Martin Bonner <martin.bonner@pitechnology.com> writes:

----Original Message---- From: David Abrahams [mailto:dave@boost-consulting.com] Sent: 27 October 2005 12:57 To: boost@lists.boost.org Subject: Re: [boost] alignment trouble with boost::optional

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Michael Shepanski <mshepanski@gmail.com> writes:

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On 10/26/05, David Abrahams <dave@boost-consulting.com> wrote:

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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).

That's an interesting heuristic. It trades away some theoretical safety for some practical utility. Should we try to warn in these cases, with a PP symbol that you can define to silence the warnings? Can we add some config macro that tells us when it is truly safe? Is there a config test we can run that will tell us at runtime? -- Dave Abrahams Boost Consulting www.boost-consulting.com