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From: Juanma Barranquero <lektu@terra.es>
Newsgroups: comp.lang.ada
Subject: Re: Assigning the value of a deferred constant?
Date: Tue, 18 Sep 2001 10:16:51 +0200
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Date: 2001-09-18T10:16:51+02:00
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On Mon, 17 Sep 2001 17:36:05 GMT, tmoran@acm.org wrote:

>  You can't elaborate the body before you're done elaborating the
>spec, but you can elaborate a different package, including its body.

Yes, that's a solution, but still it is strange to be forced to use
another package just to initialize the constant.

After all, it is not (entirely) a matter of elaboration. If in my
example I do:

  package Test is

     pragma Elaborate_Body(Test);

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

  end Test;

  package body Test is
     -- whatever
  begin
     -- Default_Byte_Order assigned here
  end Test;

i.e., I substitute a variable for the deferred constant, it works as
expected. So I suppose that what I was really asking is why

  V : T;
  C : constant T := V;
  for C'Address use V'Address;

does not make C a constant view of V. Surprisingly, I've done some
more tests (with GNAT 3.13p) and it *does* make C a constant view of
V... if done in the public part of the package. So:

  package Test is
     -- other things

     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;

works (at the cost of making the variable public and so allowing the
back-door modification of Default_Byte_Order), but

  package Test is
     -- other things

     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;

does not. The only differences are that Default_Byte_Order is now
deferred and Machine_Order is private. Is that an implementation
issue, or the semantics intended by the ARM? I'm pretty confused right
now :)

Thanks,

                                                      /L/e/k/t/u