nullptr is a null pointer constant with type nullptr_t. It implicitly converts to any pointer type but not to integer types. The type nullptr_t is defined in <stddef.h>.

The macro NULL is defined as an integer constant (typically 0 or (void*)0), which can match integer parameters in overloaded or generic contexts. nullptr has a distinct type that converts only to pointers, eliminating ambiguity in type-generic code and improving type safety.

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

void process(int *ptr) {
    if (ptr == nullptr) {
        printf("Received null pointer\n");
    } else {
        printf("Value: %d\n", *ptr);
    }
}

int main(void) {
    int x = 42;
    process(&x);
    process(nullptr);
}