# 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 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:

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` 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

strong_ordering (C++20) | the result type of 3-way comparison that supports all 6 operators and is substitutable (class)
weak_ordering (C++20) | the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)

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