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

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

Contents

• 1 Constants
• 2 Conversions
• 3 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 Notes
  • 9.4 Example
  • 9.5 See also

[edit] Constants

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

[edit] 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.

[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::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==

| 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

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

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

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

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

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

Return value

1. v.

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

[edit] 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.

[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 and is not substitutable (class) [edit] |