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From: lutz@iks-jena.de (Lutz Donnerhacke)
Newsgroups: comp.lang.ada
Subject: Re: AdaYY wish list from current projects
Date: 26 Feb 2001 11:20:24 GMT
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Date: 2001-02-26T11:20:24+00:00
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* tmoran@acm.org wrote:
>What's wrong with:
>   subtype String_Lengths is integer range 1 .. 32767;
>   type pstring(len : String_Lengths) is record
>      data : String (String_Lengths);
>   end record;
>   for pstring use record
>     len at 0 range 0 .. 15;
>   end record;

It's not portable, len should be little endian.

:   raw  : constant String := "\0\4Test";
:   test : pstring (String_Lengths'Last);
:   for test'Address use raw'Address;

warning: default initialization of "test" may modify "raw"
warning: use pragma Import for "test" to suppress initialization (RM B.1(24))

>Yes, the compiler won't do your subscript range checking for you, but

Even worse: Put_Line ("Len:" & plen'Image (test.len)); results in "Len: 1000".
And of course
   Put ("Data: >");
   for i in test.data'Range loop
      Put (test.data(i));
   end loop;
   Put_Line ("<");
will print the whole 1000 chars.

>you are going to import/export this data object to a non-Ada program
>anyway.

Ok, pragma Import does suppress the initialization:
with Ada.Text_IO, System; use Ada.Text_IO;

procedure t is
   subtype plen is Natural range 0 .. 1000;
   type pstring (len : plen := plen'Last) is record
      data : String (1 .. len);
   end record;
   for pstring use record
      len at 0 range 0 .. 15;
   end record;
   for pstring'Bit_Order use System.Low_Order_First;
   
   raw : constant String := ASCII.EOT & ASCII.NUL & "Test";
   test : pstring;
   for test'Address use raw'Address;
   pragma Import(Ada, test);
begin
   Put_Line ("Len:" & plen'Image (test.len));
   
   Put ("Data: >");
   for i in test.data'Range loop
      Put (test.data(i));
   end loop;
   Put_Line ("<");
end t;

compiles without warning and results in:
Len: 4
Data: >Test<

Great!

Using the opposite endianess causes:
t.adb:9:11: warning: multi-byte field specified with non-standard Bit_Order
t.adb:9:11: warning: bytes are not reversed (component is little-endian)

*Barf*