std::partial_ordering (3) - Linux Man Pages

std::partial_ordering: std::partial_ordering

NAME

std::partial_ordering - std::partial_ordering

Synopsis


Defined in header <compare>
class partial_ordering; (since C++20)


The class type std::partial_ordering is the result type of a three-way_comparison that


* admits all six relational operators (==, !=, <, <=, >, >=)


* does not imply substitutability: if a is equivalent to b, f(a) may not be equivalent to f(b), where f denotes a function that reads only comparison-salient state that is accessible via the argument's public const members. In other words, equivalent values may be distinguishable.
* admits incomparable values: a < b, a == b, and a > b may all be false

Constants


The type std::partial_ordering has four valid values, implemented as const static data members of its type:


Member constant Definition


less(inline constexpr) a valid value of the type std::partial_ordering indicating less-than (ordered before) relationship
                             (public static member constant)
[static]


equivalent(inline constexpr) a valid value of the type std::partial_ordering indicating equivalence (neither ordered before nor ordered after)
                             (public static member constant)
[static]


greater(inline constexpr) a valid value of the type std::partial_ordering indicating greater-than (ordered after) relationship
                             (public static member constant)
[static]


unordered(inline constexpr) a valid value of the type std::partial_ordering indicating relationship with an incomparable value
                             (public static member constant)
[static]

Conversions


std::partial_ordering is implicitly-convertible to std::weak_equality, while both std::strong_ordering and std::weak_ordering are implicitly-convertible to partial_ordering.


                       implicit conversion to std::weak_equality
operator weak_equality (public member function)


 std::partial_ordering::operator weak_equality


constexpr operator weak_equality() const noexcept;

Return value


std::weak_equality::equivalent if v is equivalent, std::weak_equality::nonequivalent if v is less, greater, or unordered.

Comparisons


Comparison operators are defined between values of this type and literal 0. This supports the expressions a <=> b == 0 or a <=> b < 0 that can be used to convert the result of a three-way comparison operator to a boolean relationship; see std::is_eq, std::is_lt, etc.
The behavior of a program that attempts to compare a partial_ordering with anything other than the integer literal 0 is undefined.


operator==
operator!=
operator< compares with zero
operator> (function)
operator<=
operator>=
operator<=>


 operator==


friend constexpr bool operator==(partial_ordering v, /*unspecified*/ u) noexcept;
friend constexpr bool operator==(/*unspecified*/ u, partial_ordering v) noexcept;

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


true if v is equivalent, false if v is less, greater, or unordered


 operator!=


friend constexpr bool operator!=(partial_ordering v, /*unspecified*/ u) noexcept;
friend constexpr bool operator!=(/*unspecified*/ u, partial_ordering v) noexcept;

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


true if v is unordered, less, or greater, and false if v is equivalent


 operator<


friend constexpr bool operator<(partial_ordering v, /*unspecified*/ u) noexcept; (1)
friend constexpr bool operator<(/*unspecified*/ u, partial_ordering v) noexcept; (2)

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


1) true if v is less, and false if v is greater, equivalent, or unordered
2) true if v is greater, and false if v is less, equivalent, or unordered


 operator<=


friend constexpr bool operator<=(partial_ordering v, /*unspecified*/ u) noexcept; (1)
friend constexpr bool operator<=(/*unspecified*/ u, partial_ordering v) noexcept; (2)

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


1) true if v is less or equivalent, and false if v is greater or unordered
2) true if v is greater or equivalent, and false if v is less or unordered


 operator>


friend constexpr bool operator>(partial_ordering v, /*unspecified*/ u) noexcept; (1)
friend constexpr bool operator>(/*unspecified*/ u, partial_ordering v) noexcept; (2)

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


1) true if v is greater, and false if v is less, equivalent, or unordered
2) true if v is less, and false if v is greater, equivalent, or unordered


 operator>=


friend constexpr bool operator>=(partial_ordering v, /*unspecified*/ u) noexcept; (1)
friend constexpr bool operator>=(/*unspecified*/ u, partial_ordering v) noexcept; (2)

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


1) true if v is greater or equivalent, and false if v is less or unordered
2) true if v is less or equivalent, and false if v is greater or unordered


 operator<=>


friend constexpr partial_ordering operator<=>(partial_ordering v, /*unspecified*/ u) noexcept; (1)
friend constexpr partial_ordering operator<=>(/*unspecified*/ u, partial_ordering v) noexcept; (2)

Parameters


v - a std::partial_ordering value to check
u - an unused parameter of any type that accepts literal zero argument

Return value


1) v.
2) greater if v is less, less if v is greater, otherwise v.

Notes


The built-in_operator_<=> between floating-point values uses this ordering: the positive zero and the negative zero compare equivalent, but can be distinguished, and NaN values compare unordered with any other value.

Example


 This section is incomplete
 Reason: no example

See also


strong_ordering the result type of 3-way comparison that supports all 6 operators and is substitutable
                (class)
(C++20)


weak_ordering the result type of 3-way comparison that supports all 6 operators and is not substitutable
                (class)
(C++20)


strong_equality the result type of 3-way comparison that supports only equality/inequality and is substitutable
                (class)
(C++20)


weak_equality the result type of 3-way comparison that supports only equality/inequality and is not substitutable
                (class)
(C++20)