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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: 'Size hack for enumerated types
Date: Sat, 05 Jul 2014 22:47:50 +0100
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Date: 2014-07-05T22:47:50+01:00
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Victor Porton <porton@narod.ru> writes:

> I have written the following code:
>
>    type Flag_Type is (Libxml_Error_Save,
>                       Libxml_Structured_Error_Save,
> 		      URI_Interning,
>                       WWW_Skip_Init_Finish);
>    for Flag_Type'Size use Interfaces.C.int'Size; -- hack
>    for Flag_Type use (Libxml_Error_Save => 1,
>                       Libxml_Structured_Error_Save => 2,
> 		      URI_Interning => 3,
>                       WWW_Skip_Init_Finish => 4);
>
> The code above is intended to be able to pass flag variables into
> certain C functions (accepting enum types).
>
> But it seems that the above code has the drawback of storing in 4 or 8
> bytes instead of one byte.

Well, you specified 'Size to be (almost certainly) 32 bits, so it's not
surprising that that's what the compiler does.

But there's a difference between the minimum number of bits required to
hold a *value* of the type (3 in your example) and the number of bits
the compiler uses when holding an *object* of the type, which will be
what is most efficient. GNAT has 'Object_Size.

> What is a better way to do this?

Just use convention C:

   type Flag_Type is (Libxml_Error_Save,
                      Libxml_Structured_Error_Save,
                      URI_Interning,
                      WWW_Skip_Init_Finish) with Convention => C;

and let the compiler worry about sizes.