CODEWITHSUNDEEP
cc-preprocessor
sergeyz
sergeyz

Reputation: 1328

Macro evaluation in c preprocessor

I'd like to do something like this:

#define NUM_ARGS() 2
#define MYMACRO0(...) "no args"
#define MYMACRO1(...) "one arg"
#define MYMACRO2(...) "two args"
#define MYMACRO(num,...) MYMACRO##num(__VA_ARGS__)
#define GENERATE(...) MYMACRO(NUM_ARGS(),__VA_ARGS__)

And I expected it to evaluate to "two args". But instead I have

MYMACRONUM_ARGS()(1,1)

Is there a way to do what I want (using visual c++) ?

P.S. Eventually I want to implement logger that dumps all of variables. The next code 

int myInt = 7;
string myStr("Hello Galaxy!");
DUMP_VARS(myInt, myStr);

will produce log record "myInt = 7; myStr = Hello Galaxy!"

Upvotes: 4

Views: 3127

Answers (1)

effeffe
effeffe

Reputation: 2891

You need another macro because macro expansion does not take place near # or ##:

#define NUM_ARGS() 2
#define MYMACRO0(...) "no args"
#define MYMACRO1(...) "one arg"
#define MYMACRO2(...) "two args"
#define MYMACRO_AUX(num,...) MYMACRO##num(__VA_ARGS__)
#define MYMACRO(num,...) MYMACRO_AUX(num, __VA_ARGS__)
#define GENERATE(...) MYMACRO(NUM_ARGS(),__VA_ARGS__)

#include <stdio.h>

int main(void)
{
    puts(GENERATE(0, 1));

    return 0;
}

If this is what you're trying to do, but complicated preprocessor tricks are not really safe, as others already said, don't do it unless you really have to.

Upvotes: 7

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