Re: std::launder and its usage in Boost

From: Peter Dimov (pdimov_at_[hidden])
Date: 2022-02-18 17:21:19

Andrey Semashev wrote:
> >>   alignas(int) unsigned char storage[sizeof(int)];
> >>   int* p1 = new (storage) int(1);
> >>   float* p2 = new (storage) float(2.0f);
> >>   int* p3 = new (storage) int(3);
> >>   std::printf("%d\n", *p1);
> >>
> >> the program is allowed to print 1...
> >
> > I don't think it is. The "will automatically apply to the new object"
> > clause applies (even in its C++14 form) and all its requirements seem to be
> met.
> > https://eel.is/c++draft/basic.life#8
> 
> The requirement "o1 and o2 are of the same type" is not satisfied, as the types
> int and float are different.
> 
> Yes, you eventually create an int in the same storage, but I don't see where the
> standard requires this exemption rule to work across creation of an object of
> an incompatible type.

Interesting interpretation. I don't see where it requires it to be invalidated by
the intermediate creation of the float, though. It says "after" but it doesn't
say "immediately after".