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From: "Marin D. Condic" <mcondic-nospam@acm.com>
Subject: Re: conditional compilation
Date: 2000/07/31
Message-ID: <39857E5F.33C40014@acm.com>#1/1
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Organization: Quadrus Corporation
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Newsgroups: comp.lang.ada
Date: 2000-07-31T13:25:36+00:00
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Matthew Woodcraft wrote:
> 
> I noticed the following statement in the Steelman requirements:
> 
> "6C. Conditional Control. There shall be conditional control structures
> that permit selection among alternative control paths.
> ...
> Only the selected branch shall be compiled when the discriminating
> condition is a translation time expression."
> 
> Am I right in thinking that the second sentence above isn't true of
> Ada-as-we-know-it (for example, if a non-selected branch has a
> compile-time error)?
> 
I know from experience with a few different embedded compilers that it
was a common optimization to remove dead code. A construct such as:

declare
    Debug : constant Boolean := False ;
begin
    if (Debug) then
        Do_Some_Stuff ;
    else
        Do_Some_Other_Stuff ;
    end if ;
end ;

would result in the call to Do_Some_Stuff being removed from the object
code. It was a reasonably effective method of getting conditional
compilation. However, I don't recall that the unreachable code could
ever be syntactically incorrect - or why you would want it to be.

Of course the weakness of this is that you can't pull this trick in a
declarative part. It would be nice to have a mechanism for controlling
that sort of thing as well. (Yes, GNAT has pragma Debug - it's just not
part of the standard - yet.)

Now I'm not sure that a true conditional compilation feature based on
lexical exclusion is really such a good idea. I've been doing a lot of
C++ work lately using Visual C++/MFC and it never ceases to amaze me
that people actually *like* this kind of thing. I'm constantly tracking
down problems related to #ifdef and #include causing multiple instances
of things getting brought in - or not. And of course, the compiler
doesn't care because its C. Would Ada really want to get caught up in
that web?

BTW: Visual C++ has a *really* nice debugger. But then again, it needs
it. :-) It would be nice to see an environment this powerful for Ada.

MDC
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Marin David Condic - Quadrus Corporation - http://www.quadruscorp.com/
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