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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Allocators and exceptions
Date: Tue, 11 Sep 2007 14:56:54 -0700
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On Sep 11, 8:33 am, I wrote:
> On Sep 10, 7:36 pm, "Randy Brukardt" <ra...@rrsoftware.com> wrote:
>
> > (2) I believe that the current wording of the standard *requires* that
> > reclamation *not* be performed in examples like this, at least if there are
> > any controlled components in the type. That's because there is no permission
> > in Ada to do finalization early - it has to be done only if the object is
> > explicitly destroyed or when the master goes out of scope -- which for an
> > allocated object is when the *type* goes out of scope.
>
> It sure seemed to me that one could create an example in which, due to
> a default initialization on a component that contains a function call
> that does some interesting stuff, the automatic reclamation Maciej is
> asking for would lead to a dangling reference or something just as
> bad.

OK, I think I have one.  GNAT compiles and runs it, although I haven't
checked the language rules carefully to make sure it's legal.

----------------------------------------------------------------------------
package Pak1 is
    type Rec1 is limited private;
    type Rec1_Acc is access all Rec1;
    function Func1 (R : Rec1_Acc) return Integer;
private
    type Rec1 is limited record
        F1 : Integer := 123;
        F2 : Integer := Func1 (Rec1'Unchecked_Access);
    end record;
end Pak1;

package body Pak1 is

    P : Rec1_Acc;

    function Func1 (R : Rec1_Acc) return Integer is
    begin
        P := R;
        if R.F1 > 100 then
            raise Constraint_Error;
        end if;
        return 1;
    end Func1;

end Pak1;

with Pak1;
procedure test84 is
    X : Pak1.Rec1_Acc := new Pak1.Rec1;
begin
    null;
end test84;
----------------------------------------------------------------------------

When the allocator "new Pak1.Rec1" is evaluated, an object of type
Rec1 is created; then F1 is set to 123; then the code attempts to
initialize F2 by calling Func1.  The parameter is the
'Unchecked_Access of the object that has already been created.  Func1
saves this in the global P.  Then it raises Constraint_Error.  If the
Constraint_Error caused the object of type Rec1 to be deallocated and
thus no longer exist, P would then be left as a dangling reference.

                     -- Adam