----- Original Message -----
From: "Eric Friedman" <ebf@[...].net> 
To: "'David Abrahams'" <dave@[...].com> ; <boost@[...].org>
Sent: Wednesday, August 07, 2002 5:15 PM
Subject: RE: [boost] Re: Mini-review request: aligned_storage.hpp


>  [snipped]
>  Three questions come to my mind:
> 
>   * Are there any implementations that do not behave as desired under
>  this undefined
>     behavior? (Or more fundamentally, what is the reason for the
>  standard's behavior
>     on this issue?)
> 
Unless utterly neccesary, we must not make any assumptions about
implementations behaviour.

>   * Is it a major obstacle that Boost.Variant will not offer
>  exception-safe assign
>     or swap unless externally wrapped with the pimpl idiom? Should
>  boost::fast_variant
>     and boost::safe_variant be introduced to encapsulate yet allow choice
>  in this issue?
> 
Personally, I would be more than happy with a variant which is not any more
exception-safe than the objects it wraps.


>   * Any ideas out there as to how to make variant::operator= and
>  variant::swap "as safe
>     as possible," yet still efficient, even if we can't guarantee strong
>  exception-safety?
> 
My personal preference in this area is to do nothing...

Consider:

T a(val0) ;
T b(val1) ;

b = a ;
// if assignment above throws, how much inconsistent is 'b' left is totally
undefined.

Therefore, with:

variant<int,T>  va (a);
variant<int,T>  vb(b);
va = vb ;

Struggling to make the above assignment exception safe is too dificult -if
at all possible-,
while doing nothing at all won't ever be worse than the case of using T
directly.


In any event, if I need to put such touchy objects in a variant, I can
always wrap the object, with a exception-safe but specific assignment
mechanism, and put the proxy in the variant instead.


Fernando Cacciola
Sierra s.r.l.
fcacciola@[...].com
www.gosierra.com



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