I’ve been playing around with C23’s new features and I came across nullptr, and I’d like to use it in place of NULL where possible, or at least in demo code to better understand proper usage.

Here is an example of some code where I tried to replace NULL with nullptr, as you might do in C++:

#include <stdio.h>
#include <stddef.h>

int main() {
    int x = 10;
    int* ptr = &x;
    if(ptr != nullptr) {
        printf("ptr not null!");
    }
}

I’m using gcc 13.2.0, which should support C23’s nullptr. Surprisingly, compilation fails with or without the -std=c2x flag.

Here is the error gcc gives:

nullptr.c: In function 'main':
nullptr.c:7:15: error: 'nullptr' undeclared (first use in this function); did you mean 'nullptr_t'?
    7 |     if(ptr != nullptr) {
      |               ^~~~~~~
      |               nullptr_t
nullptr.c:7:15: note: each undeclared identifier is reported only once for each function it appears in

Replacing nullptr with the type nullptr_t doesn’t make sense, it’s looking for an expression before a type name like nullptr_t, and indeed that is what happens.

I can get a successful compilation and expected behavior with simple casting NULL to nullptr_t:

if(ptr != (nullptr_t)NULL)

My assumption is that this is not the proper or intended usage of C23’s nullptr, and there’s something I’m missing.