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Newsgroups: comp.lang.ada
Subject: Re: expect procedure name in procedure call(newbie)
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From: Mark Lorenzen <mark.lorenzen@ofir.dk>
Date: 29 Dec 2004 14:24:45 +0100
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Date: 2004-12-29T14:24:45+01:00
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"R" <ruthless@poczta.onet.pl> writes:

> OK so i added new variable ret(Integer) and added:
> ret := testclass.Create(object, 10);
> 
> but now Create raises an expception: CONSTRAINT_ERROR : testclass.adb:5
> access check failed
> 
> where the Create looks like:
> function Create(this: rec1_Access; s: Integer) return Integer is
> begin
> this.field := s; -- this is the 5th line - access check error
> return this.field;
> end Create;
> 
> basicly  the codes are the same from my first post - I only folow Your
> instuction not to ignore the return value.
> 
> Is my access type wrong? it point to rec1 tagged record.
> thanks in advance for Your help
> best regards R

The parameter 'this' is of an access type. As the actual parameter you
pass a non-initialised variable. In Ada, all variables of access type
have the value 'null' by default. So you pass 'Create' a null value
and the dereference it.

Your example seems to be a translation of a C++ function. I would do
the following instead:

1) Make 'Create' a procedure instead of a function with the following
   signature:

   Create (This : out Rec1; S : in Integer);

2) procedure Main is
     Object : Testclass.Rec1;
   begin
     Testclass.Create(Object, 10);
   end Main;

Do not fiddle around with pointers as much as you do. It seems like
you try to program in Ada the C++ way.

- Mark Lorenzen