Analysis of information sources in references of the Wikipedia article "ヌルポインタ" in Japanese language version.
NULL to represent the zero pointer. Because of C++'s tighter type checking, the use of plain 0, rather than any suggested NULL macro, leads to fewer problems. If you feel you must define NULL, use
const int NULL = 0;const qualifier (§5.4) prevents accidental redefinition of NULL and ensures that NULL can be used where a constant is required."