How to implement a standard-compliant assert macro with an optional formatted message?

Question

What's the way to implement a standard-compliant assert macro with an optional formatted message?

What I have works in clang, but (correctly) triggers the -Wgnu-zero-variadic-macro-arguments warning if it is turned on (e.g. via -Wpedantic) when the macro is used without the optional message. Wandbox

#define MyAssert(expression, ...)                                      \
    do {                                                               \
        if(!(expression))                                              \
        {                                                              \
            printf("Assertion error: " #expression " | " __VA_ARGS__); \
            abort();                                                   \
        }                                                              \
    } while(0)

Answer 1

I have a solution which I'm not particularly proud of..

We can obtain the first argument in plain form and as a string using:

#define VA_ARGS_HEAD(N, ...) N
#define VA_ARGS_HEAD_STR(N, ...) #N

Note that in usage, in order to not get warnings, you should do VA_ARGS_HEAD(__VA_ARGS__, ) (with the extra ,) so that VA_ARGS_HEAD is never used with a single parameter (trick taken from StoryTeller's answer).

We define the following helper function:

#include <stdarg.h>
#include <stdio.h>

inline int assertionMessage(bool, const char *fmt, ...)
{
    int r;
    va_list ap;
    va_start(ap, fmt);
    r = vprintf(fmt, ap);
    va_end(ap);
    return r;
}

When the assertion has a format string, the function would work with __VA_ARGS__ as is, however when the bool is the only argument, we're missing a format string. That's why we'll add another empty string after __VA_ARGS__ when invoking it:

#define MyAssert(...)                                                          \
    do {                                                                       \
        if(!(VA_ARGS_HEAD(__VA_ARGS__, )))                                     \
        {                                                                      \
            printf("Assertion error: %s | ", VA_ARGS_HEAD_STR(__VA_ARGS__, )); \
            assertionMessage(__VA_ARGS__, "");                                 \
            abort();                                                           \
        }                                                                      \
    } while(0)

Note that assertionMessage doesn't have printf in its name. This is deliberate and intended to avoid the compiler giving format-string related warnings for its invocations with the extra "" argument. The down-side for this is that we don't get the format-string related warnings when they are helpful.