FAQ 30.09 How is remote ownership special?

It requires a deep copy, not a shallow copy.

When an object has remote ownership, the object needs the Big Three (destructor, copy constructor, and assignment operator). These routines are responsible for destroying the referent, creating a copy of the referent, and assigning the referent, respectively.

The copy semantics for remote ownership require the referent to be copied (a.k.a. deep copy) rather than just the pointer (a.k.a. shallow copy). For example, if class MyString has a pointer to an array of characters, copying the MyString object should copy the array. It is not sufficient to simply copy the pointer to the array, since that would result in two objects that both think they are responsible for deleteing the same array.

When an object contains pointers for which it does not have remote ownership, the copy semantics are usually straightforward: the copy operation merely copies the pointer. For example, an iterator object might have a pointer to a node of a linked list, but the node is owned by the list rather than by the iterator, so copying an iterator merely needs to copy the pointer; the data in the node is not copied to the new iterator.

When an object doesn't contain pointers, the copy semantics are usually straightforward: the corresponding copy operation is called on each member object. This is what the compiler does automatically if the class doesn't have any copy operations (see FAQs 30.04, 30.05), which is why the Big Three are not usually needed in these cases.