std:: partial_ordering

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
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:

Name Definition
inline constexpr std :: partial_ordering less
[static]
a valid value indicating less-than (ordered before) relationship
(public static member constant)
inline constexpr std :: partial_ordering equivalent
[static]
a valid value indicating equivalence (neither ordered before nor ordered after)
(public static member constant)
inline constexpr std :: partial_ordering greater
[static]
a valid value indicating greater-than (ordered after) relationship
(public static member constant)
inline constexpr std :: partial_ordering 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 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<=>
compares with zero or a 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)

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<

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

(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)
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