[Boost-users] [SmartPointers] weak_ptrs, raw pointers, and cycles

From: Scott Meyers (usenet_at_[hidden])
Date: 2006-08-18 13:04:58

I've recently realized that I just don't get something.  Cycles of shared_ptrs 
are bad, because they can lead to resource leaks.  To break a cycle, we're 
encouraged to use weak_ptrs.  Okay, but we could use raw pointers to break the 
cycle, too, and I can't see any advantage offered by the weak_ptr.  If A uses a 
shared_ptr to B and B uses a weak_ptr to A, B's life will be shorter than A's 
(its destructor will be invoked from A's destructor and will run to completion 
before A's does), so there is no advantage to a weak_ptr being able to tell if 
it dangles.  In this context, it never will.  So it seems that using a raw 
pointer to break cycles has no disadvantage over a weak_ptr, and it's more 
efficient (i.e., smaller and faster).  Clearly, I'm missing something.  Can 
somebody please explain what it is?

I understand that weak_ptrs have other uses (e.g., as observers), so my question 
is only about the advice to use weak_ptrs to break cycles.

Thanks,

Scott