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() { /* ... */ } }; class Bar : public virtual Foo { public: void DoSpecific() { /* ... */ } };

Answer 1

Hello,

Virtual base classes are a tool to prevent the same class being inherited multiple times when using multiple inheritance. This is typically demonstrated in the diamond problem. Consider the following classes:

class A

{

public:

int x;

};

class B : public A

{

};

class C : public A

{

};

class D : public B, public C

{

};

int main()

{

D MyObj;

std::cout << MyObj.x;

return 0;

}

The class hierarchy would look like this:

A

/ \

B C

\ /

D

hence, the title the diamond problem.

In the above code, instantiations of class D actually have two versions of the member variable x, and this code will result in a compile error due to ambiguity.

Virtual inheritance will fix the ambiguity error. The code would then look like this:

class A

{

public:

int x;

};

class B : virtual public A

{

};

class C : virtual public A

{

};

class D : public B, public C

{

};

int main()

{

D MyObj;

std::cout << MyObj.x;

return 0;

}