Re: [boost] aligned_storage in unions

Subject: Re: [boost] aligned_storage in unions
From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2010-09-22 15:04:41

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On Wednesday 22 September 2010, Larry Evans wrote:
> On 09/22/10 13:14, Larry Evans wrote:
> > On 09/22/10 13:00, Frank Mori Hess wrote:
>
> [snip]
>
> >> How about if you re-assign a variant to a different type.  Won't it
> >> re-use the same in-place storage for a different type?
> >
> > Yes, AFAICT.  Thanks.
>
> Rethought that answer because I don't (yet)
> see where pointers to the 2 different types
> exist at the same time in the operator= for

I don't see how that gets around the anti-aliasing rules.  The compiler is 
allowed to assume incompatible pointers won't _ever_ alias.  So just because 
your program does things in a certain order doesn't mean the compiler isn't 
allowed to use the anti-aliasing assumptions to reorder things in a nasty 
way.  Couldn't it look at the destruction of the old object and the 
construction of the new object as two completely independent actions it is 
free to reorder or interleave?

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