title: Qualified class member
body: <p>I've written the following code:</p>
  
  <pre><code>class A
  {
  public:
      int foo();
  };
  
  class B
  {
  public:
      int A::foo(){ return 0; }; //error: non-friend class member 'foo' cannot have a qualified name
  };
  
  int main(){ }
  </code></pre>
  
  <p>Why does this error caused? Can you get the reference from the Standard denied that construction explicitly?</p>