Re: [boost] Is there interest in an alternative to the Singletonanti-pattern?

Subject: Re: [boost] Is there interest in an alternative to the Singletonanti-pattern?
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2011-06-30 23:59:45

On Thu, Jun 30, 2011 at 4:40 PM, Gruenke, Matt <mgruenke_at_[hidden]> wrote:
> On June 28, 2011 03:29, Ben Robinson wrote:
>
>
>  T *Get_single() __attribute__ ((const,nothrow))
>  {
>    T *s = Single_inst.get();
>    if (!s)
>    {
>      if (Single_inst.try_init_lock())
>      {
>        s = Single_inst.init_locked(new T());
>      }
>      else s = Single_inst.get_blocking();
>    }
>    return s;
>  }
>
>

I think you need a memory barrier after setting s, as well as on the
if (!s) test.
The assignment to s can drift upwards and *into* the init_locked() or
get_blocking() calls, including drifting up above an unlock() inside
those functions.
Then another thread, at if (!s) will see s as set, but maybe not all
the members of T will have been initialized yet.  Yet s is returned
and used...

Or do you have a full read-write barrier at the end of init_locked()
and get_blocking().  Also what barrier, if any, is inside get()?

I'd suggest using boost::call_once().

Tony