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ifdef

Posted on 2011-03-21
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Last Modified: 2012-05-11
#ifdef TEST
            // Line 1
#else
           // Line 1
#endif

What if I need multiple lines. is it possible
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Question by:learningunix
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7 Comments
 
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Accepted Solution

by:
käµfm³d   👽 earned 1000 total points
ID: 35184506
Yes. Everything between the #ifdef and the #endif will be treated as one unit. It is much like a regular "if" statement, but you can think of the #endif as being a substitute for the curly braces.
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LVL 16

Expert Comment

by:imladris
ID: 35184509
Yup. You can have as many lines as you like. That why the #else and the #endif are there. They are used to mark the end of the conditional code. In fact, internally, C and C++ don't even "see" lines. It's just one long string of characters. To the parser, newlines, which define the end of lines, are just characters to be ignored like blanks.
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LVL 86

Expert Comment

by:jkr
ID: 35184513
Absolutely - actually, that's quite common. Most header files are secured via header guards (http://faculty.cs.niu.edu/~mcmahon/CS241/c241man/node90.html), so that's not only more than one line, but merely a whole file.
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Author Comment

by:learningunix
ID: 35185640
Ok. I thought I have to use { } for multiple lines but that is not the case

This should work

#ifdef TEST
            int a = 1;
            a++;
           printf ("%d"a);            
#else
           // Line 1
           // Line 2
#endif

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Expert Comment

by:jkr
ID: 35186117
That should definitely work. Just keep in mind that e.g. a #define statement spanning multiple lines needs a trailing backslash if you want to do something like this.
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LVL 35

Expert Comment

by:sarabande
ID: 35187657
you sometimes will see in code

#if  1
   // some code

#endif

the 1 means true and it is easily changed to #if 0 if the code shouldn't be performed anymore. using constants instead of preprocessor macros has advantages for temporary code and code limited to one source cause you don't need to define/undefine a macro like TEST..

Sara
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Author Closing Comment

by:learningunix
ID: 35227103
thx
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