Re: which pointer

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 7 Jan 2010 13:47:57 -0800 (PST)
Message-ID:
<9e072b90-060c-4530-bfff-83ed057b4525@22g2000yqr.googlegroups.com>
On Dec 11 2009, 6:45 pm, Paavo Helde <myfirstn...@osa.pri.ee> wrote:

Nick Keighley <nick_keighley_nos...@hotmail.com> wrote in news:81377349-
fdb9-4d2f-9852-c2b2a3bc1...@j14g2000yqm.googlegroups.com:

I wanted my code to look something like this

void new_call (Msg& msg)
{
       smart_ptr<Call> call_ptr ();
       CallList::add (*call_ptr); // may throw
       Call *call = call.release();
       call.process_msg (msg);
}

now that may not be quite right, but the aim was to allocate
a Call and store it in CallList. If CallList throws the
smart_ptr will destroy the Call, otherwise process the
message and the call remains in the CallList for more
message processing.

smart_ptr would not allow copy or assignment. Release
returns the pointer value and zeros its record. I was going
to call it ExplicitReleasePointer. But then I thought,surely
boost have already done this. Aha! scoped_ptr! Oops no. No
release(). The rationale for this is that scoped_ptr implies
no transfer of ownership. But I want to transfer ownership.
That I should use auto_ptr if I want transfer of ownership
semantics. But how do I coerce auto_ptr into doing the
cleanup. Should I be making my container class do the
cleanup? Or should I be using shared_ptr in both the
container class and my calling function?


If I understand correctly, then you want something like
ScopeGuard (look it up):


Actually, I think what he wants is auto_ptr. At least, it has
exactly the semantics he seems to require.

#include <ScopeGuard/ScopeGuard.h>

template <class T> struct Delete {
        void operator()(T *t) const { delete t;}
};

void new_call (Msg& msg)
{
       Call* call_ptr = new Call();
       // assume ownership
       ScopeGuard guard = MakeGuard( Delete<Call>(), call_ptr);

       // pass object to container, container will assume
       // ownership iff the call does not throw.
       CallList::add(call_ptr); // may throw

       // give up my ownership
       guard.Dismiss();

       // call a method on the object owned by the container
       call_ptr->process_msg (msg);
}


That looks to me to be more hassle than it's worth. And if he
ever decides that he needs a factory function, which will return
an auto_ptr, then the above becomes even more complicated.

I suppose the logic is that ownership is shared so you need
a shared pointer.


Yes, a cleaner way would be to use boost::shared_ptr both in
the container and in the caller function.


If the ownership really is shared, yes. But from what he's
explained so far, I don't think it is, and a shared_ptr would
not have the desired semantics.

--
James Kanze

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