NAME¶

std::partial_ordering - std::partial_ordering

Synopsis¶

Defined in header <compare>
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

Constants¶

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

Member constant Definition
less(inline constexpr) a valid value of the type std::partial_ordering
[static] indicating less-than (ordered before) relationship
(public static member constant)
a valid value of the type std::partial_ordering
equivalent(inline constexpr) indicating equivalence (neither ordered before nor
[static] ordered after)
(public static member constant)
greater(inline constexpr) a valid value of the type std::partial_ordering
[static] indicating greater-than (ordered after) relationship
(public static member constant)
unordered(inline constexpr) a valid value of the type std::partial_ordering
[static] indicating relationship with an incomparable value
(public static member constant)

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.

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==
operator<
operator> compares with zero or a partial_ordering
operator<= (function)
operator>=
operator<=>

operator==

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

Parameters¶

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

operator<

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

Parameters¶

v - a std::partial_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, 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 ) (1)
noexcept;
friend constexpr bool operator<=( /*unspecified*/ u, partial_ordering v ) (2)
noexcept;

Parameters¶

v - a std::partial_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 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 ) (1)
noexcept;
friend constexpr bool operator>( /*unspecified*/ u, partial_ordering v ) (2)
noexcept;

Parameters¶

v - a std::partial_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, 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 ) (1)
noexcept;
friend constexpr bool operator>=( /*unspecified*/ u, partial_ordering v ) (2)
noexcept;

Parameters¶

v - a std::partial_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 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, (1)
/*unspecified*/ u ) noexcept;
friend constexpr partial_ordering operator<=>( /*unspecified*/ u, (2)
partial_ordering v ) noexcept;

Parameters¶

v - a std::partial_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.

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.

Example¶

This section is incomplete
Reason: no example

See also¶

strong_ordering the result type of 3-way comparison that supports all 6 operators
(C++20) and is substitutable
(class)
weak_ordering the result type of 3-way comparison that supports all 6 operators
(C++20) and is not substitutable
(class)