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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 three-way comparison that.

  • admits only equality and inequality comparisons (no less-than/greater-than)
  • 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.

std::weak_equality is the weakest comparison category type: it is not implicitly-convertible to any other comparison category, but the other four comparison categories (std::strong_equality, std::partial_ordering, std::weak_ordering, std::strong_ordering) are implicitly-convertible 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 equivalents
(public static member constant)
nonequivalent(inline constexpr)
[static]
 a valid value of the type std::weak_equality indicating non-equivalence
(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 three-way 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

See also

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

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