Thread: 'NULL' : undeclared identifier

When working with pointers when I use separate files (and sometimes when I use 1 file) I get:

error C2065: 'NULL' : undeclared identifier

I did some searching but none of these worked:

Code:

#define NULL

Code:

#define NULL 0

Code:

#define NULL ((void *) 0)

Code:

#ifndef NULL
#ifdef __cplusplus
#define NULL    0
#else
#define NULL    ((void *)0)
#endif
#endif

Code:

#include <cstdlib>

In fact, compiling using any these usually crashes the program.