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

From: David Abrahams (dave_at_[hidden])
Date: 2005-10-26 07:18:10

Michael Shepanski <mshepanski_at_[hidden]> writes:

>  The code below fails a compile-time assertion in boost 1.32.0 on MSVC 7.1.
>
> This is because, internally, boost::optional uses boost::alignment_of, which
> produces the number 12, which is then passed as a template parameter to
> boost::type_with_alignment, which won't accept 12.
>  Not sure where the blame lies. Is it with boost::alignment_of, for
> producing
> 12 in the first place? Perhaps not, since 12 is a multiple of 4, and the
> doco
> says boost::alignment_of is allowed to produce any multiple of the actual
> alignment value. So is the fault with boost::type_with_alignment, for not
> accepting 12? Or is there something wrong with the app?

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...

>   #include <boost/optional.hpp>
>
> class D
> {
> virtual ~D();
> };
>
> class E : virtual public D
> {
> virtual ~E();
> unsigned long long m_i;
> };
>
> boost::optional<E> s_optE;
>
> Cheers,
> --- Michael Shepanski
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-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com