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std::weak_ordering

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

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

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