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Date: Sat, 5 May 2018 14:23:02 -0700 (PDT)
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Subject: Recommendation of safe subset of Ada to use?
From: joakimds@kth.se
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Xref: reader02.eternal-september.org comp.lang.ada:52017
Date: 2018-05-05T14:23:02-07:00
List-Id: <comp.lang.ada>

Jere gave the following example in the other thread about how to get Ada ac=
ross the chasm:

1.  Dangling references:  Keeping a reference to an object past its lifetim=
e=20

Ada:=20
***********************************************=20
with Ada.Text_IO; use Ada.Text_IO;=20

procedure jdoodle is=20
    type Integer_Access is access all Integer;=20
   =20
    function Inner(Value : aliased in out Integer) return Integer_Access is=
=20
    begin=20
        return Value'Access;=20
    end Inner;=20
   =20
    function Outer return Integer_Access is=20
        Value : aliased Integer :=3D 0;=20
    begin=20
        return Inner(Value);=20
    end Outer;=20
   =20
    Ptr : Integer_Access :=3D Outer;  -- !!! Dangling reference=20
begin=20
    Put_Line("Hello World");=20
end jdoodle;=20
***********************************************=20
Hello World=20

gcc -c jdoodle.adb=20
gnatbind -x jdoodle.ali=20
gnatlink jdoodle.ali -o jdoodle=20

It's a 20 line application that demonstrates a dangling pointer in Ada. Tha=
t's not supposed to be able to happen unless one goes outside of Ada's type=
 system by using Unchecked_Deallocation, Unchecked_Conversion or System.Add=
ress_To_Access_Conversion. I've tried the example with the GNAT compiler an=
d it does not detect the issue. I do not believe this is a GNAT bug. Aliase=
d parameters were part of the solution to be able to safely reference eleme=
nts in containers and thereby avoid unnecessary copying. By making this pos=
sible was a hole in Adas type system introduced? It means that one cannot s=
afely use all the features of Ada and be sure of memory safety instead one =
should stick to a subset of Ada. One subset that comes to mind is SPARK. An=
other is for example sticking to Ada95 or Ada 2005. Or maybe one should jus=
t ban usage of aliased parameters but then what should one do with the stan=
dard containers that one probably uses throughout one's application. I am c=
onfused. Anybody that can shed light?

/Joakim