[cpp-threads] Some concerns with N2324

[Lawrence Crowl]

On 7/16/07, Peter Dimov <pdimov at mmltd.net> wrote:
> Lawrence Crowl wrote:
> > On 7/15/07, Roger Orr <rogero at howzatt.demon.co.uk> wrote:
>
> >> (4) Should we prefer a type to an enumeration?
> >>
> >> We wondered whether the 'memory_order' could to be a set of
> >> typed objects, like with new(nothrow), rather than an enumeration.
> >> However this might cause problems with the C API.
> >> Or would this be a possible application of strong enum (N2213)?
> >
> > I would like to ensure that use of the base functions is compilable
> > from C.
>
> In N2195 I propose separate types for each ordering constant. It's still
> possible to define them as enums having distinct values for C compatibility
> if the C functions are not intrinsics:
>
> ...
>
> The symbols __relaxed, __acquire, __release, __acq_rel and __ordered are
> shown in the synopsis as macros for presentation purposes. The
> implementation is allowed to not define them as macros, subject to the
> requirements below.
>
> ...
>
> Constraints
>
> typedef unspecified _Relaxed;
> typedef unspecified _Acquire;
> typedef unspecified _Release;
> typedef unspecified _Acq_Rel;
> typedef unspecified _Ordered;
>
> The types _Relaxed, _Acquire, _Release, _Acq_Rel and _Ordered are
> unspecified distinct POD types.
>
> #define __relaxed see below
> #define __acquire see below
> #define __release see below
> #define __acq_rel see below
> #define __ordered see below
>
> The symbols __relaxed, __acquire, __release, __acq_rel and __ordered are
> values of type _Relaxed, _Acquire, _Release, _Acq_Rel and _Ordered,
> respectively. It is unspecified whether they are lvalues or rvalues.
>
> [Note: two possible definitions of _Relaxed and __relaxed might be
>
> struct _Relaxed {};
> #define __relaxed _Relaxed()
>
> and
>
> enum _Relaxed { __relaxed };
>
> --end note]

Syntactically, this works, but the ABI for the two are
generally not the same, and we need the same ABI.

-- 
Lawrence Crowl