In C what is a virtual base class

0 votes

I'm curious what a "virtual base class" is and what it signifies.

Let me provide an example.

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

class Bar : public virtual Foo
{
public:
    void DoSpecific() { /* ... */ }
};
Jun 9 in C++ by Nicholas
• 2,060 points
10 views

1 answer to this question.

0 votes

When employing multiple inheritance, virtual base classes are used to prevent several "instances" of a particular class from appearing in an inheritance hierarchy.

Consider the following example:

class A { public: void Foo() {} };
class B : public A {};
class C : public A {};
class D : public B, public C {};

The preceding class hierarchy yields the "dreaded diamond," which looks like this:

  A
 / \
B   C
 \ /
  D

A D instance will consist of B, which contains A, and C, which also includes A. 

So you have two "instances" of A (for lack of a better term).

There is the possibility of ambiguity in this circumstance. 

This is what happens when you do this:

D d;
d.Foo(); // is this B's Foo() or C's Foo() ??

Virtual inheritance exists to address this issue. 

While you use the virtual keyword when inheriting a class, you're informing the compiler that you only want one instance.

class A { public: void Foo() {} };
class B : public virtual A {};
class C : public virtual A {};
class D : public B, public C {};

This signifies that the hierarchy has just one "instance" of A. 

Hence

D d;
d.Foo(); // no longer ambiguous

answered Jun 10 by Damon
• 3,380 points

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