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From: vilot@wanginst.UUCP (Michael Vilot)
Newsgroups: comp.lang.ada
Subject: Re: Elaboration of Parameterised Record Types
Message-ID: <553@wanginst.UUCP>
Date: Sun, 9-Nov-86 14:37:19 EST
Article-I.D.: wanginst.553
Posted: Sun Nov  9 14:37:19 1986
Date-Received: Mon, 10-Nov-86 21:29:04 EST
References: <386@cernvax.UUCP>
Reply-To: vilot@wanginst.UUCP (Michael Vilot)
Organization: Wang Institute, Tyngsboro, Ma.  01879
Keywords: subtype range attributes
Summary: Another DEC Ada bug
List-Id: <comp.lang.ada>

In article <386@cernvax.UUCP> paul@cernvax.UUCP writes:
>
[ a reference from the Vax-Ada Programmer's Run-time Reference Manual]
>However, this seems to conflict with the ARM (Ada Reference Manual) ...
[ Section 7.6 ]
>
>   max_possible_string : CONSTANT:=some_value; --Implem defined
>   SUBTYPE index IS INTEGER RANGE 0..max_possible_string;
>   TYPE text(this_string_maximum : index) IS
>   RECORD
>
>   "From a user's point of view, a TEXT is a variable length string. Each
>   TEXT object has a maximum length, which must be given when the object is
>   declared, and a current value, which is a string of some length between
>   zero and the maximum." 
>                ^^^^^^^
>
>Which maximum??? Dec have assumed max_possible_string. But if this should
>be so, why is it necessary to declare, and supply a value for,
>"this_string_maximum"? 
>
>On the contrary, it would appear that "this_string_maximum" is what the
>(admittedly ambiguous) ARM intended to say. 
>
>My question therefore is:
>Is my understanding faulty or is my NUMERIC_ERROR the result of a coding
>bug in the DEC Vax-Ada compiler, based on a DEC misinterpretation of the
>ARM at this point? I would have thought that DEC should allocate storage
>for the record according to the value of the parameter
>"this_string_maximum" and not according to the type of
>"this_string_maximum". 
>
>In practice, many programs will work under either interpretation and will
>give the same external behaviour. Internally however, the DEC
>interpretation allocates much more space. Lastly, the DEC interpretation
>cannot cope with examples such as the one below.

The DEC implementation assigns 'LENGTH according to the BASE type, not
the subtype.  Hence the over-allocation of storage.

The real workaround (on DEC  Ada) is to  use a named  number, and  not
rely on the attribute.

(Portable?  Of  COURSE  it's portable! 
 It works on any  DEC  implementation! :-)
-- 
Michael J. Vilot			 ...!decvax!wanginst!vilot (UUCP)
Wang Institute of Graduate Studies	vilot%wanginst@CSNet-Relay (CSNet)
Tyng Road, Tyngsboro, MA 01879		MVilot@ADA20            (ARPA)