Difference between revisions of "cpp/utility/compare/weak ordering"
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m (Formatting) |
(The property described in this sentence is called "connected" or "total" i.e. "< is connected" I added a link to this property's Wikipedia article.) |
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* admits all six relational operators ({{tt|1===}}, {{tt|1=!=}}, {{tt|<}}, {{tt|1=<=}}, {{tt|>}}, {{tt|1=>=}}) | * admits all six relational operators ({{tt|1===}}, {{tt|1=!=}}, {{tt|<}}, {{tt|1=<=}}, {{tt|>}}, {{tt|1=>=}}) | ||
{{cpp/utility/compare/substitutable|no}} | {{cpp/utility/compare/substitutable|no}} | ||
| − | * does not allow incomparable values: exactly one of {{c|a < b}}, {{c|1=a == b}}, or {{c|a > b}} must be {{c|true}} | + | * [https://en.wikipedia.org/wiki/Connected_relation does not allow incomparable values]: exactly one of {{c|a < b}}, {{c|1=a == b}}, or {{c|a > b}} must be {{c|true}}. |
===Constants=== | ===Constants=== | ||
Revision as of 10:01, 26 January 2023
| Defined in header <compare>
|
||
| class weak_ordering; |
(since C++20) | |
The class type std::weak_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.
- does not allow incomparable values: exactly one of a < b, a == b, or a > b must be true.
Contents |
Constants
The type std::weak_ordering has three valid values, implemented as const static data members of its type:
| Member constant | Definition |
| less(inline constexpr) [static] |
a valid value of the type std::weak_ordering indicating less-than (ordered before) relationship (public static member constant) |
| equivalent(inline constexpr) [static] |
a valid value of the type std::weak_ordering indicating equivalence (neither ordered before nor ordered after) (public static member constant) |
| greater(inline constexpr) [static] |
a valid value of the type std::weak_ordering indicating greater-than (ordered after) relationship (public static member constant) |
Conversions
std::weak_ordering is implicitly-convertible to std::partial_ordering, while std::strong_ordering is implicitly-convertible to weak_ordering.
| operator partial_ordering |
implicit conversion to std::partial_ordering (public member function) |
std::weak_ordering::operator partial_ordering
| constexpr operator partial_ordering() const noexcept; |
||
Return value
std::partial_ordering::less if v is less, std::partial_ordering::greater if v is greater,
std::partial_ordering::equivalent if v is equivalent.
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::weak_ordering is an associated class of the arguments.
The behavior of a program that attempts to compare a weak_ordering with anything other than the integer literal 0 is undefined.
| operator==operator<operator>operator<=operator>=operator<=> |
compares with zero or a weak_ordering (function) |
operator==
| friend constexpr bool operator==(weak_ordering v, /*unspecified*/ u) noexcept; |
(1) | |
| friend constexpr bool operator==(weak_ordering v, weak_ordering w) noexcept = default; |
(2) | |
Parameters
| v, w | - | std::weak_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 or greater2) true if both parameters hold the same value, false otherwise
operator<
| friend constexpr bool operator<(weak_ordering v, /*unspecified*/ u) noexcept; |
(1) | |
| friend constexpr bool operator<(/*unspecified*/ u, weak_ordering v) noexcept; |
(2) | |
Parameters
| v | - | a std::weak_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 or equivalent2) true if
v is greater, and false if v is less or equivalent
operator<=
| friend constexpr bool operator<=(weak_ordering v, /*unspecified*/ u) noexcept; |
(1) | |
| friend constexpr bool operator<=(/*unspecified*/ u, weak_ordering v) noexcept; |
(2) | |
Parameters
| v | - | a std::weak_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 greater2) true if
v is greater or equivalent, and false if v is less
operator>
| friend constexpr bool operator>(weak_ordering v, /*unspecified*/ u) noexcept; |
(1) | |
| friend constexpr bool operator>(/*unspecified*/ u, weak_ordering v) noexcept; |
(2) | |
Parameters
| v | - | a std::weak_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 or equivalent2) true if
v is less, and false if v is greater or equivalent
operator>=
| friend constexpr bool operator>=(weak_ordering v, /*unspecified*/ u) noexcept; |
(1) | |
| friend constexpr bool operator>=(/*unspecified*/ u, weak_ordering v) noexcept; |
(2) | |
Parameters
| v | - | a std::weak_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 less2) true if
v is less or equivalent, and false if v is greater
operator<=>
| friend constexpr weak_ordering operator<=>(weak_ordering v, /*unspecified*/ u) noexcept; |
(1) | |
| friend constexpr weak_ordering operator<=>(/*unspecified*/ u, weak_ordering v) noexcept; |
(2) | |
Parameters
| v | - | a std::weak_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.
Example
| This section is incomplete Reason: no example |
See also
| (C++20) |
the result type of 3-way comparison that supports all 6 operators and is substitutable (class) |
| (C++20) |
the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class) |
