In C++, what is a virtual base class?

Question

I want to know what a \"virtual base class\" is and what it means.

Let me show an example:

class Foo
{
public:
    void DoSomething() { /* .         


        

Answer 1

About the memory layout

As a side note, the problem with the Dreaded Diamond is that the base class is present multiple times. So with regular inheritance, you believe you have:

  A
 / \
B   C
 \ /
  D

But in the memory layout, you have:

A   A
|   |
B   C
 \ /
  D

This explain why when call D::foo(), you have an ambiguity problem. But the real problem comes when you want to use a member variable of A. For example, let's say we have:

class A
{
    public :
       foo() ;
       int m_iValue ;
} ;

When you'll try to access m_iValue from D, the compiler will protest, because in the hierarchy, it'll see two m_iValue, not one. And if you modify one, say, B::m_iValue (that is the A::m_iValue parent of B), C::m_iValue won't be modified (that is the A::m_iValue parent of C).

This is where virtual inheritance comes handy, as with it, you'll get back to a true diamond layout, with not only one foo() method only, but also one and only one m_iValue.

What could go wrong?

Imagine:

• A has some basic feature.
• B adds to it some kind of cool array of data (for example)
• C adds to it some cool feature like an observer pattern (for example, on m_iValue).
• D inherits from B and C, and thus from A.

With normal inheritance, modifying m_iValue from D is ambiguous and this must be resolved. Even if it is, there are two m_iValues inside D, so you'd better remember that and update the two at the same time.

With virtual inheritance, modifying m_iValue from D is ok... But... Let's say that you have D. Through its C interface, you attached an observer. And through its B interface, you update the cool array, which has the side effect of directly changing m_iValue...

As the change of m_iValue is done directly (without using a virtual accessor method), the observer "listening" through C won't be called, because the code implementing the listening is in C, and B doesn't know about it...

Conclusion

If you're having a diamond in your hierarchy, it means that you have 95% probability to have done something wrong with said hierarchy.