Why can't a weak_ptr be constructed from a unique_ptr?

Question

If I understand correctly, a weak_ptr doesn't increment the reference count of the managed object, therefore it doesn't represent ownership. It simply lets you access an object, the lifetime of which is managed by someone else. So I don't really see why a weak_ptr can't be constructed from a unique_ptr, but only a shared_ptr.

Can someone briefly explain this?

Answer 1

If you think about it, a weak_ptr must refer to something other than the object itself. That's because the object can cease to exist (when there are no more strong pointers to it) and the weak_ptr still has to refer to something that contains the information that the object no longer exists.

With a shared_ptr, that something is the thing that contains the reference count. But with a unique_ptr, there is no reference count, so there is no thing that contains the reference count, thus nothing to continue to exist when the object is gone. So there's nothing for a weak_ptr to refer to.

There would also be no sane way to use such a weak_ptr. To use it, you'd have to have some way to guarantee that the object wasn't destroyed while you were using it. That's easy with a shared_ptr -- that's what a shared_ptr does. But how do you do that with a unique_ptr? You obviously can't have two of them, and something else must already own the object or it would have been destroyed since your pointer is weak.

Answer 2

std::weak_ptr can't be used unless you convert it to std::shared_ptr by the means of lock(). if the standard allowed what you suggest, that means that you need to convert std::weak_ptr to unique in order to use it, violating the uniqueness (or re-inventing std::shared_ptr)

In order to illustrate, look at the two pieces of code:

std::shared_ptr<int> shared = std::make_shared<int>(10);
std::weak_ptr<int> weak(shared);

{
*(weak.lock()) = 20; //OK, the temporary shared_ptr will be destroyed but the pointee-integer still has shared  to keep it alive
}

Now with your suggestion:

std::unique_ptr<int> unique = std::make_unique<int>(10);
std::weak_ptr<int> weak(unique);

{
*(weak.lock()) = 20; //not OK. the temporary unique_ptr will be destroyed but unique still points at it! 
}

That has been said, you may suggest that there is only one unique_ptr, and you still can dereference weak_ptr (without creating another unique_ptr) then there is no problem. But then what is the difference between unique_ptr and shared_ptr with one reference? or moreover, what is the difference between a regular unique_ptr and C-pointers an get by using get?

weak_ptr is not for "general nonowning resources", it has a very specific job - The main goal of weak_ptr is to prevent circular pointing of shared_ptr which will make a memory leak. Anything else needs to be done with plain unique_ptr and shared_ptr.