Question 10.26

How can I write a macro which takes a variable number of arguments?

One popular trick is to define and invoke the macro with a single, parenthesized ``argument'' which in the macro expansion becomes the entire argument list, parentheses and all, for a function such as printf:

	#define DEBUG(args) (printf("DEBUG: "), printf args)

	if(n != 0) DEBUG(("n is %d\n", n));

The obvious disadvantage is that the caller must always remember to use the extra parentheses.

gcc has an extension which allows a function-like macro to accept a variable number of arguments, but it's not standard. Other possible solutions are to use different macros (DEBUG1, DEBUG2, etc.) depending on the number of arguments, to play games with commas:

	#define DEBUG(args) (printf("DEBUG: "), printf(args))
	#define _ ,

	DEBUG("i = %d" _ i)

It is often better to use a bona-fide function, which can take a variable number of arguments in a well-defined way. See questions 15.4 and 15.5.

Read sequentially: prev next up top