The Io language has a similar gotcha - when I cloned one of my prototypes that contained a List member, I was surprised to find that all the clones shared the s

The Io language has a similar gotcha - when I cloned one of my prototypes that contained a List member, I was surprised to find that all the clones shared the same List! It actually makes sense - it was the reference to the list that was copied, but all the references pointed to the same list. The problem with always defaulting to deep copy in a language where all object slots are by reference is "where do you stop?" Do you copy all the objects known to that object? If the object holds a data file or a resource, do you deep copy that too? What about object graphs? What about two objects mutually holding references to each other? What if the object holds a reference to the global application object? So the most common way to do it is default to only a shallow copy. It's up to the user to define a deep copy if they need it because only the user knows what members are semantically "part of" the parent object and what are "pointers" to unowned objects.