How does the ::lock() function work on std::weak_ptr?

  c++, smart-pointers, weak-ptr

When you have a weak_ptr, and you do:

std::weak_ptr<int> wp; // Pretend it's assigned to a shared_ptr
if (!wp.expired()) // Equivalent to use_count() != 0
{ /* Here we're not 100% sure the raw pointer is accessible because a shared_ptr from another thread
   could have been destroyed, and is in the process of decrementing the use_count from 1 to 0 and calling the deleter.*/

}

On the other hand:

std::weak_ptr<int> wp; // Pretend it's assigned to a shared_ptr
if (wp.expired()) Equivalent to use_count() == 0
{ /* Here we are sure that the raw pointer is invalid because of the interesting property that once the use_count is 0 it can't get incremented anymore */

}

The correct way to access a pointer if the resource is still valid is by calling lock, which returns a shared_ptr to the resource if it’s valid, or an empty/null shared_ptr if the resource is not valid:

if (std::shared_ptr shp = wp.lock())
{// We know the resource is valid, we can use it
}

The thing I don’t get is that the lock function is defined, according to the docs, as:

Effectively returns expired() ? shared_ptr<T>() : shared_ptr<T>(*this), executed atomically. 

So if expired() (equivalent to use_count() == 0) returns true, then we know for sure that the pointer can’t be dereferenced, we get back an empty shared_ptr. On the other hand, if it’s expired() returns false we get back a shared_ptr to the resource, but the thing is that, as I mentioned above, when expired() returns true we know the pointer’s bad, but when it returns false it can be a false negative, but we get returned a shared_ptr. When we check that returned shared_ptr another thread could still be in the process of writing to the control block, right?

When the docs say "executed atomically" does it mean that calling lock() method uses a mutex every time you want to access the weak_ptr, or does it use an atomic operation? If so, what is the performance cost of this?

I don’t think it uses atomic types for the counters because I heard about a trick that’s often implemented which I think is a method of avoiding using atomic types/operations:

Adding one for all shared_ptr instances is just an optimization (saves
one atomic increment/decrement when copying/assigning shared_ptr
instances) Link.

Source: Windows Questions C++

LEAVE A COMMENT