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c++c++11shared-ptrweak-ptr

Does weak_ptr is always expired after shared_ptr reset?

I have found a similar question but my example is slightly different.

Here is a very simple example:

int main()
{
   auto sp = std::make_shared<T>();
   std::weak_ptr<T> wp = sp;
   sp.reset();
   assert(wp.expired());
}

Is it guaranteed that in the above case weak_ptr is always expired after shared_ptr reset ?

Solution

  • Yes, this is guaranteed. Calling

    sp.reset();

    is the equivalent of

    shared_ptr().swap(*this)

    Which means that sp is now back to a default constructed state. If we look at use_count in the standard we have

    Returns: the number of shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

    And since it is empty, use_count is 0 and expried will be true as expried

    Checks whether the managed object has already been deleted. Equivalent to use_count() == 0.