NAME¶

std::equality_comparable,std::equality_comparable_with - std::equality_comparable,std::equality_comparable_with

Synopsis¶

Defined in header <concepts>
template< class T > (since
concept equality_comparable = (1) C++20)
__WeaklyEqualityComparableWith<T, T>;
template< class T, class U >

concept equality_comparable_with =
std::equality_comparable<T> &&
std::equality_comparable<U> &&
__ComparisonCommonTypeWith<T, U> && (2) (since
std::equality_comparable< C++20)
std::common_reference_t<
const std::remove_reference_t<T>&,
const std::remove_reference_t<U>&>> &&

__WeaklyEqualityComparableWith<T, U>;
template< class T, class U >

concept __WeaklyEqualityComparableWith =
requires(const std::remove_reference_t<T>& t,
const std::remove_reference_t<U>& u) { (exposition
{ t == u } -> boolean-testable; (3) only*)
{ t != u } -> boolean-testable;
{ u == t } -> boolean-testable;
{ u != t } -> boolean-testable;

};
template< class T, class U >
(until
concept __ComparisonCommonTypeWith = C++23)
std::common_reference_with< (exposition
const std::remove_reference_t<T>&, only*)

const std::remove_reference_t<U>&>;
template< class T, class U, class C =
std::common_reference_t<const T&, const U&> >

concept _ComparisonCommonTypeWithImpl =
std::same_as<std::common_reference_t<const T&, const
U&>,
std::common_reference_t<const U&, const
T&>> && (4)
requires {
requires std::convertible_to<const T&, const C&> (since
|| C++23)
std::convertible_to<T, const C&>; (exposition
requires std::convertible_to<const U&, const C&> only*)
||
std::convertible_to<U, const C&>;
};
template< class T, class U >
concept __ComparisonCommonTypeWith =

_ComparisonCommonTypeWithImpl<std::remove_cvref_t<T>,
std::remove_cvref_t<U>>;

1) The concept std::equality_comparable specifies that the comparison operators ==
and != on T reflects equality: == yields true if and only if the operands are equal.
2) The concept std::equality_comparable_with specifies that the comparison operators
== and != on (possibly mixed) T and U operands yield results consistent with
equality. Comparing mixed operands yields results equivalent to comparing the
operands converted to their common type.
3) The exposition-only concept __WeaklyEqualityComparableWith specifies that an
object of type T and an object of type U can be compared for equality with each
other (in either order) using both == and !=, and the results of the comparisons are
consistent.
4) The exposition-only concept __ComparisonCommonTypeWith specifies that two types
share a common type, and a const lvalue
or a non-const rvalue
(since C++23) of either type is convertible to that common type.

Semantic requirements

These concepts are modeled only if they are satisfied and all concepts they subsume
are modeled.

In the following paragraphs, given an expression E and a type C, CONVERT_TO<C>(E) is
defined as:

* static_cast<C>(std::as_const(E)). (until C++23)
* static_cast<const C&>(std::as_const(E)) if that is a valid
expression, (since C++23)
* static_cast<const C&>(std::move(E)) otherwise.

1) std::equality_comparable<T> is modeled only if, given objects a and b of type T,
bool(a == b) is true if and only if a and b are equal. Together with the requirement
that a == b is equality-preserving, this implies that == is symmetric and
transitive, and further that == is reflexive for all objects a that are equal to at
least one other object.
2) std::equality_comparable_with<T, U> is modeled only if, let
* t and t2 be lvalues denoting distinct equal objects of types const
std::remove_reference_t<T> and std::remove_cvref_t<T> respectively,
* u and u2 be lvalues denoting distinct equal objects of types const
std::remove_reference_t<U> and std::remove_cvref_t<U> respectively,
* C be std::common_reference_t<const std::remove_reference_t<T>&, const
std::remove_reference_t<U>&>,

the following expression is true:

* bool(t == u) == bool(CONVERT_TO<C>(t2) == CONVERT_TO<C>(u2)).
3) __WeaklyEqualityComparableWith<T, U> is modeled only if given
* t, an lvalue of type const std::remove_reference_t<T> and
* u, an lvalue of type const std::remove_reference_t<U>,

the following are true:

* t == u, u == t, t != u, u != t have the same domain;
* bool(u == t) == bool(t == u);
* bool(t != u) == !bool(t == u); and
* bool(u != t) == bool(t != u).
4) __WeaklyEqualityComparableWith<T, U> is modeled only if:

The corresponding common_reference_with concept is modeled. (until C++23)
Let

* C be std::common_reference_t<const T&, const U&>,
* t1 and t2 be equality-preserving expressions that are lvalues of
type std::remove_cvref_t<T>,
* u1 and u2 be equality-preserving expressions that are lvalues of
type std::remove_cvref_t<U>, (since C++23)

the following conditions hold:

* CONVERT_TO<C>(t1) equals CONVERT_TO<C>(t2) if and only if t1
equals t2; and
* CONVERT_TO<C>(u1) equals CONVERT_TO<C>(u2) if and only if u1
equals u2.

Equality preservation

Expressions declared in requires expressions of the standard library concepts are
required to be equality-preserving (except where stated otherwise).

Implicit expression variations

A requires expression that uses an expression that is non-modifying for some
constant lvalue operand also requires implicit expression variations.