Virtual inheritance

[1][2] It is used when inheritance represents restriction of a set rather than composition of parts.

As declared above, a call to bat.Eat is ambiguous because there are two Animal (indirect) base classes in Bat, so any Bat object has two different Animal base class subobjects.

Explicit qualification not only uses an easier, uniform syntax for both pointers and objects but also allows for static dispatch, so it would arguably be the preferable method.

(In the first code example we see that Eat is not overridden in either Mammal or WingedAnimal, so the two Animal subobjects will actually behave the same, but this is just a degenerate case, and that does not make a difference from the C++ point of view.)

The ability to share a single instance of the Animal parent between Mammal and WingedAnimal is enabled by recording the memory offset between the Mammal or WingedAnimal members and those of the base Animal within the derived class.

Suppose a pure virtual method is defined in the base class.