编译器错误 C2911

“member”: 无法在当前范围中声明或定义

在命名空间、类或函数内部，只能定义同一命名空间、类或函数的成员，或者由同一命名空间、类或函数括起的成员。

以下示例生成 C2911：

// C2911.cpp
struct A;

namespace M {
   struct D;
}

namespace N {
   struct C;

   namespace O {
      struct B;
   }

   // in N
   struct ::A {};   // C2911  A is member of global NS
   struct O::B{};   // OK B is in O, O is inside of N
   struct C {};     // OK C is member of N
   struct M::D {};  // C2911 D is member of M, M not enclosed by N
}