[nullablepointer.requirements]

A Cpp17NullablePointer type is a pointer-like type that supports null values.

A type P meets the Cpp17NullablePointer requirements if

(1.1)
P meets the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, Cpp17Swappable, and Cpp17Destructible requirements,
(1.2)
the expressions shown in Table 36 are valid and have the indicated semantics, and
(1.3)
P meets all the other requirements of this subclause.

A value-initialized object of type P produces the null value of the type.

The null value shall be equivalent only to itself.

A default-initialized object of type P may have an indeterminate or erroneous value.

[Note 1:

Operations involving indeterminate values can cause undefined behavior, and operations involving erroneous values can cause erroneous behavior ([basic.indet]).

— end note]

An object p of type P can be contextually converted to bool .

The effect shall be as if p != nullptr had been evaluated in place of p.

No operation which is part of the Cpp17NullablePointer requirements shall exit via an exception.

In Table 36, u denotes an identifier, t denotes a non-const lvalue of type P, a and b denote values of type (possibly const) P, and np denotes a value of type (possibly const) std::nullptr_t.

Table 36 — Cpp17NullablePointer requirements [tab:cpp17.nullablepointer]

🔗 Expression	Return type	Operational semantics
🔗 P u(np);		Postconditions: u == nullptr
🔗 P u = np;
🔗 P(np)		Postconditions: P(np) == nullptr
🔗 t = np	P&	Postconditions: t == nullptr
🔗 a != b	decltype(a != b) models boolean-testable	!(a == b)
🔗 a == np	decltype(a == np) and decltype(np == a) each model boolean-testable	a == P()
🔗 np == a
🔗 a != np	decltype(a != np) and decltype(np != a) each model boolean-testable	!(a == np)
🔗 np != a