需要一些帮助来编写EMPTY_OR宏

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

如果我了解您要正确执行的操作，请删除sizeof，然后检查字符串中的第一个字符是否为\\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;
}