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From: noren@dinl.uucp (Charles Noren)
Newsgroups: comp.lang.c++,comp.lang.c
Subject: Conditional #if preprocessor expressions
Message-ID: <1849@dinl.mmc.UUCP>
Date: 17 Apr 91 16:47:45 GMT
Reply-To: noren@dinl.UUCP (Charles Noren)
Organization: Martin Marietta I&CS, Denver CO.
Lines: 82

This is my day to ask stupid questions....

I have defined a macro symbol, INCL to be:

  #define INCL apple

I then use INCL in the following expressions:

  #if INCL == apple
  #include "apple.h"
  #endif

  #if INCL == orange
  #include "orange.h"
  #endif

My expectation is that only the include file "apple.h"
is included.  However, the include file "orange.h" is
included as well.  Why is this? 
(I know there must be an obvious answer...)
BTW, some examples in nihcl use this construct.  See
ex11-1.c under the ex directory.

My environment:

 o  Sun Spac station 2.
 o  SunOS 4.1.1
 o  Saber C++ 1.0
 o  also compiled under the default Sun cc.

Sample program:

======main.c==================================
/*
 * File: main.c
 *
 */

#define INCL apple

#if INCL == apple
#include "apple.h"
#endif

#if INCL == orange
#include "orange.h"
#endif

#include <stdio.h>

char* myString = FRUIT;

main()
{
    printf("myString = [%s].\n", myString);
}
==============================================

======apple.h=================================
/* File: apple.h  */

#define FRUIT "apple"

==============================================

======orange.h================================
/* File: orange.h  */

#define FRUIT "orange"

==============================================


Thanks,


-- 
Chuck Noren
NET:     dinl!noren@ncar.ucar.edu
US-MAIL: Martin Marietta I&CS, MS XL8058, P.O. Box 1260,
         Denver, CO 80201-1260
Phone:   (303) 977-1646