Difference between revisions of "cpp/utility/compare/partial ordering"
m (templated substituability) |
(note FP as an example) |
||
| Line 137: | Line 137: | ||
{{c|true}} if {{tt|v}} is {{tt|greater}} or {{tt|equivalent}}, and {{c|false}} if {{tt|v}} is {{tt|less}} or {{tt|unordered}} | {{c|true}} if {{tt|v}} is {{tt|greater}} or {{tt|equivalent}}, and {{c|false}} if {{tt|v}} is {{tt|less}} or {{tt|unordered}} | ||
}} | }} | ||
| + | |||
| + | ===Notes=== | ||
| + | The [[cpp/language/operator_comparison#Three-way_comparison|built-in operator <=>]] between floating-point values uses this ordering: the positive zero and the negative zero compare {{tt|equivalent}}, but can be distinguished, and NaN values compare {{tt|unordered}} with any other value. | ||
===Example=== | ===Example=== | ||
Revision as of 11:05, 26 December 2017
| 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 |
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) [static] |
a valid value of the type std::partial_ordering indicating less-than (ordered before) relationship (public static member constant) |
| equivalent(inline constexpr) [static] |
a valid value of the type std::partial_ordering indicating equivalence (neither ordered before nor ordered after) (public static member constant) |
| greater(inline constexpr) [static] |
a valid value of the type std::partial_ordering indicating greater-than (ordered after) relationship (public static member constant) |
| unordered(inline constexpr) [static] |
a valid value of the type std::partial_ordering indicating relationship with an incomparable value (public static member constant) |
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.
| operator weak_equality |
implicit conversion to std::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 zero. 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
| operator==operator!=operator<operator>operator<=operator>= |
compares with zero (function) |
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; 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 less, and false if v is greater, equivalent, 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 less or equivalent, and false if v is 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 greater, and false if v is less, equivalent, 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 greater or equivalent, and false if v is less or unordered
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
| (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 and is not substitutable (class) |
