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Date: 22 Sep 92 20:22:07 GMT
From: pitt.edu!gvls1!aviary!dmarshal@gatech.edu  (Dave Marshall)
Subject: Re: conditional compiles ?
Message-ID: <1743@aviary.Stars.Reston.Unisys.COM>
List-Id: <comp.lang.ada>

In article <1992Sep22.115353.27412@umbc3.umbc.edu>, dipto@umbc4.umbc.edu (Dipto
 Chakravarty) writes:
> 
> I am new to Ada, and was wondering how to do conditional compiles under
> an Ada program. Conditional compiles using #ifdef and #endif, etc.,
> allows us to maintain a large software which has several system specific 
> routines using C. 
> 
> Please reply by email, as I am not a regular "rn" reader. Thanx in advance.
> 
> .


It's not possible to use conditional compilation in Ada like you can in C.

However, by using a Boolean parameter to a function, procedure, or generic
package or subprogram, you can achieve the same effect.  (Examples below.)

Paragraph 2 of Section 10.6 of the LRM states:

A compiler may find that some statements or subprograms will never be executed,
 
for example, if their execution depends on a condition known to be FALSE.  The
corresponding object machine code can then be omitted.  This rule permits the
effect of conditional compilation within the language.

Examples:

(I didn't compile these, so don't sue me if they're not exactly right.)

function MY_FUNCTION ( A : in INTEGER, DEBUG: in BOOLEAN) is

begin

  if DEBUG then

-- some debug code

  end if;

-- and so on

end MY_FUNCTION;

The procedure example is equally trivial.

I think that the way to give a compiler the best chance to optimize the
code away is to use the conditional parameter as a parameter for
instantiating a generic package or subprogram.

generic
  DEBUG : BOOLEAN;
package MY_PACKAGE is

  procedure DO_WAH;

end MY_PACKAGE;

Now implement the body of MY_PACKAGE and surround your conditional code
with "if"s on DEBUG.  When you instantiate the package, the compiler
should be able to do what's right.

package MY_DEBUG_TEST is new MY_PACKAGE ( DEBUG => TRUE);

OR

package MY_FINAL_PRODUCT is new MY_PACKAGE ( DEBUG => FALSE);

You can do the same thing with procedures.

generic
  DEBUG : BOOLEAN;
procedure MY_TEST_PROCEDURE;

-- 
                                           Dave Marshall
                                           dmarshal@stars.reston.unisys.com