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

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) |

`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) |

constexpr operator partial_ordering() const noexcept; |

`std::partial_ordering::less`

if `v`

is `less`

, `std::partial_ordering::greater`

if `v`

is `greater`

, `std::partial_ordering::equivalent`

if `v`

is `equivalent`

.

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) |

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

friend constexpr bool operator==(weak_ordering v, weak_ordering w) noexcept = default; | (2) |

v, w | - | `std::weak_ordering` values to check |

u | - | an unused parameter of any type that accepts literal zero argument |

1)

`true`

if `v`

is `equivalent`

, `false`

if `v`

is `less`

or `greater`

2)

`true`

if both parameters hold the same value, `false`

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

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

v | - | a `std::weak_ordering ` value to check |

u | - | an unused parameter of any type that accepts literal zero argument |

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`

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

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

v | - | a `std::weak_ordering ` value to check |

u | - | an unused parameter of any type that accepts literal zero argument |

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`

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

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

v | - | a `std::weak_ordering ` value to check |

u | - | an unused parameter of any type that accepts literal zero argument |

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`

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

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

v | - | a `std::weak_ordering` value to check |

u | - | an unused parameter of any type that accepts literal zero argument |

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`

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

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

v | - | a `std::weak_ordering` value to check |

u | - | an unused parameter of any type that accepts literal zero argument |

1)

`v`

.
2)

`greater`

if `v`

is `less`

, `less`

if `v`

is `greater`

, otherwise `v`

.
(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, is not substitutable, and allows incomparable values (class) |

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