Suppose we have a class-hierarchy whose base class B requires a clone() method to be defined in its derivations. Whilst this works perfectly for most of them there is this one derivation X that holds an optional reference to a resource/object that can only be referenced from one instance of X. Whilst the rest of X can be cloned without problems this reference would have to be set to null/nil/nullptr.

What is the correct way to handle this situation?

• X::clone() could throw an UncloneableException. Though this would be correct, it is not satisfying.
• Set the reference to null/nil/nullptr and leave no notice of it. The user of X has to know about Xs "particularities".
• Require derivations of B to define B::nearestClone(): the user of B has to be aware that the return value might not be a 1:1 clone.
• Something else?