std::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
  • 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

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::partial_ordering

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

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

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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