C++ 命名空间

inline namespace

You can define and specialize members of an inline namespace as if they were also members of the enclosing namespace.
inline namespace 表示该命名空间也是上级命名空间的成员。
参考: Inline namespace definitions (C++11)

例如：

namespace A {
#if USE_INLINE_B
   inline
#endif
   namespace B {
      int foo(bool) { return 1; }
   }
   int foo(int) { return 2; }
}

int main(void) {
   return A::foo(true); // 如果USE_INLINE_B为0，则该句出错：找不到定义。
}

Unnamed / anonymous namespaces vs static in namespace

Refer to: Unnamed / anonymous namespaces vs static in namespace