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From: stt@houdini.camb.inmet.com (Tucker Taft)
Subject: Re: constraint-errors
Date: 1996/10/07
Message-ID: <DywwM1.F8K.0.-s@inmet.camb.inmet.com>#1/1
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newsgroups: comp.lang.ada
Date: 1996-10-07T00:00:00+00:00
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Paul O Svelmoe (zingbust@selway.umt.edu) wrote:
: Can someone please give me some general advice on what causes
: constraint-errors?  For 2 years I have been plagued by them every time I
: try to do any programming at all in Ada.  The only way I get by is by
: continuing to try different things until something finally works, but I
: never really understand what caused the crash in the first place.  

If you look in Chapter 11 of the Ada reference manual, you will find
a list of causes of predefined exceptions (in the section entitled
"Suppressing Checks" -- admittedly a non-intuitive place to look for
this information).  Here are the most common causes:

   1) Attempting to dereference a null pointer
   2) Indexing beyond the end of an array
   3) Choosing a field of the "wrong" variant of a record 
   4) Going outside the bounds of an integer range.
   5) Forgetting to properly initialize a variable.
      
There are other possibilities, but these are probably the most common.

: ... Right
: now I'm using a stack implemented as a linked list and a simple test 
: 'while not Is_Empty(sorted_stack)' is causing a constraing error.  I know
: you can't diagnose that particular error just from that but I really want
: to try and gain insight into this thing.  Can anyone point me in the right
: direction?  Thanks so much.

If you are using linked lists, then by far the most common cause of
a constraint_error is dereferencing a null pointer.  In the above case,
I suspect that "sorted_stack" is a null pointer, and Is_Empty doesn't
check for that.

: Paul Svelmoe
: zingbust@selway.umt.edu

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA