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
