I have seen below macro in topmost header file:
You shouldn’t have seen that, the standard library defines it in <cstddef> (and <stddef.h>). 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.
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 (ยง9.2 [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.