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From: howard@hulder.css.gov (Howard Turner)
Newsgroups: comp.lang.ada
Subject: Re: What should this do?
Keywords: Variable-length strings constraint exception
Message-ID: <758@esosun.UUCP>
Date: 28 Jun 91 21:40:23 GMT
References: <1991Jun28.193513.14271@afit.af.mil>
Sender: news@esosun.UUCP
Reply-To: howard@hulder.css.gov (Howard Turner)
Organization: Science Aplications Int'l Corp.
In-reply-to: dlindsle@afit.af.mil (David T. Lindsley)
List-Id: <comp.lang.ada>


	dlindsle@afit.af.mil (David T. Lindsley) writes:
	I have a question about dynamic strings.  I was looking at a package
	on simtel20 that did the following:

	             subtype INDEX is natural range 0..100;
	             type DYN_STRING (SIZE : INDEX := 0) is private; 
	
	--           private
	--                 type DYN_STRING (size : index := 0) is
	--                    record
	--                        DATA : string (1..size);
	--                    end record;

	the second declaration had a note to the effect of "uncomment and use
	this if you've got a VALIDATED compiler".

	Now as far as I can tell, during elaboration, a declaration of the form

		S : DYN_STRING;

	will result in the attempt to allocate an array constrained to (1..0),
	which should raise an exception.  At least, that's what it does under
	VAX Ada, but not on Verdix.  (Both generate warnings.)

	It seems to me this has to be a bug in one of the compilers.  My question
	is: whose?  Should this, or should it not, raise an exception?  (The
	LRM references weren't any help.)


Ahemm...
While working on a recent project, I ran into much the same problem.
One of the persons involved (Bob Smith, smith@prc.unisys.com) found
the following and sent it to me.  Bob set me right.  Thanks Bob.  


	Bob Smith (quite a while ago) writes:
	...
	You might try reading LRM 3.6.1 paragraph 4:

	  If any of the discrete ranges defines a null range, any array thus 
	  constrained is a null array, having no components.

	Then take a look at 3.5 paragraph 3:
	
	  The range L .. R specifies the values from L to R inclusive if the 
	  relation L <= R is true. . . .  A null range is a range for which
	  the relation R < L is TRUE; no value belongs to a null range.
	...


While it does not explicitly mention strings, remember that the type 
string is defined as an array of characters.

Hopes this helps

Howard Turner
SAIC
619.458.2654
howard@esosun.css.gov