Error: operator "defined" requires an identifier-CodePudding

I have been searching for an hour about this seemingly obvious question, and read several other posts including this one with the same title, but I am still struggling to find a convincing answer. Please forgive my ignorance in advance, and consider this code:

#define MY_MACRO 1
#define IMPLEMENT(x) (defined(x) && (x))

#if IMPLEMENT(MY_MACRO)
#define TESTVAL 1
#else
#define TESTVAL 0
#endif

#include <stdio.h>

int main()
{
    printf("Value is %d\n", TESTVAL);
    return 0;
}

Which doesn't compile and the error messages are:

error: operator "defined" requires an identifier
error: missing '(' in expression

One of the answers in the linked thread says the argument of IMPLEMENT is not known at compile-time, so we cannot use it with '#if defined'. But I don't think this is the case here. Can anyone explain why this doesn't compile?

Bonus question: is there any trick to safely replace the line #if defined(x) && x?

CodePudding user response：

Can anyone explain why this doesn't compile?

IMPLEMENT(MY_MACRO) is expanding MY_MACRO to 1 and then replacing it for defined(1). defined(1) is invalid. 1 is not an identifier.

is there any trick to replace the line #if defined(x) && x?

Yes, just use #if x. If it's not defined, then it's zero. Just #if x, no need for a defined.

#if MY_MACRO