C++ macro with lambda argument using 2+ captured elements generates error

Question

foo(const std::function<void()>& functor) {
    ....
}

#define MACRO_EXAMPLE(functor) foo(functor)

int main() {
    int i = 0, j = 0;
    MACRO_EXAMPLE([i](){}); // works fine
    MACRO_EXAMPLE([i, j](){}); // error: macro "MACRO_EXAMPLE" passed 2 arguments, but takes just 1
  });
}

Why? How to make that a macro understands lambdas?

Answer 1

Add one more round of parenthesis:

MACRO_EXAMPLE(([i, j](){}));
//            ^          ^

Otherwise the part before , is interpreted as macro's first parameter, and the part after , is interpreted as the macro's second parameter.

Answer 2

As an alternative to @SingerofTheFall's answer (which fixes the problem when invoking the macro), you can also fix the problem in the macro itself, by making it variadic:

#define MACRO_EXAMPLE(...) foo(__VA_ARGS__)

This works by allowing the preprocessor to parse the lambda at , tokens into multiple arguments, but then uses all of these arguments and the separating commas again, so the net effect is what you want.