C++ Virtual Base Class

Virtual base classes, used in virtual inheritance, is a way of preventing multiple "instances" of a given class appearing in an inheritance hierarchy when using multiple inheritance.

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

D d;
d.Foo();    // There will be two instances of A, and is this B's foo() or C's?

With virtual, there is only one instance of A

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

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

References: https://stackoverflow.com/questions/21558/in-c-what-is-a-virtual-base-class