Re: put of access type

[sjw <simon.j.wright@mac.com>]

On Aug 19, 8:00 pm, Rob Solomon <use...@drrob1-noreply.com> wrote:
> >> I would like to output a pointer to the screen.  (I am just learning
> >> Ada).  
>
> >> How do I put(Any_Access_Type)
>
> >You probably meant the address of the pointed object. Here you go. Let Ptr
> >be a pointer to some object
>
> >(with System.Storage_Elements;  use System.Storage_Elements;)
>
> >   Integer_Address'Image (To_Integer (Ptr.all'Address)));
>
> >Ptr.all'Address is the address of the object. To_Integer converts the
> >address to a plain integer type (Integer_Address defined in
> >System.Storage_Elements). This type can be output as any other.
>
> I am trying to understand this.  
>
> Back when I learned Modula-2, I learned that pointer types are
> assignment compatible with System.Address and I understand them as
> being the same thing.  They  differ in context, not value.
>
> I see that access types are being used the same as pointer types, but
> if I understand correctly, are actually quite different.  But I don't
> understand how.  ie, how are access types and pointers different?

For simple types (integer; simple records) there's probably no
difference. For types with constraints/discriminants, bets are off:
consider

  S : access String := "foo";

S will most likely (this is compiler-dependent) *not* point to the 'f'
in "foo"; it might point to a structure containing

   4-byte integer, lower bound of string (= 1)
   4-byte integer, upper bound of string (= 3)
   1-byte character = f
   1-byte character = o
   1-byte character = o

> What is the difference between integer_address and address?

integer_address is some sort of integer, with 'image, "=", "-" etc;
address is private, with no such operations (it does have comparators;
see package System).

> I need help with this code:
>
> procedure adr is
>
> c: integer;
> x: aliased integer;
> p1, p2 : access integer;
> str : string (1..255);
>
> Begin
>   p1 := x'access;
>   p2 := p1;
>   str := integer_address'image(to_integer(p1.all'address));
>   c := integer(to_integer(p2.all'address));
>
> some put statements
> end adr;
>
> This compiles w/ GNAT under Ubuntu 9.04, but I am getting
> constraint_error, range check failed.
>
> I don't understand why

integer_address'image is going to be up to about 10 (32-bit addresses)
or 20 (64-bit addresses) characters long, and you are assigning it to
a string of length 255; that won't work.

Either pass the value directly to the put routine -- put
(integer_address'image(... -- or (probably less useful to you?) use a
declare block:

declare
   str : constant string := integer_address'image(....
begin
   put(str);