virtual inheritance

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;

Solution

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();

};

---

class C : public Base
{

public:

//...

};

class D : public Base
{
public:

    //...

};

---

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

Comments

[Thomas Eding]

Wrong could very well be Correct. It depends on the situation. Normal MI and virtual MI both have their places, even in the presense of (not-dreadful) diamond patterns that arise. Just remember that there is no size that fits all.

[Kevin]

I think you're drawing attention to the wrong thing - with your "Wrong" and "Right" classes. They're identical for a reason - they're not the problem. While you correctly have the virtual inheritance for the parents C and D, you basically gloss over it - C and D were the ones that were "wrong" and need to be fixed, not the class deriving from both of them.

[Scott Hutchinson]

This could use a more explicit conclusion. What does it mean to say "virtual inheritance, which will know to refer to the right Ambig() function"? How does it know? It's not obvious to me how this solves the problem.

[Scott Hutchinson]

This is much clearer for me: stackoverflow.com/a/21607/5652483