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

I have seen below macro in topmost header file:

You shouldn't have seen that, the standard library defines it in (and ). And, IIRC, according to the standard, redefining names defined by standard header files results in undefined behaviour. So from a purely standardese viewpoint, you shouldn't do that.

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I've seen people do the following, for whatever reason their broken mind thought of:

struct X{
  virtual void f() = NULL;
}

(As in [incorrectly]: "set the virtual table pointer to NULL")

This is only valid if NULL is defined as 0, because = 0 is the valid token for pure-virtual functions (§9.2 [class.mem]).

That said, if NULL was correctly used as a null pointer constant, then nothing should break.

However, beware that, even if seemingly used correctly, this will change:

void f(int){}
void f(char*){}

f(0); // calls f(int)
f(nullptr); // calls f(char*)

However, if that was ever the case, it was almost certainly broken anyways.