15 Library introduction [library]

15.5 Library-wide requirements [requirements]

15.5.3 Requirements on types and expressions [utility.requirements] Cpp17NullablePointer requirements [nullablepointer.requirements]

A Cpp17NullablePointer type is a pointer-like type that supports null values.
A type P satisfies the Cpp17NullablePointer requirements if:
  • P satisfies the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructible requirements,
  • lvalues of type P are swappable,
  • the expressions shown in Table 30 are valid and have the indicated semantics, and
  • P satisfies 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 value.
Operations involving indeterminate values may cause undefined behavior.
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 30, 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 30Cpp17NullablePointer requirements
Return type
Operational semantics
P u(np);
Ensures: u == nullptr
P u = np;
Ensures: P(np) == nullptr
t = np
Ensures: t == nullptr
a != b
contextually convertible to bool
!(a == b)
a == np
contextually convertible to bool
a == P()
np == a
a != np
contextually convertible to bool
!(a == np)
np != a