命名空间范围内的外部 - gcc vs clang vs msvc

Question

我用最新的 gcc、clang 和 MSVC 测试了下面看似奇怪的代码示例； clang 和 gcc 都给出 link 错误，但是 MSVC 编译和 links 没有任何问题。哪一个是正确的？

// foo.h
#pragma once

namespace A
{
    class foo
    {
    public:
        foo();
        void print();
    };
}

// foo.cpp
#include <iostream>
#include "foo.h"

int* p = nullptr;

using namespace A;

foo::foo()
{
    p = new int(5);
}

void foo::print()
{
    extern int* p;
    std::cout << *p;
}

#include "foo.h"

int main()
{
    A::foo f;
    f.print();
}

gcc 和 clang：

foo.cpp:(.text+0x35): undefined reference to 'A::p'

Answer 1

GCC 和 Clang 都符合标准。示例和解释在标准中给出 [basic.namespace]/4:

The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as specified in [namespace.memdef]). Such a redeclaration has the same enclosing namespaces as the original declaration. [ Example:

namespace Q {
  namespace V {
    void f();                   // enclosing namespaces are the global namespace, Q, and Q::V
    class C { void m(); };
  }
  void V::f() {                 // enclosing namespaces are the global namespace, Q, and Q::V
    extern void h();            // ... so this declares Q::V::h
  }
  void V::C::m() {              // enclosing namespaces are the global namespace, Q, and Q::V
  }
}

— end example]

命名空间范围内的外部 - gcc vs clang vs msvc

extern in namespace scope - gcc vs clang vs msvc

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