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 ac

Answer 1

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 shared = std::make_shared(10);
std::weak_ptr 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 unique = std::make_unique(10);
std::weak_ptr 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.