Re: [boost] alignment trouble with boost::optional

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2005-10-28 02:57:41

----Original Message----
From: David Abrahams [mailto:dave_at_[hidden]]
Sent: 27 October 2005 12:57
To: boost_at_[hidden]
Subject: Re: [boost] alignment trouble with boost::optional

> Michael Shepanski <mshepanski_at_[hidden]> writes:
> 
>> On 10/26/05, David Abrahams <dave_at_[hidden]> wrote:
>>> 
>>> 
>>> I think the problem is that type_with_alignment can't find a type
>>> that has an alignment of 12. Producing an error in that case is the
>>> right behavior. We could add the type whose alignment_of<> is 12 to
>>> the reperotoire of types being tested by type_with_alignment...
>> 
>> 
>> 
>> Don't we need to be sure that type_with_alignment<> can accept *any*
>> number that alignment_of<> produces -- or, conversely, make sure that
>> alignment_of<> only produces numbers that type_with_alignment<> can
>> accept? (I'm assuming that we want boost::optional to work all the
>> time, not just for my example.)
> 
> Sure, that's what we want.  Can you figure out *how*?  That's the
> problem.

In summary:

1. alignment_of<T> produces a number that is a multiple of the required
alignment.
2. type_with_alignment<> cannot deal with all the numbers that
alignment_of<> produces.

Does anyone know of any platform (including platforms that are no longer in
use), where the alignment requirement was /not/ a power of two?  If not,
then alignment_of<> "just" needs to eliminate the factors from its result
which are not a power of two.  (That would also allow us to eliminate the
special case for long double under Borland).

-- 
Martin Bonner
Martin.Bonner_at_[hidden]
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: [phone]