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From: kilgallen@eisner.decus.org (Larry Kilgallen)
Subject: Re: Invoking parental methods (was: Java vs Ada 95)
Date: 1996/10/31
Message-ID: <1996Oct31.094136.1@eisner>#1/1
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references: <ROGOFF.96Oct14095012@sccm.Stanford.EDU>
 <DzBM79.52n.0.-s@inmet.camb.inmet.com> <5536en$2vk@nntpa.cb.lucent.com>
 <1996Oct29.070337.1@eisner> <5599k7$bjr@nntpa.cb.lucent.com>
x-nntp-posting-user: KILGALLEN
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organization: LJK Software
newsgroups: comp.lang.ada
Date: 1996-10-31T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <5599k7$bjr@nntpa.cb.lucent.com>, ka@socrates.hr.att.com (Kenneth Almquist) writes:
>> In article <5536en$2vk@nntpa.cb.lucent.com>, ka@socrates.hr.att.com (Kenneth Almquist) writes:
>>> procedure p(param: access child_obj) is
>>>    type super is access all parent_obj;
>>> begin
>>>    ...
>>>    p(super(param));  -- call the parent's p
>>>    ...
>>> end p;
>>
>> Declaring a pointer type within p does not meet the requirement
>> of minimizing the change required when an intermediate generation
>> gets inserted and there are many such invocations.  Why can't it
>> be declared at the top of the package ?
> 
> Because of the accessibility rules, which are designed to prevent
> dangling references to local variables.  The code below illustrates
> the problem that they are designed to solve.  As the comment notes,
> Ada prevents a dangling reference from being created by raising
> Contraint_Error when an attempt is made to convert the argument to
> and Obj_Ptr.
> 
> If you want to minimize the changes required when inserting an
> intermediate generation, declare "subtype Super is Parent_Ptr;" at
> the top of the package, and the declare "type Super_Ptr is access
> all Super;" in each of the individual routines.
> 				Kenneth Almquist
> 
> 
> procedure Outer is
>     type Obj is record
>         I : Integer;
>     end record;
>     type Obj_Ptr is access all Obj;
> 
>     Dangling_Reference : Obj_Ptr;
> 
>     procedure Create_Dangling_Reference(P : access Obj) is
>         type T is access all Obj;
>         P2 : T;
>         P3 : Obj_Ptr;
>     begin
>         P2 := T(P);        -- Legal.
>         P3 := Obj_Ptr(P);  -- Raises Constraint_Error.
>         Dangling_Reference := P3;
>     end Create_Dangling_Reference;
> 
>     procedure Inner is
>         Data : aliased Obj;
>     begin
>         Create_Dangling_Reference(Obj'access);
>     end Inner;
> begin
>     Inner;
>     -- Dangling_Reference would point to a nonexistent object if
>     -- execution were to reach this point.  Fortunately, execution
>     -- does not reach this point because Constraint_Error is raised
>     -- above.
>     Dangling_Reference.I := 0;
> end Outer;

But the dangling reference seems to be precipitated by the
the pointer object, not by the the pointer type.
What is wrong with:

    type super is access all parent_obj; -- this line may get changed

    procedure p_first(param: access child_obj) is
    begin
       ...
       p_first(super(param));  -- call the parent's p_first
       ...
    end p_first;

    procedure p_second(param: access child_obj) is
    begin
       ...
       p_second(super(param));  -- call the parent's p_second
       ...
    end p_second;

    procedure p_third(param: access child_obj) is
    begin
       ...
       p_third(super(param));  -- call the parent's p_third
       ...
    end p;

Larry Kilgallen