std::weak_equality
Defined in header <compare>


class weak_equality; 
(since C++20)  
The class type std::weak_equality
is the result type of a threeway comparison that
 admits only equality and inequality comparisons (no lessthan/greaterthan)
 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.
std::weak_equality
is the weakest comparison category type: it is not implicitlyconvertible to any other comparison category, but the other four comparison categories (std::strong_equality, std::partial_ordering, std::weak_ordering, std::strong_ordering) are implicitlyconvertible to std::weak_equality
.
Constants
The type std::weak_equality
has only two valid values, implemented as const static data members of its type: std::weak_equality::equivalent and std::weak_equality::nonequivalent:
Member constant  Definition 
equivalent(inline constexpr) [static] 
a valid value of the type std::weak_equality indicating equivalence (public static member constant) 
nonequivalent(inline constexpr) [static] 
a valid value of the type std::weak_equality indicating nonequivalence (public static member constant) 
Comparisons
Comparison operators are defined between values of this type and literal 0. This supports the expressions a <=> b == 0 and a <=> b != 0 used to convert the result of a threeway comparison operator to a boolean relationship; see std::is_eq and std::is_neq.
The behavior of a program that attempts to compare a weak_equality
with anything other than the integer literal 0 is undefined.
operator==operator!=operator<=> 
compares with zero (function) 
operator==
friend constexpr bool operator==(weak_equality v, /*unspecified*/ u) noexcept; friend constexpr bool operator==(/*unspecified*/ u, weak_equality v) noexcept; 

Parameters
v    a std::weak_equality 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 nonequivalent
operator!=
friend constexpr bool operator!=(weak_equality v, /*unspecified*/ u) noexcept; friend constexpr bool operator!=(/*unspecified*/ u, weak_equality v) noexcept; 

Parameters
v    a std::weak_equality value to check

u    an unused parameter of any type that accepts literal zero argument 
Return value
false if v
is equivalent
, and true if v
is nonequivalent
operator<=>
friend constexpr weak_equality operator<=>(weak_equality v, /*unspecified*/ u) noexcept; friend constexpr weak_equality operator<=>(/*unspecified*/ u, weak_equality v) noexcept; 

Parameters
v    a std::weak_equality value to check

u    an unused parameter of any type that accepts literal zero argument 
Return value
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 and is not 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) 