You don’t specify the language you’re using, but I would suggest that one approach would be to have the base class should have a protected method cloneBase, and then for each class which could support cloning have a sealed concrete derivative which implements a covariantly-generically-typed cloning interface, with itself as the generic type parameter. It may be useful to have this interface also include a method which implementations will use to return themselves, typed according to the generic parameter.

If AnimalBase would have no difficulty supporting cloning, but some derived classes may have non-cloneable objects, then Animal should be a sealed class which derives from AnimalBase and implements ICloneable<Animal>. If BirdBase derives from animal, Bird would be a derivative of that which implements ICloneable<Bird>. If Phoenix derives from Bird but can’t support cloning, neither it nor any derivative would implement any ICloneable interface.

Using this approach, code which needs something that is cloneable and derives from AnimalBase could accept a parameter of type ICloneable<AnimalBase>; such a type could accept either Animal or Bird, even though those types are unrelated. Code which needs to use an ICloneable<AnimalBase> as an AnimalBase could use its Self member.

Alternatively, one could implement an IsCloneable property/method and then have a Clone method which is only guaranteed to work if IsCloneable returns true. Such an approach would provide no way for a type to indicate at compile time that it needed something that can be cloned, but it would avoid the need for extra sealed “leaf” types. While there is some benefit to having methods express their requirements via their parameters, there is also benefit to avoiding the creation of otherwise-unnecessary types.

Under the constraints of the question, I would have an interface such as this (pseudocode):

public interface Cloneable<T> {
  boolean isCloneable();
  T clone();
}

The isCloneable() method would return true if the current object can be cloned, while the clone() method would perform the actual clone. This is a kludgy hack for the fact that a subclass might not be cloneable despite implementing the Cloneable interface. If your programming language already provides a Cloneable interface you may need to add your own to make this work.

In fact this entire exercise serves to prove that a Cloneable interface is a bad idea. It places a constraint on any class that directly or indirectly implements the interface, and potentially any class contained in Cloneable objects.

Your question makes the assumption that cloning must be supported. However, the ideal way of copying an object is not to have a Cloneable interface, but to bake the idea of “copying” directly into a class without using the type system to define what is and is not cloneable. That is out of the scope of your question, but it might be worth exploring as a separate exercise (and perhaps follow-on question).

Sounds like one particular instance of X is the “alpha” X and is, in effect, a singleton. Breaks some design principles but might still be reasonable.

The strict purist approach is that alphaX is not clonable, nor persistable, etc. so nobody can break the singleton contract. This is actually tricky and has generated many articles and books. Your options are to make all Xs non-cloneable, or for just alphaX to throw an exception. Can the user know if their particular X is the alpha?

However, if this is more like nodes in a tree where there is one special root node, but clone still makes some sense (whether you clone the parent and children is a good question) you could clone but set that reference to null.

So, “it depends”. Sorry that this isn’t definitive. 🙂