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From: "G.B." <bauhaus@notmyhomepage.invalid>
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Subject: =?UTF-8?Q?Re:_How_to_get_Ada_to_=e2=80=9ccross_the_chasm=e2=80=9d?=
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Date: Sat, 5 May 2018 18:47:54 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:52011
Date: 2018-05-05T18:47:54+02:00
List-Id: <comp.lang.ada>

On 04.05.18 21:05, Jere wrote:

> 1.  Dangling references:  Keeping a reference to an object past its lifetime
> 
> Ada:
> ***********************************************
> with Ada.Text_IO; use Ada.Text_IO;
> 
> procedure jdoodle is
>      type Integer_Access is access all Integer;
>      
>      function Inner(Value : aliased in out Integer) return Integer_Access is
>      begin
>          return Value'Access;
>      end Inner;
>      
>      function Outer return Integer_Access is
>          Value : aliased Integer := 0;
>      begin
>          return Inner(Value);
>      end Outer;
>      
>      Ptr : Integer_Access := Outer;  -- !!! Dangling reference
> begin
>      Put_Line("Hello World");
> end jdoodle;

Anything that can be done to prevent the above effect
should be welcome, if it is representative of what the
2012 RM allows. Or is this new *aliased* parameter thing
some I-know-what-I-am-doing Ada?

So, do explicitly *aliased* parameters indeed break all accessiblity
rules of Ada?  I noticed that it is mentioned in the RM alongside
parameters that are passed by reference already because their
type is a by-reference type. I'd expect then, that one
would drop *aliased* for those kinds of type, thus


     27. with Ada.Text_IO; use Ada.Text_IO;
     28.
     29. procedure jdoodle2 is
     30.     type T is tagged
     31.        record
     32.            Data :  Integer;
     33.        end record;
     34.
     35.     type T_Access is access all T;
     36.
     37.     function Inner(Value : in out T) return T_Access is
     38.     begin
     39.         return Value'Access;
                        |
         >>> non-local pointer cannot point to local object

     40.     end Inner;
     41.
     42.     function Outer return T_Access is
     43.         Value : aliased T := T'(Data => 0);
     44.     begin
     45.         return Inner(Value);
     46.     end Outer;
     47.
     48.     Ptr : T_Access := Outer;
     49. begin
     50.     Put_Line("Hello World");
     51. end jdoodle2;


The same dangling pointer effect reappears, however, when I put
*aliased* back, for no apparent reason:

     function Inner(Value : aliased in out T) return T_Access is
     begin
         return Value'Access;
     end Inner;