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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: CONSTRAINT ERROR? access check failed
Date: Thu, 1 Mar 2018 18:56:32 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:50758
Date: 2018-03-01T18:56:32+01:00
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On 03/01/2018 03:38 PM, Mehdi Saada wrote:
> Work finished ! Thank you.
> But I would like to know why I get a warning at compile time with
> type Link (Num : LINE_NUMBER := 0; Length : NATURAL := 0) is
>        record
> 	 NEXT : access LINK := null;
> 	 Line : String (1 .. Length) := (1..Length => ' ');
>        end record;
> As if it could cause a stack overflow (was written as is). Yet I don't get at each compilation, surprisingly.

Because you have defaults for your discriminants, you can declare

V : Link;

V is then unconstrained. You can change its discriminants through whole-record 
assignment:

V := (Num => 23, Length => 42, Next => null, Line => (1 .. 42 => 'S') );

The way GNAT handles this is to allocate space for the largest variant, and only 
use the part for the current discriminant. Apparently GNAT doesn't use a stack 
large enough to handle a string of length Integer'Last.

Why is Num a discriminant? There's no reason for a discriminant that isn't used 
in the record definition.

-- 
Jeff Carter
"Facts do not cease to exist because they are ignored."
Aldous Huxley
134