Quick Q: Why does shared_ptr needs to hold reference counting for weak_ptr?
Quick A: To know when to deallocate the control block.
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Why does shared_ptr needs to hold reference counting for weak_ptr?
The reference count controls the lifetime of the pointed-to-object. The weak count does not, but does control (or participate in control of) the lifetime of the control block.
If the reference count goes to 0, the object is destroyed, but not necessarily deallocated. When the weak count goes to 0 (or when the reference count goes to 0, if there are no weak_ptrs when that happens), the control block is destroyed and deallocated, and the storage for the object is deallocated if it wasn't already.
The separation between destroying and deallocating the pointed-to-object is an implementation detail you don't need to care about, but it is caused by using make_shared.
If you do
shared_ptr<int> myPtr(new int{10});you allocate the storage for the int, then pass that into the shared_ptr constructor, which allocates storage for the control block separately. In this case, the storage for the int can be deallocated as early as possible: as soon as the reference count hits 0, even if there is still a weak count.
If you do
auto myPtr = make_shared<int>(10);then make_shared might perform an optimisation where it allocates the storage for the int and the control block in one go. This means that the storage for the int can't be deallocated until the storage for the control block can also be deallocated. The lifetime of the int ends when the reference count hits 0, but the storage for it is not deallocated until the weak count hits 0.
Is that clear now?
Be careful.