# std::weak_ordering

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Defined in header `<compare>` |  |
class weak_ordering; |  | (since C++20)

The class type `std::weak_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.
- does not allow incomparable values: exactly one of a < b, a == b, or a > b must be true.

### Constants

The type `std::weak_ordering` has three 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::weak_ordering` indicating less-than (ordered before) relationship (public static member constant)
equivalent(inline constexpr) [static] | a valid value of the type `std::weak_ordering` indicating equivalence (neither ordered before nor ordered after) (public static member constant)
greater(inline constexpr) [static] | a valid value of the type `std::weak_ordering` indicating greater-than (ordered after) relationship (public static member constant)

### Conversions

`std::weak_ordering` is implicitly-convertible to std::partial_ordering, while std::strong_ordering is implicitly-convertible to weak_ordering.

operator partial_ordering | implicit conversion to std::partial_ordering (public member function)

## std::weak_ordering::operator partial_ordering

constexpr operator partial_ordering() const noexcept; |  |

### Return value

std::partial_ordering::less if `v` is `less`, std::partial_ordering::greater if `v` is `greater`, std::partial_ordering::equivalent if `v` is `equivalent`.

### 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::weak_ordering` is an associated class of the arguments.

The behavior of a program that attempts to compare a `weak_ordering` with anything other than the integer literal ​0​ is undefined.

operator==operator<operator>operator<=operator>=operator<=> | compares with zero or a `weak_ordering` (function)

## operator==

friend constexpr bool operator==( weak_ordering v, /*unspecified*/ u ) noexcept; | (1) |
friend constexpr bool operator==( weak_ordering v, weak_ordering w ) noexcept = default; | (2) |

### Parameters

v, w | - | `std::weak_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` or `greater`
2) true if both parameters hold the same value, false otherwise

## operator<

friend constexpr bool operator<( weak_ordering v, /*unspecified*/ u ) noexcept; | (1) |
friend constexpr bool operator<( /*unspecified*/ u, weak_ordering v ) noexcept; | (2) |

### Parameters

v | - | a `std::weak_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` or `equivalent`
2) true if `v` is `greater`, and false if `v` is `less` or `equivalent`

## operator<=

friend constexpr bool operator<=( weak_ordering v, /*unspecified*/ u ) noexcept; | (1) |
friend constexpr bool operator<=( /*unspecified*/ u, weak_ordering v ) noexcept; | (2) |

### Parameters

v | - | a `std::weak_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`
2) true if `v` is `greater` or `equivalent`, and false if `v` is `less`

## operator>

friend constexpr bool operator>( weak_ordering v, /*unspecified*/ u ) noexcept; | (1) |
friend constexpr bool operator>( /*unspecified*/ u, weak_ordering v ) noexcept; | (2) |

### Parameters

v | - | a `std::weak_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` or `equivalent`
2) true if `v` is `less`, and false if `v` is `greater` or `equivalent`

## operator>=

friend constexpr bool operator>=( weak_ordering v, /*unspecified*/ u ) noexcept; | (1) |
friend constexpr bool operator>=( /*unspecified*/ u, weak_ordering v ) noexcept; | (2) |

### Parameters

v | - | a `std::weak_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`
2) true if `v` is `less` or `equivalent`, and false if `v` is `greater`

## operator<=>

friend constexpr weak_ordering operator<=>( weak_ordering v, /*unspecified*/ u ) noexcept; | (1) |
friend constexpr weak_ordering operator<=>( /*unspecified*/ u, weak_ordering v ) noexcept; | (2) |

### Parameters

v | - | a `std::weak_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`.

### 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)
partial_ordering (C++20) | the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)

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