15 Library introduction [library]

15.5 Library-wide requirements [requirements]

15.5.3 Requirements on types and expressions [utility.requirements]

15.5.3.3 Cpp17NullablePointer requirements [nullablepointer.requirements]

1
#
A Cpp17NullablePointer type is a pointer-like type that supports null values.
A type P satisfies the Cpp17NullablePointer requirements if:
  • (1.1)
    P satisfies the Cpp17EqualityComparable, Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructible requirements,
  • (1.2)
    lvalues of type P are swappable,
  • (1.3)
    the expressions shown in Table 30 are valid and have the indicated semantics, and
  • (1.4)
    P satisfies all the other requirements of this subclause.
2
#
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.
[ Note
:
Operations involving indeterminate values may cause undefined behavior.
— end note
 ]
3
#
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.
4
#
No operation which is part of the Cpp17NullablePointer requirements shall exit via an exception.
5
#
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 30 — Cpp17NullablePointer requirements
Expression
Return type
Operational semantics
P u(np);
Ensures: u == nullptr
P u = np;
P(np)
Ensures: P(np) == nullptr
t = np
P&
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