++ of C in ada

++ of C in ada

[nicolas.b]

How can i implement the operator ++ in ada :

type T_Ptr is access all T_Item;
How can i implement : procedure Increment (Ptr : in out T_Ptr);
thanks

Re: ++ of C in ada

[Ludovic Brenta]

"nicolas.b" <nicolas.blanpain@fr.thalesgroup.com> writes:
> How can i implement the operator ++ in ada :
>
> type T_Ptr is access all T_Item;
> How can i implement : procedure Increment (Ptr : in out T_Ptr);
> thanks

Don't do this.

In C, arrays and pointers are interchangeable, and pointer++ is used
to traverse an array.

In Ada, arrays and pointers are different.  You cannot increment a
pointer, but you can increment an array index:

type T_Item_Array is array (Positive range <>) of T_Item;

procedure Proc (A : in out T_Item_Array) is
begin
   for J in A'Range loop
      A (J) := ...
   end loop;
end Proc;

The above demonstrates how Ada handles arrays of varying sizes,
without the need for pointers (the compiler uses pointers behind the
scenes, and also ensures you don't go past the end of the array,
forget to deallocate your array, pass the wrong array size to Proc, or
other such mistakes).

If you are not doing arrays, the answer to your question is probably
still "don't do this".  If you explain what you are trying to do, we
will explain the proper Ada way (or ways) of doing it.  In general,
pointer arithmetic is useful only when interfacing directly with your
hardware.

HTH

-- 
Ludovic Brenta.

Re: ++ of C in ada

[Jeffrey Carter]

nicolas.b wrote:
> How can i implement the operator ++ in ada :
> 
> type T_Ptr is access all T_Item;
> How can i implement : procedure Increment (Ptr : in out T_Ptr);

The fact that you're asking this indicates that you're at a stage in 
your exposure to Ada when you will probably have no need whatsoever for 
access types. Access types are rarely needed in Ada unless you're 
creating a dynamic data structure, and even then you don't need pointer 
arithmetic.

If you have some problem for which you think access types and pointer 
arithmetic are required, let us know what the problem is and I suspect 
we can show you why you're wrong, and what the Ada way is. In the 
extremely rare case that you're not wrong, we can show you what to do.

-- 
Jeff Carter
"C++: The power, elegance and simplicity of a hand grenade."
Ole-Hjalmar Kristensen
90

Re: ++ of C in ada

[Marc A. Criley]

nicolas.b wrote:
> How can i implement the operator ++ in ada :
> 
> type T_Ptr is access all T_Item;
> How can i implement : procedure Increment (Ptr : in out T_Ptr);
> thanks
> 

See package Interfaces.C.Pointers.  Section B.3.2 in the Ada LRM 
(http://www.adapower.com/rm95/RM-B-3-2.html).

Think twice before going with this approach, though.  Is it really 
necessary, or are you just carrying through a C/C++ idiom?

-- Marc A. Criley
-- www.mckae.com
-- DTraq - XPath In Ada - XML EZ Out

Re: ++ of C in ada

[Martin Krischik]

nicolas.b wrote:

> How can i implement the operator ++ in ada :
> 
> type T_Ptr is access all T_Item;
> How can i implement : procedure Increment (Ptr : in out T_Ptr);
> thanks

Other allready pointed out that what you are asking it is not a good idea.
May I suggest the following readings:

http://en.wikibooks.org/wiki/Programming:Ada:Control#for_loop_on_arrays
http://en.wikibooks.org/wiki/Programming:Ada:Types:array

Martin

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com

Re: ++ of C in ada

[Robert A Duff]

"nicolas.b" <nicolas.blanpain@fr.thalesgroup.com> writes:

> How can i implement the operator ++ in ada :
> 
> type T_Ptr is access all T_Item;
> How can i implement : procedure Increment (Ptr : in out T_Ptr);

You can implement such a thing using System.Storage_Elements and
System.Address_To_Access_Conversions.

Why do you want to use address arithmetic on T_Ptr?

I'm sure you'll get lots of responses saying you *don't* want to do
that, but there are some rare cases where it might be a good idea.

- Bob

Re: ++ of C in ada

[Craig Carey]

On 24 Jul 2005 11:15:17 -0700, "nicolas.b"
<nicolas.blanpain@fr.thalesgroup.com> wrote:

>How can i implement the operator ++ in ada :
>
>type T_Ptr is access all T_Item;
>How can i implement : procedure Increment (Ptr : in out T_Ptr);
>thanks


A simple solution is to adapt GNAT source code. Then a problem for
a newbie might be: how to choose Linux and avoid daylight savings
bugs, unless Cygwin "cvs" runs. Here is some code I had already
written:


with System;  with Ada.Unchecked_Conversion;
                                    --  Similar to GNAT's i-cpoint.adb
package LL_Byte_Pointers is               --  Low level byte pointers

   function "+" (Left : in System.Address; Right : in Integer)
         return System.Address;

   pragma Convention (Intrinsic, "+");

end LL_Byte_Pointers;


package body LL_Byte_Pointers is

   generic function AUC renames Ada.Unchecked_Conversion;
   subtype Int is Integer;

   type Addr is mod System.Memory_Size;

   function To_System_Address is new AUC (Addr, System.Address);
   function To_Addr is new AUC (System.Address, Addr);
   function To_Addr is new AUC (Int, Addr);

   Elmt_Size   : constant Int := (String'Component_Size
                        + System.Storage_Unit - 1) / System.Storage_Unit;

               --  Strings are packed byte arrays. X'Address counts bytes
   pragma Assert (String'Component_Size = 8);
   pragma Assert (System.Storage_Unit = 8);

   function "+" (Left : in System.Address; Right : in Int)
         return System.Address is
   begin
      return To_System_Address (
                  To_Addr (Left) + To_Addr (Elmt_Size * Right));
   end "+";


end LL_Byte_Pointers;

Checked in the September 2004 MinGW GNAT compiler:

L:/mingw/bin/gnatmake ll_byte_pointers.adb -gnata -gnato -gnatq -gnatf
   -gnatU -m -i -O0 -g -gnatVa -gnatwacfHlpru -gnatR3s
   -bargs -E -p -we -s -static -largs -v -v -L. -lwin32ada -luser32...

(Older and newever versions of that compiler don't run, and all versions
seem unusable on large programs. The May 2005 MinGW GNAT lacks a gnat1
file, and seemingly an e-mail address of the pacakger).

---

There was a list of 191 FreeBSD Ports that would be deleted, at the
 "announce@freebsd.org" list (on 29 July 2005). 

With Debian GNU/Linux, the failures seem to occur at my end (no fonts
in KDE, and a while back it was ill-explained data loss until IBM did
something for the opensource project). 

A FreeBSD GNAT Ada cross-compiler seems to be a need.
I found that the FreeBSD Linuxulator can compete with and beat,
a native Linux distro (such a Debian system).


Craig Carey
research@ijs.co.nz