virtual inheritance [duplicate]

Question

What is the meaning of "virtual" inheritance?

I saw the following code, and didn't understand the meaning of the keyword virtual in the following context:

class A {}; class B : public virtual A; 

Answer 1

Virtual inheritance is used to solve the DDD problem (Dreadful Diamond on Derivation).

Look at the following example, where you have two classes that inherit from the same base class:

class Base {  public:   virtual void  Ambig();  }; 

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class C : public Base {  public:  //...  };  class D : public Base { public:      //...  }; 

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Now, you want to create a new class that inherits both from C and D classes (which both have inherited the Base::Ambig() function):

class Wrong : public C, public D {  public:  ...  }; 

While you define the "Wrong" class above, you actually created the DDD (Diamond Derivation problem), because you can't call:

Wrong wrong; wrong.Ambig();  

This is an ambiguous function because it's defined twice:

Wrong::C::Base::Ambig() 

And:

Wrong::D::Base::Ambig() 

In order to prevent this kind of problem, you should use the virtual inheritance, which will know to refer to the right Ambig() function.

So - define:

class C : public virtual Base  class D : public virtual Base  class Right : public C, public D