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Date: Thu, 17 Feb 2011 18:27:35 -0500
From: Hyman Rosen <hyrosen@mail.com>
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Newsgroups: comp.lang.ada
Subject: Re: What's wrong with C++?
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Date: 2011-02-17T18:27:35-05:00
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On 2/17/2011 6:17 PM, J-P. Rosen wrote:
> Le 18/02/2011 00:03, Hyman Rosen a écrit :
>> It also occurs to me that the ability to control their internal
>> representation makes them seem an awful lot like integers!
> I think you might be missing a fundamental point about the typing model
> of Ada here.
>
> There are two levels: an abstract type that pertains to the problem
> domain, and an internal representation (which eventually boils down to
> bits). In Ada, you work in the problem domain, independently of the
> representation.
>
> Normally, the compiler chooses the internal representation for you.
> However, if you need a certain representation, you can impose it on the
> compiler; nevertheless, you still work in the abstract domain.
>
> A typical example of this is what we discussed about the 'Pos attribute:
> it is defined as the relative position of an enum in the list where it
> is defined. It has nothing to do whatsoever with its internal
> representation (although, for efficiency reasons, they are the same /by
> default/).

I do understand. I don't know Ada, but I would assume that controlling
internal representation is generally used to match object layout (not
value) against external requirements. (Or to make your arrays of booleans
be arrays of bits, I guess.) It seems more unusual, when you want your
enumerators to have specific internal values, to also claim that they're
not "really" integers.