Re: [Boost-users] Should we usually, if not always, create and use weak pointers weak_ptrs with shared pointers shared_ptrs?

From: Nat Goodspeed (nat_at_[hidden])
Date: 2008-05-30 14:57:40

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.