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Date: 22 Oct 92 16:56:50 GMT
From: visicom!amstel!rlk@nosc.mil  (Bob Kitzberger)
Subject: Re: Platform specific sources
Message-ID: <rlk.719773010@amstel>
List-Id: <comp.lang.ada>

tim@trident.datasys.swri.edu (Timothy J. Barton) writes:

>Is there a "superior" method for supporting platform specific
>features in the same source code using Ada?

IMHO, the superior method is to encapsulate target-specific code
into a (sub)unit (package, procedure, etc.) that can be separately compiled.

Two main approaches:

A. Subunits

    package My_System is

      package Target_Specifics is separate;
    ...

  Then, have 'n' version of the separate package Target_Specifics, in
  separate, uniquely named files (Target_sparc.ada, Target_mc680x0.ada,
  etc.)  Of course, only compile the file that matches the target you
  are building your system for.

  (caveat: there are performance issues related to use of subunits.
   See "Optimization Issues Relating to Subunits", Tom Burger, Ada Letters
   May/June 1992.)

B. Packages

       with Target_Specifics;
       package My_System is ...

   As above, write a version of Target_Specifics for each target
   you are supporting.
   


For some target dependencies, it can be difficult to encapsulate the
dependencies into units or subunits.  For finer granularity of
conditional compilation, use constants:

     type Target is ( MC680x0, 80x86, SPARC);

     This_Target : constant Target := MC680x0;

     ...

     if This_Target = SPARC then
        ...
     etc.

  Reasonable compilers will optimize away code inside of conditional
  statements, if the conditional statically evaluates to false.


        .Bob.
----------------
Bob Kitzberger          VisiCom Laboratories, Inc.
rlk@visicom.com         10052 Mesa Ridge Court, San Diego CA 92121 USA
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