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-z

Answer 1

The basic solution is to use << on cerr:

#define MyAssert(expression, msg)                                  \
do {                                                               \
    if(!(expression))                                              \
    {                                                              \
        std::cerr << msg;                                          \
        abort();                                                   \
    }                                                              \
} while(0)

This solution uses C++ streams, so you can format the output as you see fit. Actually this is a simplification of a C++17 solution that I'm using to avoid temporaries (people tend to use + instead of << with this solution, triggering some efficiency warnings).

Use it then like this:

MyAssert(true, "message " << variable << " units");

I think the optionality is bogus here, as you are outputting "Assertion error:" meaning that you expect a message.