Re: which pointer

Paavo Helde <>
Fri, 11 Dec 2009 12:45:02 -0600
Nick Keighley <> wrote in news:81377349-

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):

#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

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

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

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


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