Copy constructor for a class with unique_ptr to an abstract class as a member

Question

I have a class (C) with a vector of unique_ptrs to an abstract class (A) as a member. This is because C must work with all classes of type A, i.e. its children.

The problem is that I cannot figure out how to write a copy constructor for C, since the type of the objects that the pointers are pointing to are not known at compile time. It actually seems impossible to me. Can anyone confirm that it is impossible? Do you have any suggestions on how to solve the problem? Is it too awful to have a class without a copy constructor?

Leis

Is it too awful to have a class without a copy constructor? No, it's fine. For example unique_ptr doesn't have it as you may have noticed. – Expulsive

One solution involves adding a member function virtual A* clone() = 0; to A. – Jankowski

Do what Pubby said, but only if you need C to be copy constructible. There's nothing wrong with having non-copyable classes, it depends entirely on your use case. – Ladoga

What do you imagine this copy will do? If you intend to also make a copy of all the objects being pointed to by the unique_ptrs then the only way is that A will have to include a cloning function. – Eliaseliason

Answer 1

You did not say whether you have control of the code for the abstract class and the classes derived from it. If you do, then the easiest way is to provide a pure virtual method Clone in the abstract class and implement it in derived classes. This method should handle creating the right copies. Unfortunately, because unique_ptr is not copyable you need to iterate through your vector and create copies by calling Clone.

Cowslip

Answer 2

Well, since std::unique_ptr<T> is not copyable, and thus std::vector<std::unique_ptr<T>> is not copyable, and thus C which has std::vector<std::unique_ptr<T>> as a member should not be default-copyable.

You can, of course, implement copy-constructor that makes deep copy of T, but it depends on what T actually is.

Amblyoscope

It can be done for non-abstract classes. See #16030581 – Leis

@Furihr that also requires the type to be complete, I think – Eliaseliason