std::strong_equality
Defined in header <compare>


class strong_equality; 
(since C++20)  
The class type std::strong_equality
is the result type of a threeway comparison that
 admits only equality and inequality comparisons (no lessthan/greaterthan)
 implies substitutability: if a is equivalent to b, f(a) is also 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 are indistinguishable.
Contents 
[edit] Constants
The type std::strong_equality
has four valid values, implemented as const static data members of its type:
Member constant  Definition 
equivalent(inline constexpr) [static] 
a valid value of the type std::strong_equality indicating equality, equal to equal (public static member constant) 
equal(inline constexpr) [static] 
a valid value of the type std::strong_equality indicating equality, equal to equivalent (public static member constant) 
nonequivalent(inline constexpr) [static] 
a valid value of the type std::strong_equality indicating nonequality, equal to nonequal (public static member constant) 
nonequal(inline constexpr) [static] 
a valid value of the type std::strong_equality indicating nonequality, equal to nonequivalent (public static member constant) 
[edit] Conversions
std::strong_equality
is implicitlyconvertible to std::weak_equality, and std::strong_ordering is implicitlyconvertible to strong_equality.
operator weak_equality 
implicit conversion to std::weak_equality (public member function) 
std::strong_equality::operator weak_equality
constexpr operator weak_equality() const noexcept; 

Return value
std::weak_equality::equivalent if v
is equivalent
or equal
, std::weak_equality::nonequivalent if v
is nonequivalent
or nonequal
.
[edit] 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 strong_equality
with anything other than the integer literal 0 is undefined.
operator==operator<=> 
compares with zero or a strong_equality (function) 
operator==
friend constexpr bool operator==(strong_equality v, /*unspecified*/ u) noexcept; 
(1)  
friend constexpr bool operator==(strong_equality v, strong_equality w) noexcept = default; 
(2)  
Parameters
v, w    std::strong_equality values to check

u    an unused parameter of any type that accepts literal zero argument 
Return value
v
is equivalent
or equal
, false if v
is nonequivalent
or nonequal
equal
is equal to equivalent
and nonequal
is equal to nonequivalent
, but equal
is not equal to nonequal
.
operator<=>
friend constexpr strong_equality operator<=>(strong_equality v, /*unspecified*/ u) noexcept; friend constexpr strong_equality operator<=>(/*unspecified*/ u, strong_equality v) noexcept; 

Parameters
v    a std::strong_equality value to check

u    an unused parameter of any type that accepts literal zero argument 
Return value
v
[edit] Example
This section is incomplete Reason: no example 
[edit] 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 not substitutable (class) 