Need some help writing an EMPTY_OR macro - c++

Question

I'm trying to code a macro called EMPTY_OR, which will return the first argument, but if it's empty, it will return the second one.
Here's what I have so far:
#include <iostream>
#define EMPTY_OR(x, y) ( (sizeof(#x) > sizeof("")) ? (x) : (y) )
#define TEST(x, y) EMPTY_OR(y, x)
using namespace std;
int main()
{
int four = TEST(4, );
int eight = TEST(4, 8);
cout << "four: " << four << endl;
cout << "eight: " << eight << endl;
return 0;
}
It's close, but it doesn't work because the first line of the main function expands to the following:
( (sizeof("") > sizeof("")) ? () : (4) )
The condition is never true, so () is never evaluated. I shouldn't care about it, but the compiler does, and it shows an error.
How can I solve it with the most straightforward and standard-compliant (or at least MSVC-compliant) way?

Answer 1

If I'm understanding what you're trying to do correctly, I'd remove the sizeof and just check if the first character in the string is \0;
#define EMPTY_OR(x, y) ( #x[0] ? (x+0) : (y) )

Answer 2

Here's a solution adapted from this article and without Boost that works on anything that I can think of that you can pass:
#define CAT(a, b) CAT_(a, b)
#define CAT_(a, b) a##b
#define IF(cond, t, f) CAT(IF_, cond)(t, f)
#define IF_1(t, f) t
#define IF_0(t, f) f
#define COMMA(x) ,
#define ARG3(a, b, c, ...) c
#define HAS_COMMA(x) ARG3(x, 1, 0,)
#define EMPTY(x) EMPTY_(HAS_COMMA(COMMA x ()))
#define EMPTY_(x) HAS_COMMA(CAT(EMPTY_, x))
#define EMPTY_1 ,
#define EMPTY_OR(x, y) IF(EMPTY(x), y, x)
#define TEST(x, y) EMPTY_OR(y, x)
#include <iostream>
using namespace std;
int main()
{
int t = TEST(1==, ) 1;
int f = TEST(1==, 0==) 1;
cout << "t: " << t << endl;
cout << "f: " << f << endl;
return 0;
}