Have you ever wanted to write a “for each” loop over all the args of a variadic macro? Or have you ever wanted to overload a macro on the number of arguments? (If you’re saying to yourself, “good grief, why?” — I’ll describe a use case at the bottom of this post.)
I learned how to do this today, and I wanted to blog about it to cement the technique in my own mind. (And I hereby put all the code I show here into the public domain.)
Simple variadic macros
The first piece of magic you need to do something like this is
__VA_ARGS__. This allows you to write macros that take an arbitrary number of arguments, using
... to represent the macro’s parameters:
__VA_ARGS__ is a standard feature of C99, and I’ve known about it for a long time. I’ve also known about GCC (and Clang’s) extension, which attaches special meaning to
##__VA_ARGS__ if it’s preceded by a comma–it removes the comma if
##__VA_ARGS__ expands to nothing. If I change my macro definition to:
…I can now call
eprintf("hello, world"); without a complaint from the compiler.
But it’s not enough
That doesn’t let me do a “for each” loop, though. All the args that I pass are expanded, but I can’t do anything with them, individually. I have no names for my macro’s parameters–just the anonymous ….
I went poking around, not expecting to find a solution, but I was pleasantly surprised.