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Old 04-01-2005, 02:53 PM   #1
Registered: Jul 2004
Location: Spain
Posts: 111

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Question about macros

Hello there, take a look at this code:
#define macro(a,a1,b,b1) \
#ifdef a \
#endif \
#ifdef b \
b1; \
I can't compile this code, i'm sure it's wrong, but it says what i want to do, i.e. :
macro(Q, printf("Q is defined\n"), W, printf("W is defined\n"));

/* This will be expanded to */ 

#ifdef Q
printf("Q is defined\n");
#ifdef W
printf("W is defined\n");
So, how must i rewrite my macro?

Thanks in advance.
Old 04-01-2005, 04:15 PM   #2
Senior Member
Registered: Aug 2002
Location: Groningen, The Netherlands
Distribution: Debian
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It's a bit of an unusual thing you are trying here:
You defined a preprocessor macro that expands to new preprocessor directives.

This would require the preprocessor to run recursively over your source code. Though I've never thought of this idea, I would be surprised if the preprocessor works this way.

You could however try to run the preprocessor twice: the first time youself by running the "cpp" command, and have the compiler run it the second time the normal way. I think this would be frowned upon by many people (including me), but it may work.

Last edited by Hko; 04-01-2005 at 04:17 PM.
Old 04-01-2005, 05:48 PM   #3
jim mcnamara
Registered: May 2002
Posts: 964

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cpp will expand macros inside include files.
You could try putting thiose commands inside nested include files.

Although as to why you are doing this I'm with Hko.
There must be a better way.
Old 04-02-2005, 03:17 AM   #4
Registered: Jul 2004
Location: Spain
Posts: 111

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So, what's the better way to do that?

Old 04-03-2005, 04:34 AM   #5
Registered: Jul 2004
Location: Spain
Posts: 111

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It's already done, i've changed the code's layout. Thanks.

Since these constant are well-known and there are only two, i've done the following:

#ifdef Q
#define macro(a,b) \
#elif defined(W)
#define macro(a,b) \


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