Re: Type safety on wikipedia

["jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>]

Florian Weimer wrote:
> Now, suppose that X is a pool-specific access value for some type T,
> and Free is a corresponding instance of Ada.Unchecked_Deallocation.
> Suppose that
>
>
>    Free (X);
>
> has just been exected.  Suppose the next thing to be evaluated is
>
>    declare
>       Y : T := X.all;
>    begin
>       ...


Let's look at an actual program doing an equivalent action:

with Ada.Text_Io;

procedure Access_Test is
   type Int_Access is access Integer;
   P : Int_Access;
begin
   P := null;
   Ada.Text_Io.Put_Line(Integer'Image(P.All));
end Access_Test;


The problem is one of attempting to dereference a null access object.
The above program compiles without error.

When run, I get the following error message:

raised CONSTRAINT_ERROR: access_test.adb:8 access check failed

It appears that Ada's runtime checks detect an erroneous problem.
I would say that a program that terminates with a CONSTRAINT_ERROR
has not exhibited the same kind of improper behavior as a program that
does not detect the problem.

In C, while it is an error to de-reference a null pointer, the runtime
system
does nothing to detect the problem. C programs continue with really
nasty
garbage values.

Jim Rogers