std::strong_equality

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Defined in header <compare>
class strong_equality;		(since C++20)

The class type std::strong_equality is the result type of a three-way comparison that

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

• implies substitutability: if a is equivalent to b, f(a) is also 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 are indistinguishable.

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 (public static member constant)
equal(inline constexpr) [static]	a valid value of the type std::strong_equality indicating equality (public static member constant)
nonequivalent(inline constexpr) [static]	a valid value of the type std::strong_equality indicating non-equality (public static member constant)
nonequal(inline constexpr) [static]	a valid value of the type std::strong_equality indicating non-equality (public static member constant)

Conversions

std::strong_equality is implicitly-convertible to std::weak_equality, and std::strong_ordering is implicitly-convertible 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.

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 strong_equality with anything other than the integer literal 0 is undefined.

operator==operator!=operator<=>

compares with zero (function)

operator==

friend constexpr bool operator==(strong_equality v, /*unspecified*/ u) noexcept; friend constexpr bool 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

true if v is equivalent or equal, false if v is nonequivalent or nonequal

operator!=

friend constexpr bool operator!=(strong_equality v, /*unspecified*/ u) noexcept; friend constexpr bool 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

false if v is equivalent or equal, and true if v is nonequivalent or 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

See also

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