17 Language support library [support]

17.2 Common definitions [support.types]

17.2.3 Null pointers [support.types.nullptr]

1
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The type nullptr_t is a synonym for the type of a nullptr expression, and it has the characteristics described in [basic.fundamental] and [conv.ptr].
[Note 1: 
Although nullptr's address cannot be taken, the address of another nullptr_t object that is an lvalue can be taken.
— end note]
2
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The macro NULL is an implementation-defined null pointer constant.168
See also: ISO/IEC 9899:2018, 7.19
168)168)
Possible definitions include 0 and 0L, but not (void*)0.