[Bug c++/95349] Using std::launder(p) produces unexpected behavior where (p) produces expected behavior

ed at catmur dot uk gcc-bugzilla@gcc.gnu.org
Fri May 29 13:53:55 GMT 2020


--- Comment #12 from Ed Catmur <ed at catmur dot uk> ---
(In reply to Richard Biener from comment #11)
> Note that for C++ types you can apply memcpy to the placement new is not
> needed since object re-use terminates lifetime of the previous object and
> starts lifetime of a new one.

Under P0593R6 it has the effect of implicitly creating objects on demand.
Effectively it is supposed to "curse" the double and "bless" the subsequent
uint64_t. Invoking P0593 may be jumping the gun since it's still in LWG, but
Richard (Smith) wants it retroactively applied to C++20 IS as a DR, and that
could still happen.

> Note that while your example performs memcpy dances you are probably
> after a solution that elides all generated code?

Sure, I assume that memcpy of anything smaller than a page will be elided :)

> Note that I do not belive making your examples work as you intend is
> possible in an actual implementation without sacrifying all
> type-based alias analysis.

Ouch. You might be asked to if and when P0593 goes in (again, assuming I've
understood it correctly). Would it be appropriate to find out what Ville

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