Mandy's co-worker recently learned about virtual inheritance in C++. It's become their new favorite button to push, and they only use virtual inheritance.

Virtual inheritance is one workaround for the "diamond problem", where you have an inheritance tree that "recombines" via multiple inheritance: when a child class has a Hapsburg-esque parentage, it's hard to decide which inherited methods are being invoked or overridden.

In C++, imagine you have class A, which gets derived into classes B and C. Class D then uses multiple inheritance to inherit from B and C. In practice, this means that D inherits from A twice, and would have two internal copies of all of A's members.

If, on the other hand, you have B and C inherit virtually (class B : public virtual A), then class D will only actually inherit a single copy of A.

If you're reading this and going, "Boy, this just sounds like another reason to avoid multiple inheritance," you're probably right. The important thing, though, is inheriting virtually impacts how your child classes will inherit. It only matters if you have child classes.

Which brings us back to Mandy's co-worker. He read somewhere that defaulting to virtual inheritance all the time was safer, so he did that all the time. All. The. Time.

Including in this case:

class SomeSpecificDerivedClass final : public virtual SomeBaseClass

The class here is marked final. It can't be inherited from. The use of virtual accomplishes nothing.

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