Variadic macros tricks

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.

What happened when I decided to learn this technique. I’m trying to spare you… :-) Image credit: xkcd.com

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:

Nice. __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.

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