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From: Juanma Barranquero <lektu@terra.es>
Newsgroups: comp.lang.ada
Subject: Assigning the value of a deferred constant?
Date: Mon, 17 Sep 2001 09:53:03 +0200
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Date: 2001-09-17T09:53:03+02:00
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On Mon, 17 Sep 2001 09:37:43 +0200, Juanma Barranquero
<lektu@terra.es> wrote:

>More or less. I've been doing:
>
>   type Byte_Order is (Little_Endian, Big_Endian);
>
>   Default_Byte_Order : Byte_Order;

And vaguely related: Is there any way to declare a deferred constant
and assign its value in the body of the declaring package, during
elaboration? Something like:

  package Test is

     pragma Elaborate_Body;

     type Byte_Order is (Little_Endian, Big_Endian);
     Default_Byte_Order : constant Byte_Order;

  private

     Machine_Order : Byte_Order := Big_Endian;

     Default_Byte_Order : constant Byte_Order := Machine_Order;
     for Default_Byte_Order'Address use Machine_Order'Address;

  end Test;


  with Unchecked_Conversion;

  package body Test is

     type Word is mod 2 ** 32;
     subtype Chr4 is String (1..4);

     function UC is new Unchecked_Conversion(Word, Chr4);

     Test_Word : constant Word := 16#33323130#;
     Test_Chr4 : constant Chr4 := "0123";

  begin
     if UC(Test_Word) = Test_Chr4 then
        Machine_Order := Little_Endian;
     else
        Machine_Order := Big_Endian;
     end if;
  end Test;


That's not working (on Windows, with GNAT 3.13p), but I don't know if
it should or shouldn't :(

I know I could have different bodies for different architectures, or
just substitute a function for the deferrent constant, doing instead:

  function Default_Byte_Order returns Byte_Order;
  pragma Inline(Default_Byte_Order);

or something similar. But the question remains: Is there an easy
and/or portable way to have a public constant view of a private
variable?

Thanks,

                                                       /L/e/k/t/u