Namespace using declaration (bug in GCC/VS2010)?

问题

namespace A{
   int i;
}

int main(){
   using A::i;
   using A::i;
}

VS2010 - compiles fine

gcc (ideone) - compiles fine

Comeau - gives error ""ComeauTest.c", line 10: error: "i" has already been declared in the current scope using A::i;"

$7.3.3/8 - "A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed."

The example right there indicates that the code is indeed ill-formed.

So, is this a bug in GCC and VS2010?

EDIT 2:

Remove the multiple using directives as it was unrelated to the query on hand.

回答1:

The example you refer to is known to be inconsistent. The committee hasn't yet fixed this.

So, is this a bug in GCC and VS2010?

I don't think it's a bug in either of GCC/VS2010/Clang or Comeau. It appears to be a bug in the C++ Standard. In these cases, compile writers have to make up their mind on what is most viable. If you remove the example in question, then 3.3/4 states the example is valid: "Given a set of declarations in a single declarative region, each of which specifies the same unqualified name, ... they shall all refer to the same entity, or all refer to functions and function templates; or ...".

The question arises, as discussed in the linked issue, what 7.3.3/8 refers to when it says "declarations", which the committee didn't reach consensus about. And so, until then 3.3/4 applies for GCC/VS2010 and Clang, while Comeau chooses to use some other semantics.

回答2:

Yes you are right. This is indeed a bug in g++, MSVC++ and Clang. Comeau has got it correct.

As you said 7.3.3/8 says

A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed

The following code snippet is also given.

void f()
{
    using A::i;
    using A::i;  //error: double declaration
}

Similarly your code is ill-formed too.

来源：https://stackoverflow.com/questions/4252451/namespace-using-declaration-bug-in-gcc-vs2010

标签

c++

namespaces

using-declaration