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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: OO, C++, and something much better!
Date: 1997/01/23
Message-ID: <EACHUS.97Jan23104846@spectre.mitre.org>#1/1
X-Deja-AN: 211715305
references: <mheaney-ya023280001601972303180001@news.ni.net>
 <E4DvAz.G06@syd.csa.com.au>
organization: The Mitre Corp., Bedford, MA.
newsgroups: 
 comp.lang.c++,comp.lang.smalltalk,comp.lang.eiffel,comp.lang.ada,comp.object,comp.software-eng
Date: 1997-01-23T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <E4DvAz.G06@syd.csa.com.au> donh@syd.csa.com.au (Don Harrison) writes:

  > That's one view. Another view is to consider such 'subtypes' as
  > types in their own right. This is more honest, IMO. Using this
  > definition, neither Ada nor Eiffel is 'type-safe'.

   I'm not trying to start a flame war here but the distinctions
between types and subtypes in Ada are subtle, and semantic games like
this don't help.

   If you call a tail a leg, how many legs does a dog have?

   When it comes to computer language definitions, only the reference
manual can do redefinitions like that for technical terms.  In Ada,
subtype is a technical term, and is clearly very different from a
type: A type declaration creates an (anonymous) type, and a (named and
possibly constrained) first subtype.  A subtype declaration creates
only a name for a subtype of some already defined type.  In fact all
of the differences between:

   type Foo is new Integer range 1..10;
 
      and 

   subtype Foo is Integer range 1..10;

      are because the first declaration actually creates a type and
several operations on that type, and the second declaration creates a
shorthand for a subtype mark and possibly a constraint.  As a language
lawyer, I feel compelled to point out that a subtype declaration in
Ada can define attributes--but so can object declarations.  In the
example above, Foo'First is 1, not Integer'First.  In any case
attributes which are functions are operations of the (anonyomus) type,
not of the subtype, so a type and all its subtypes always share the
same operations:

    subtype Foo is Integer range 1..10;
    Bar: Integer := 1234;
    Bar_Image: String := Foo'Image(Bar);
   
    gives Bar_Image the value " 1234".  Yes, the subtype declaration
did declare a function Foo'Image, but its parameter is of subtype
Foo'Base.  The terminology is tricky here, Foo'Base is not a subtype
of Foo or of Integer, it is a subtype of the anonymous type created by
the declaration of Integer in Standard.  Normal (but fuzzy) usage is
to say that the type of the parameter to Foo'Image is Integer, not Foo.

    There are places in Ada where a subtype_mark is permitted but not
a subtype_indication, which is a subtype_mark with an explicit
constraint.  However, this is for stylistic reasons and to simplify
the order of elaboration.  In fact the big difference between Ada
subtypes and C macros used as type names is that in Ada the
expressions are evaluated at the point of the subtype declaration.

     In Ada it is not unusual to see subtype declarations like:

     subtype Short is Really_Long_Package_Name.Short;

     used solely as a renaming.  To quote RM95 8.5(6 & 7): "A subtype
defined without any additional constraint can be used to achieve the
effect of renaming another subtype (including a task or protected
subtype) as in

      "subtype Mode is Ada.Text_IO.File_Mode;"

--

					Robert I. Eachus

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