How to return an empty string if __VA_ARGS__ is empty in a C++ macro?

Question

I have the following macro:

#define MyAssert(condition, ...) CallMyAssertFunctionInCpp(condition, __VA_ARGS__)

__VA_ARGS__ is the error message of type char *, I would like to make it equal to "" if __VA_ARGS__ is empty.

How to do that? I'm not sure I can do condition in macro but maybe there is a trick?

if it's C++ then why don't just use templates? It's much easier and cleaner — phuclv, Sep 14 '21 at 08:59
IIRC it's not possible using macros. Almost all problems with function-like macros can be solved by using actual functions instead. For example this one. where you can have one templated overload taking arguments and one taking no arguments. — Some programmer dude, Sep 14 '21 at 08:59
***Why*** do you have to use a macro? What is the actual and underlying problem you need to solve with a macro like this? What are the full list of requirements and limitations? — Some programmer dude, Sep 14 '21 at 09:12
https://stackoverflow.com/questions/11761703/overloading-macro-on-number-of-arguments and overload for zero and one argument. — KamilCuk, Sep 14 '21 at 09:45
If you really want macro trick: https://godbolt.org/z/6vMGW3P64 — VainMan, Sep 14 '21 at 09:56

score 3 · Answer 1 · answered Sep 14 '21 at 09:07

As suggested in comments, it's easy to achieve using C++ templates:

#include <utility>

const char * CallMyAssertFunctionInCpp(bool condition, int arg);

template <typename... T>
const char * MyAssert(bool condition, T... args) {
    if constexpr (sizeof...(T) == 0) {
        return "";
    } else {
        return CallMyAssertFunctionInCpp(condition, std::forward<T>(args)...);
    }
}

void example() {
    MyAssert(true);
    MyAssert(true, 42);
}

How to return an empty string if __VA_ARGS__ is empty in a C++ macro?

1 Answers1