Re: Smart pointer for observe only behavior

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 11 Jul 2008 23:27:55 -0400

Message-ID:

<daniel_t-972CF7.23275511072008@earthlink.vsrv-sjc.supernews.net>

Soumen <soumen78@gmail.com> wrote:

"Daniel T." <danie...@earthlink.net> wrote:

Soumen <soume...@gmail.com> wrote:

I've requirement to observe a raw pointer (i.e. I should be
able to query if the pointer I'm using is still valid or not
but when the observer goes out of scope, the resource -- memory
-- shouldn't be released). Is there any boost way (or using any
other smart pointer) to achieve this? Since I've a raw pointer,
probably I cannot use weak_ptr.

Is there a way to do this using shared_ptr with custom deleter?

No. You would have to write some sort of observable pointer. I've
done this before but be advised that such a pointer is quite
smart and kind of expensive. You would need two classes, a
"master_pointer" that owns the object in question and
"observer_pointers". observer_pointers log themselves with the
master_pointer as holders of pointers to the object and the
master_pointer notifies all the observers when the object is
deleted so they will know the object is no longer valid.

Could you please elaborate a bit more?

Think of a Pointer class that has a "kill" method. When any owner of a
Pointer object calls kill, the object is deleted and all the Pointers
that point to the same object are set to NULL.

Is it not shared_ptr and weak_ptr combo type soln?

I don't know.

OK. The pointer I want to observe say is of class A and class A
is singleton. the creation and destruction of A not managed by
different module (legacy). But my module (new) needs to use
some member functions of A. My module cannot manage A but is
dependent on A. During course of execution, there're chances
that A's object is recreated. And I need to be aware of it. Is
there a solution?

Yes and the solution is simple. Don't keep any pointers to the A
singleton. Every time you need to call a member-function on the
singleton object, get the pointer from the A::instance()
member-function.

Not quite clear as in my problem domain I need to know if A's
object is recreated or not. If recreated, I need to do something,
else not.

In that case, keep a pointer to the last instance of A that you used,
call A::instance() again and compare the pointers to see if they are the
same...