On Thu, Aug 17, 2023 at 2:01 PM Frederick Virchanza Gotham via
Std-Proposals <std-proposals_at_[hidden]> wrote:
>
> On Thursday, August 17, 2023, Tom Honermann wrote:
> > The code has undefined behavior since
> > no object of std::array<T,len> actually
> > exists at the address returned by
> > pretend_is_std_array().
>
>
> Yeah but something ABI-compatible with it does. But even if it were zero-initialised memory, that can be an array of int's all set to zero.
>
>
> > While the code might exhibit the behavior you
> > want today, there is no guarantee that a future
> > compiler upgrade won't include optimizer
> > changes that cause it to behave differently.
> > Violating the C++ object model is not
> > something I would recommend.
>
>
> Change the pointer to 'a pointer to volatile' and then use std::launder on it.
>
> By the way I've only ever seen one example of C++ code that behaves differently when you use 'launder', and it involved a constructor using 'placement new' to invoke a constructor belonging to another class on the current object, something like:
>
> MyClass::MyClass(void)
> {
> ::new(this) SomeOtherClass();
> }
>
> The use of 'launder' prevented caching of the vtable -- which can also be achieved by making the object 'volatile' like I did in a puzzle I composed on Codewars:
>
> https://www.codewars.com/kata/6359c81e00fba2852618a1cb/
>
> The object model in C++ isn't as wild and wacky as some people make it out to be. There isn't and optimiser up on a lighthouse with a sniper rifle waiting for the perfect opportunity to invoke UB.

You are on a mailing list discussing the C++ object model as defined
by the standard. Within that context, we *treat* the standard as if
there is a sniper "waiting for the perfect opportunity to invoke UB".
Examining how specific systems behave might be useful for defining
what things ought to be UB, but UB is not something that the standard
would ever merely ignore.

Put simply, unless you're suggesting that the standard changes what is
and is not UB, what individual compilers do isn't all that important.