Variadic Macro Assertions

Variadic Macro Assertions

C++ allows #define preprocessor macros to have variable arguments using the “...” and “__VA_ARG__” special tokens. Our yassert macro changes to:

    #define yassert(cond, ...) \
       ( (cond) || \
          aussie_yassert_fail(#cond, __FILE__, __LINE__, __VA__ARG__) )

And we change our “aussie_yassert_fail” to have an extra optional “message” parameter.

    bool aussie_yassert_fail(char* str, char* fname, int ln, char *mesg=0);

This all works fine if the yassert macro has 2 arguments (condition and extra message) but we get a bizarre compilation error if we omit the extra message (i.e. just a basic assertion with a condition). The problem is that __VA_ARG__ expands to nothing (because there's no optional extra message argument), and the replacement text then has an extra “,” just hanging there at the end of the argument list, causing a syntax error.

Fortunately, the deities who define C++ standards noticed this problem and added a solution in C++17. There's a dare-I-say “hackish” way to fix it with the __VA__OPT__ special token. This is a special token whose only purpose is to disappear along with its arguments if there's zero arguments to __VA_ARG__ (i.e. it takes the ball and goes home if there's no-one else to play with). Hence, we can hide the comma from the syntax parser by putting it inside __VA_OPT__ parentheses. The final version becomes:

    #define yassert(cond, ...) \
       ( (cond) || \
          aussie_yassert_fail(#cond, __FILE__, __LINE__ \
              __VA_OPT__(,) __VA__ARG__) )

Note that the comma after __LINE__ is now inside of a __VA_OPT__ special macro. Actually, that's not the final, final version. We really should add “__func__” in there, too, to report the function name. Heck, why not add __DATE__ and __TIME__ while we're at it? Why isn't there a standard __DEVELOPER__ macro that adds my name?

I really need someone from the C++ committee to wash my mouth out with soap. I mean, __VA_OPT__ is not a hack; it's an internationally standardized feature. Sorry, my mistake!

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