> there is no "array of int16_t" object at x

I replied to language.lawyer earlier bit forgot to reply all so it's not on
the list - is there no proper way at all in C++ to read/get a reference to
the top 16 bit of a int32_t?

> the usual lvalue-to-rvalue conversion then reads the value of x

Indeed, but I don't think this was possible on an std::atomic<T> as it is
neither copy or move assignable, and the formalisations of the model did
not consider it possible to non-atomically read or write to an atomic
location (except for the initialisation write)


On Thu, 26 May 2022, 10:18 Jens Maurer via Std-Discussion, <
std-discussion_at_[hidden]> wrote:

> On 26/05/2022 10.02, language.lawyer--- via Std-Discussion wrote:
> >> std::pair<int16_t, int16_t> mixed_size() {
> >> int32_t x = 0;
> >>
> >> {
> >> auto x_atomic = std::atomic_ref<int32_t>(x);
> >> x_atomic.store(0xabbafafa, std::memory_order_relaxed);
> >> // atomic_ref lifetime ends so we can use x again
> >> }
> >>
> >> auto x_parts = reinterpret_cast<int16_t*>(&x);
> >> int16_t& x_left = x_parts[0];
> >> int16_t& x_right = x_parts[1];
> >
> > The last two lines have undefined behavior.
>
> ... irrespective of anything "atomic"; there is
> no "array of int16_t" object at x, so you're
> violating the pointer arithmetic and/or aliasing
> rules.
>
>
> The following example:
>
> > std::int32_t mixed_atomicity() {
> > int32_t x = 0;
> >
> > {
> > auto x_atomic = std::atomic_ref<int32_t>(x);
> > x_atomic.store(42, std::memory_order_relaxed);
> > // atomic_ref lifetime ends so we can use x again
> > }
> >
> > // Obviously reading 42 is sane here, but there is no codified
> semantics
> > // governing non-atomically reading an atomic location in any
> circumstance
> > int32_t read = x;
> > returnread;
> > }
>
>
> just uses the plain non-atomic semantics; the atomic store stored a value
> into x (in addition to all the concurrency effects the atomic store might
> have), and the usual lvalue-to-rvalue conversion then reads the value of x.
>
>
> Jens
>
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