From cppreference.com
classpartial_ordering;(since C++20)The class type std::partial_ordering is the result type of a
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.
: 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:
Name Definition less
[static]
a valid value indicating less-than (ordered before) relationship
(public static member constant) equivalent
[static]
a valid value indicating equivalence (neither ordered before nor ordered after)
(public static member constant) greater
[static]
a valid value indicating greater-than (ordered after) relationship
(public static member constant) unordered
[static]
a valid value indicating relationship with an incomparable value
(public static member constant)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.
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
,
, etc.
These functions are not visible to ordinary
or
, and can only be found by
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==
friendconstexprbooloperator==(partial_orderingv,/*unspecified*/u)noexcept; (1) friendconstexprbooloperator==(partial_orderingv,partial_orderingw)noexcept=default; (2) 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 unordered
2)true if both parameters hold the same value, false otherwise
operator<
friendconstexprbooloperator<(partial_orderingv,/*unspecified*/u)noexcept; (1) friendconstexprbooloperator<(/*unspecified*/u,partial_orderingv)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<=
friendconstexprbooloperator<=(partial_orderingv,/*unspecified*/u)noexcept; (1) friendconstexprbooloperator<=(/*unspecified*/u,partial_orderingv)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>
friendconstexprbooloperator>(partial_orderingv,/*unspecified*/u)noexcept; (1) friendconstexprbooloperator>(/*unspecified*/u,partial_orderingv)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>=
friendconstexprbooloperator>=(partial_orderingv,/*unspecified*/u)noexcept; (1) friendconstexprbooloperator>=(/*unspecified*/u,partial_orderingv)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<=>
friendconstexprpartial_orderingoperator<=>(partial_orderingv,/*unspecified*/u)noexcept; (1) friendconstexprpartial_orderingoperator<=>(/*unspecified*/u,partial_orderingv)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
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
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)