Let’s consider this example code:
#include <stddef.h>
int *f(void) { return NULL; }
int g(int x) { return x == NULL ? 3 : 4; }
We want f to compile without warnings, and we want g to cause an error or a warning (because an int variable x was compared to a pointer).
In C, #define NULL ((void*)0) gives us both (GCC warning for g, clean compile for f).
However, in C++, #define NULL ((void*)0) causes a compile error for f. Thus, to make it compile in C++, <stddef.h> has #define NULL 0 for C++ only (not for C). Unfortunately, this also prevents the warning from being reported for g. To fix that, C++11 uses built-in nullptr instead of NULL, and with that, C++ compilers report an error for g, and they compile f cleanly.