c++ and injected base name

Question

The following code does not compile in gcc:

namespace One{
   class A{
   };
};

namespace Two{
   class A{
      public:
         void what(){
            cout << "Two::A says what!" << endl;
         }
   };

   class B : public One::A{
      public:
         B(){
            A xx;
            xx.what();
         }
   };

};

And it gives:

gccbug.cpp: In constructor ‘Two::B::B()’:
gccbug.cpp:23: error: ‘class One::A’ has no member named ‘what’

Now, I was told that this is correct behavior (due to injected base name of One::A making A refer to One::A). However, this code compiles in C# (well, after changing a few things), so this seems to be c++ specific.

What I'm wondering is.. why? Is there a specific purpose for injecting the base name "One::A" as "A"?

Answer 1

The only reason I can think of is that in C++ you are likely to refer to the base class name in the initializer list of the constructor, like this:

namespace Two {

  /*...*/

  class B : public One::A {
  public:
     B():A()
     {
        /*...*/
     }
   };
}

Of course the purpose then is different from the one in your example, because you actually declare a local variable inside the constructor, whereas in my example, the A() refers to the object of type A that is implicit in the definition of class B due to inheritance.

However, the situation of my example is more likely to occur, so I guess they thought let's not require the namespace to be made explicit in this case. As a consequence, any reference to A without a namespace is interpreted as referring to the base class, rather than any other class named A, even if it is in the same namespace as the declaration of B.

Answer 2

Is there a specific purpose for injecting the base name "One::A" as "A"?

Yes. It is so that you could write this:

namespace N
{
   class A
   {
       A *a;
   };
}

In the absence of injected-name, you've to write N::A *a which is not nice.

Note that it is because of injected-name, the following lines are allowed:

A::A *a1; //ok
A::A::A *a2; //ok 
A::A::A::A *a3; //ok 
A::A::A::A::A *a4; //ok 
//and so on

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