Subj : Re: A question about atomic_ptr
To   : comp.programming.threads
From : Peter Dimov
Date : Fri Apr 15 2005 01:42 am

Joe Seigh wrote:
> On Thu, 14 Apr 2005 16:53:13 +0300, Peter Dimov <pdimov@gmail.com>
> wrote:
>> Joe Seigh wrote:
>>> The atomic swap is one part.  The other part is safely incrementing
>>> the reference count without "owning" it first.  There's the DWCAS
>>> solution, the PPC solution, and various GC based solutions using
>>> RCU, SMR, etc... And of course the 2CAS solution which only works on
>>> MC68020 and MC68030 processors.
>>
>> You mean the race between
>>
>>     atomic_ptr<T> p2( p1 );
>>
>> and
>>
>>     p1 = 0;
>>
>> ?
>
> Yes.

I must be missing something because I think this can be fixed easily:

atomic_ptr<>::assign( atomic_ptr & other ) // other is unshared

    atomic_ptr temp( *this );

    atomically set *this to other
    set other to empty

    // destroy temp

The 'atomically set *this to other' is hard for shared_ptr because it stores 
two pointers, but not a problem for a single pointer implementation.

operator=( atomic_ptr const & other ) is of course

    atomic_ptr temp( other );
    this->assign( other );

What am I missing?

.