Published Jul 22, 2019
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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