FAQ 17.09 How can a class Y be a kind-of another class X without inheriting the bits of X ?

FAQ 17.09 How can a class Y be a kind-of another class X without inheriting the bits of X?

It's doable, but C++ has to be outfoxed.

The only mechanism provided by C++ that defines the kind-of relationship also forces inheriting the bits of the base class. If the base class's data structures are inappropriate for certain derived classes, it's necessary to outfox C++ by deferring the definition of the bits to the lower levels of the class hierarchy.

One way to do this is to define an ABC that possesses no (or few) internal data structures, then define the data structures in the concrete derived classes. In this way, the derived classes define the kind-of relationship, but they don't have any bits imposed on them by the base class. For example, both X and Y would inherit from this common ABC, but they would not inherit from each other. Strictly speaking, this does not achieve the original goal (Y is not a kind-of X), but it is normally close enough. The reason is that the bulk of the system can (hopefully) be written to use the ABC rather than X or Y directly, and inheritance allows both X and Y to be passed into the bulk of the system.