qualification-in-a-class-declarations-class-head