std::weak_ordering - cppreference.com (original) (raw)

| | | | | --------------------- | | ------------- | | 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.

Contents

• 1 Constants
• 2 Conversions
• 3 std::weak_ordering::operator partial_ordering
  • 3.1 Return value
  • 3.2 Comparisons
• 4 operator==
  • 4.1 Parameters
  • 4.2 Return value
• 5 operator<
  • 5.1 Parameters
  • 5.2 Return value
• 6 operator<=
  • 6.1 Parameters
  • 6.2 Return value
• 7 operator>
  • 7.1 Parameters
  • 7.2 Return value
• 8 operator>=
  • 8.1 Parameters
  • 8.2 Return value
• 9 operator<=>
  • 9.1 Parameters
  • 9.2 Return value
  • 9.3 Example
  • 9.4 See also

[edit] Constants

The type std::weak_ordering has three valid values, implemented as const static data members of its type:

[edit] Conversions

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

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.

[edit] 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==

| 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

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

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

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

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

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

Return value

1. v.

2. greater if v is less, less if v is greater, otherwise v.

[edit] Example

[edit] See also

| | the result type of 3-way comparison that supports all 6 operators and is substitutable (class) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class) [edit] |