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 D are abstract. B is from a third party.

B has a non-pure, virtual method that G needs to implement (to be a D). Can I and is it good practice to redefine/override a virtual function to be pure virtual?

Example:

class B // from a third party { public:    virtual void foo(); };  class D : public B { public:    void foo() override = 0; // allowed by gcc 4.8.2    virtual void bar() = 0; };  class G : public D { public:    // forgot to reimplement foo    void bar() override; };  int main() {    G test;  // compiler error is desired } 

To the question of "can I?" gcc allows it, but I do not have the terms/vocabulary to verify the behavior is part of the standard or is undefined and happens to work today.

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).