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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Conditional compilation in Ada?
References: <cndgus$9g6$1@korweta.task.gda.pl>
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>a small change in a fundamental data structure. Both versions of the
>data structure are mostly handled the same way (within thousands of
>lines of source code), but there are some 10 or 20 (number still
>growing) places where minor changes are necessary. What is worse,
  You can use an "if" statement.
if Is_Version1 then ...
else ...
  If you define Is_Version1 at the top, or in another small package, as
  Is_Version1 : constant Boolean := True;
then an intelligent compiler will not generate any code for the impossible
Is_Version1 = False condition or the if-test itself.  That is, it will be
identical in effect to C conditional compilation.
  If the "minor changes" are similar, you could change the processing in
the many places in the huge module to instead call a subroutine and then
only make changes/ifs in the body of that subroutine (or subroutines).