NAME¶

std::weak_ordering - std::weak_ordering

Synopsis¶

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) a valid value of the type std::weak_ordering indicating
[static] less-than (ordered before) relationship
(public static member constant)
equivalent(inline constexpr) a valid value of the type std::weak_ordering indicating
[static] equivalence (neither ordered before nor ordered after)
(public static member constant)
greater(inline constexpr) a valid value of the type std::weak_ordering indicating
[static] 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> compares with zero or a weak_ordering
operator<= (function)
operator>=
operator<=>

operator==

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

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) (1)
noexcept;
friend constexpr weak_ordering operator<=>(/*unspecified*/ u, weak_ordering v) (2)
noexcept;

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¶

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)
partial_ordering the result type of 3-way comparison that supports all 6 operators,
(C++20) is not substitutable, and allows incomparable values
(class)