Re: Rules for Representation of Subtypes

[bobduff@world.std.com (Robert A Duff)]

In article <mheaney-ya023180002609962246250001@news.ni.net>,
Matthew Heaney <mheaney@ni.net> wrote:
>"If the size of a type of a first named subtype T has been specified by a
>length clause, the T'Size yields the specified size, AND THE SIZE OF EVERY
>SUBTYPE OF T IS ALSO THIS VALUE [my emphasis]."
>
>If my interpretation is correct (I'm waiting for an opinion, Bob...), then
>this would solve my original problem, which was

I'm not sure if I'm the Bob being addressed, but...   I think your
interpretation is correct.  However, in Ada 95, such a rule can't work.
Consider:

    type T is range 0..15;
    for T'Size use 4;
    subtype Bigger is T'Base range -15..15;

Clearly, Bigger'Size cannot be 4.

Also, note that the size of an object can be bigger than the size of its
subtype (in both Ada 83 and 95).  In other words, specifying the size of
an object gives much more exact control, and is desirable in many
low-level situations (despite the fact that it is rather verbose).

Also, any rule that begins "if the so-and-so attribute is specified..."
is suspect, because you don't want the meaning of 'Whatever to depend on
whether the user chose it or the compiler chose it.  For example, if I
print out Foo'Whatever, and find out it's 123, and then I add "for
Foo'Whatever use 123;", the meaning of my program shouldn't change.

Unfortunately, though, Robert is correct that the new rules cause some
problems with porting code from Ada 83 to Ada 95.  On the other hand,
there were already problems porting between Ada 83 compilers, so I don't
know.  Sigh.

The Java language designers had it much easier -- they simply say that
the predefined types are 8, 16, 32, and 64 bits, and if you're running
on a 36-bit machine, tough.

- Bob