[cpp-threads] Comments on D3193 atomics C compat

[Anthony Williams]

Hi,

I've been reading the latest C compatibility paper D3193, and the notes 
on the reflector.

It is unfortunate that C are insisting on keeping the "_Atomic int" 
style, as this requires a keyword and cannot be built in the library on 
top of existing compiler support for atomics with a different syntax. 
Both _Atomic(int) and atomic<int> can be built on the support from an 
existing compiler, whatever the syntax is.

I do not see _Atomic(x) in the latest draft of D3193. Has it been 
removed? If so, why? Are we relying on the atomic_xyz typedefs for C 
compatibility?

I am not sure how it helps C compatibility to make it implementation 
defined whether or not atomic_int is a typedef for atomic<int> or a base 
class of atomic<int>. That can easily lead to non-portable C++ --- if I 
write my code on an implementation where atomic_int is a typedef then I 
can freely mix atomic<int> and atomic_int all over the place, but this 
may not then compile on an implementation where atomic_int is a base class.

Anthony
-- 
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