Re: C's sizeof

[Simon Wright <simon@pushface.org>]

Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> Simon Wright <simon@pushface.org> writes:
>
>> The (draft) ARM Annex B.3[1] says at para 73 - Note 7 -
>>
>>     "To obtain the effect of C's sizeof(item_type), where Item_Type is
>>     the corresponding Ada type, evaluate the expression:
>>     size_t(Item_Type'Size/CHAR_BIT)."
>>
>> Should that not be
>>
>>     size_t((Item_Type'Size + CHAR_BIT - 1)/CHAR_BIT)
>>
>> ?
>
> I think it should be more like:
>
> pragma Assert(Item_Type'Base'Size mod CHAR_BIT = 0);
> pragma Assert(Item_Type'Size = Item_Type'Base'Size);
> size_of_item: constant size_t := size_t(Item_Type'Size/CHAR_BIT);
> pragma Assert(size_of_item*CHAR_BIT = Item_Type'Size);
>
> See Jeff Carter's comment.
>
> Maybe some of those asserts are overkill, but if you're interfacing to
> C, you don't want to mess around with subranges that C doesn't
> understand.

So should the ARM suggest that sizeof() for scalar types could be
obtained by

   size_t((Item_Type'Base'Size + CHAR_BIT - 1)/CHAR_BIT)

? No, because (again, with GNAT) it doesn't work for enumerations.