Re: On alignment

[Jason Merrill <jason at redhat dot com>]

On 02 May 2003 15:07:54 +0200, Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:

> Tom Tromey <tromey@redhat.com> writes:
>
> [...]
>
> | What will eventually go wrong with what we have now?
> | 
> |     template<typename T>
> |     struct aligner
> |     {
> |       T field;
> |     };
> | 
> |     #define ALIGNOF(TYPE) (__alignof__ (((aligner<TYPE> *) 0)->field))
>
> This, as I understand it from my previous attempt, won't give you the
> right answer (for double, it would give 8 instead of 4).

It will give you the right answer on the trunk (and I just checked in a
patch so it will give you the right answer in 3.3 as well).

> What is wrong with the following code I posted earlier?
>
>     template<typename T>
>       struct alignment {
>
>          struct helper {
>             T datum;
>             helper();
>          };
>
>          static helper instance;
>
>          enum { value = __alignof__ (instance) };
>       };

The alignment of a non-field instance of a class can be greater that the
alignment of the class itself.  Just saying __alignof (helper) should work,
though I'm not sure it's guaranteed to work.  Also, why do you give helper
a constructor?

Tom's example above is directly asking the question they want an answer to;
as long as it works, that's the best way to do it, and it does work now.

Jason