Why is the (virtual) destructor of the derived class not called when deleting an array through a pointer to the base class?

Note that whilst a Cat is an Animal, an array of Cats is not an array of Animals. In other words, arrays are invariant in C++, not covariant like they are in some other languages.

So you are up-casting this array and this later confuses the compiler. You must do array delete[] in this case on the correct, original, type – Cat*.

Note that you would have similar issues for the same reason if you allocated an array of 2 or more Cats, cast this to an Animal* and then tried to use the second or subsequent Animal.