There isn't a guarantee that NULL has a bit pattern that's the same as a true (void)0 C-style null pointer either. I didn't look it up directly but apparently

There isn't a guarantee that NULL has a bit pattern that's the same as a true (void)0 C-style null pointer either. I didn't look it up directly but apparently NULL is allowed to be nullptr instead of 0, and nullptr converts to void in varargs arguments. I think C/C++ also allow actual null pointers to have different bit patterns than the integer zero, so in C++03 passing NULL to terminate a varargs argument list would technically be incorrect, or maybe implementation-defined.