std::partial_ordering
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partial_ordering
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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.
Contents
[edit] Constants
The type std::partial_ordering has four valid values, implemented as const static data members of its type:
Name
Definition
inline constexpr std::partial_ordering less
[static]
(public static member constant)
inline constexpr std::partial_ordering equivalent
[static]
(public static member constant)
inline constexpr std::partial_ordering greater
[static]
(public static member constant)
inline constexpr std::partial_ordering unordered
[static]
(public static member constant)
[edit] Conversions
std::partial_ordering cannot be implicitly converted to other comparison category types, while both std::strong_ordering and std::weak_ordering are implicitly-convertible to partial_ordering.
[edit] 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.
These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::partial_ordering is an associated class of the arguments.
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>operator<=operator>=operator<=>
partial_ordering (function)
operator==
friend constexpr bool operator==( partial_ordering v, /*unspecified*/ u ) noexcept;
(1)
friend constexpr bool
operator==( partial_ordering v, partial_ordering w ) noexcept = default;
(2)
operator==( partial_ordering v, partial_ordering w ) noexcept = default;
Parameters
v, w
-
std::partial_ordering values to check
u
-
an unused parameter of any type that accepts literal zero argument
Return value
1) true if
v is equivalent, false if v is less, greater, or unordered2) true if both parameters hold the same value, false otherwise
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 unordered2) 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 unordered2) 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 unordered2) 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 unordered2) 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.
[edit] 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.
[edit] Example
This section is incomplete
Reason: no example
Reason: no example
[edit] See also
(C++20)
(class) [edit]
(C++20)
(class) [edit]