Can I override a virtual function with a pure virtual one?

Question

I have three classes: B, D and G. D is a B and G is a D. Both B and

Answer 1

You asked:

Can I override a virtual function with a pure virtual one?

The answer is: Yes, you can. From the C++11 standard:

10.4 Abstract classes

5 [ Note: An abstract class can be derived from a class that is not abstract, and a pure virtual function may override a virtual function which is not pure. —end note ]

Answer 2

If you compile the code with a more modern compiler then you'll get the following error messages that explain the problem

prog.cc:23:6: error: variable type 'G' is an abstract class
   G test;  // compiler error is desired
     ^
prog.cc:10:9: note: unimplemented pure virtual method 'foo' in 'G'
   void foo() override = 0; // allowed by gcc 4.8.2
        ^
1 error generated.

As for the Standard then (10.3 Virtual functions)

11 A virtual function declared in a class shall be defined, or declared pure (10.4) in that class, or both; but no diagnostic is required (3.2).