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 threeway 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 comparisonsalient state that is accessible via the argument's public const members. In other words, equivalent values may be distinguishable.
 does not allow incomparable values: at least one of a < b, a == b, and 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 lessthan (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 greaterthan (ordered after) relationship (public static member constant) 
Conversions
std::weak_ordering
is implicitlyconvertible to std::weak_equality and std::partial_ordering, while std::strong_ordering is implicitlyconvertible to weak_ordering.
operator weak_equality 
implicit conversion to std::weak_equality (public member function) 
std::weak_ordering::operator weak_equality
constexpr operator weak_equality() const noexcept; 

Return value
std::weak_equality::equivalent if v
is equivalent
, std::weak_equality::nonequivalent if v
is less
or greater
.
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 threeway comparison operator to a boolean relationship; see std::is_eq, std::is_lt, etc.
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>=operator<=> 
compares with zero (function) 
operator==
friend constexpr bool operator==(weak_ordering v, /*unspecified*/ u) noexcept; friend constexpr bool operator==(/*unspecified*/ u, weak_ordering v) noexcept; 

Parameters
v    a std::weak_ordering value to check

u    an unused parameter of any type that accepts literal zero argument 
Return value
true if v
is equivalent
, false if v
is less
or greater
operator!=
friend constexpr bool operator!=(weak_ordering v, /*unspecified*/ u) noexcept; friend constexpr bool operator!=(/*unspecified*/ u, weak_ordering v) noexcept; 

Parameters
v    a std::weak_ordering value to check

u    an unused parameter of any type that accepts literal zero argument 
Return value
true if v
is less
or greater
, and false if v
is 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
v
is less
, and false if v
is greater
or equivalent
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
v
is less
or equivalent
, and false if v
is greater
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
v
is greater
, and false if v
is less
or equivalent
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
v
is greater
or equivalent
, and false if v
is less
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
greater
if v
is less
, less
if v
is greater
, otherwise v
.
Example
This section is incomplete Reason: no example 
See also
(C++20) 
the result type of 3way comparison that supports all 6 operators and is substitutable (class) 
(C++20) 
the result type of 3way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class) 
(C++20) 
the result type of 3way comparison that supports only equality/inequality and is substitutable (class) 
(C++20) 
the result type of 3way comparison that supports only equality/inequality and is not substitutable (class) 