Virtual Base Class in C++

Question

I have a query regarding the virtual base class. In order to resolve the "dreaded diamond of death" /ambiguity problem in multiple inheritance, virtual base class is introduced.

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

What will happen when a keyword virtual is not used in class C declaration. Could you please explain me in detail?

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

Answer 1

If your inheritance is not virtual then A members will be present twice in D class.

If A had a field named _a , then, in D , writing B::_a or C::_a would refer to two different memory zones. If your inheritance is virtual then you have only one memory zone.

Answer 2

If you are using virtual than there will be no ambiguity when you call foo() using the instance of Class D . and if you are not using virtual than it will be ambiguity.. But be careful that virtual inheritance cost more so use it carefully..

Answer 3

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

If the inheritance from the A class to B marked virtual but not A class to C, then C++ will create a single virtual A (D inherits B, B inherits A) and a nonvirtual A (D inherits C, C inherits A). Therefore, your code will not solve the diamond problem:

D d_object;
A &a_ref = d_object; // error raised -> B::A or C::A