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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Preprocessor functionality equivalent ideas needed
Date: 12 Dec 2003 05:52:32 +0000
Organization: Pushface
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Date: 2003-12-12T05:52:32+00:00
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Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de> writes:

> There are two more ways I know of, one uses generics for listing
> the configuration-specific things that your program needs. The other
> uses tagged types, that is O-O types. Again for OSs,
> 
> generic
>    type OS is private;
> 
>    with function available_disk_space (op_sys: OS) return Natural;
>    --  free blocks
> 
> package System_Dependent is
>    procedure Installation_Setup;
>    procedure Unpack;
> end System_Dependent;
> 
> If you now write
> 
> with BSD;
> package Everywhere_Working_Installation is
>   new Specifics (available_disk_space => BSD.free_blocks_count);
> 
> you can use the procedures in package Everywhere_Working_Installation
> in your program.

A technique we've found handy is an extension of the "generic
signature package" idea, for use when your code won't be written in a
generic:

generic
   with procedure Actual_P (I : Integer);
   with function Actual_Q return Boolean;
package Signature is
   procedure P (I : Integer) renames Actual_P;
   function Q return Boolean renames Actual_Q;
end Signature;

That way you don't have to use the names P, Q in the actual package
(indeed, the actual subprograms could come from more than one
package).

-- 
Simon Wright                               100% Ada, no bugs.