std::equality_comparable, std::equality_comparable_with
Defined in header <concepts>


template< class T > concept equality_comparable = __WeaklyEqualityComparableWith<T, T>; 
(1)  (since C++20) 
template< class T, class U > concept equality_comparable_with = 
(2)  (since C++20) 
template< class T, class U > concept __WeaklyEqualityComparableWith = 
(3)  (exposition only*) 
(4)  
template< class T, class U > concept __ComparisonCommonTypeWith = 
(until C++23) (exposition only*) 

template< class T, class U, class C = std::common_reference_t<const T&, const U&> > 
(since C++23) (exposition only*) 

std::equality_comparable
specifies that the comparison operators ==
and !=
on T
reflects equality: ==
yields true if and only if the operands are equal.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.__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. __ComparisonCommonTypeWith
specifies that two types share a common type, and a const lvalue or a nonconst rvalue (since C++23) of either type is convertible to that common type.[edit] 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:

(until C++23) 

(since C++23) 
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 equalitypreserving, 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.
t
andt2
be lvalues denoting distinct equal objects of types const std::remove_reference_t<T> and std::remove_cvref_t<T> respectively 
u
andu2
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)).

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).
The corresponding 
(until C++23) 
Let
the following conditions hold:

(since C++23) 
[edit] Equality preservation
Expressions declared in requiresexpressions of the standard library concepts are required to be equalitypreserving (except where stated otherwise).
[edit] Implicit expression variations
A requiresexpression that uses an expression that is nonmodifying for some constant lvalue operand also implicitly requires additional variations of that expression that accept a nonconstant lvalue or (possibly constant) rvalue for the given operand unless such an expression variation is explicitly required with differing semantics. These implicit expression variations must meet the same semantic requirements of the declared expression. The extent to which an implementation validates the syntax of the variations is unspecified.