Zhiyang Ong wrote:

Should we usually, if not always, create and use weak pointers weak_ptrs with shared pointers shared_ptrs?

Is setting all weak_ptrs to null the only reason for using weak_ptrs?

Ummm... let me try to paraphrase your question. I think you're asking: When are weak_ptrs useful? shared_ptr is based on reference counting, and the classic problem with reference counts is circular chains. If instance A contains a shared_ptr to instance B, and instance B contains a shared_ptr to instance A, both are effectively immortal. Such cycles are often unintentional. weak_ptr can be used to avoid that situation. If instance B holds a weak_ptr to instance A, rather than a shared_ptr, then instance B's weak_ptr no longer guarantees the survival of instance A. When the last shared_ptr to instance A is destroyed, instance A will be deleted. If instance A's shared_ptr is the only remaining shared_ptr to instance B, instance B will be deleted as well. Why use weak_ptr, though? Why not just store a simple A* in B? The usefulness of weak_ptr arises when instance A's shared_ptr is NOT the last remaining shared_ptr to instance B. In that case, instance A will be deleted but instance B will survive. Some time later, a method on instance B will try to traverse B's (now invalid) pointer to instance A. If it's a simple A*, B's method has no way of knowing that instance A is now dead, and you'll get a segfault. If it's a weak_ptr<A>, then B's method can detect the absence of instance A and respond appropriately. If that's not the question you're asking, then I'm sorry, I didn't understand you.