Re: considerations regarding active issue #711 (contradiction in empty shared_ptr)

From:

Greg Herlihy <greghe@mac.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sat, 7 Jun 2008 14:44:57 CST

Message-ID:

<8e838943-e3db-4faa-a472-5fa95257908e@s33g2000pri.googlegroups.com>

On Jun 6, 7:39 pm, Rodolfo Lima <rodl...@gmail.com> wrote:

I fail to see an useful use case of the empty/non-null smart pointer,
it just doesn't make sense to me. I mean, an empty/non-null smart
pointer is no better than a raw pointer, why use the former when the
latter is simpler?

Because a program might need to pass a pointer to, say, a class member
object (or any object that is part of an encompassing allocation) when
an interface calls for a shared_ptr.

It boils down to what really is a smart pointer.
One fundamental concept is that it manage the pointer life time, be it
directly or indirectly through another smart pointer (the aliasing
case). For me the empty/non-null case just doesn't fit in this
concept, as being empty implies it doesn't manage anything, so there's
no point in having a stored pointer (which would be unmanaged, which
is the contrary of the original fundamental concept).

Yes, the stored pointer of an "empty" shared_ptr is not managed by the
empty shared_ptr - but is instead managed (indirectly) by the
shared_ptr it aliases.

Furthermore, under either proposed fix for #711, the assert() in your
original program can fail (when the aliased shared_ptr is not empty).
For example:

     // aliasing shared_ptr
     shared_ptr<int> aux(new int);
     int i;
     shared_ptr<int> sptr(aux, &i);
     weak_ptr<int> wptr;

     if (sptr)
     {
         wptr = sptr;
         assert(wptr.lock() == sptr); // assertion failure!
     }

Essentially, attempting to monitor an empty, aliased shared_ptr with a
weak_ptr will always fail - no matter how Issue #711 is resolved.

Greg

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