Is it safe to #define NULL nullptr?

Question

I have seen below macro in many topmost header files:

#define NULL 0  // C++03

In all over the code, NULL and 0 are u

Answer 1

You shouldn't be defining it at all, unless you're writing your own version of ; it certainly shouldn't be in "many topmost header files".

If you are implementing your own standard library, then the only requirement is

18.2/3 The macro NULL is an implementation-defined C++ null pointer constant

so either 0 or nullptr is acceptable, and nullptr is better (if your compiler supports it) for the reason you give.