C++ named requirements: EqualityComparable

The type must work with == operator and the result should have standard semantics.

Requirements

The type T satisfies EqualityComparable if given expressions a, b and c of type T or (since C++11) const T

Expression	Type	Effects
a == b	meets BooleanTestable (until C++20)	Establishes an equivalence relation, that is, it satisfies the following properties: For all values of a, a == a yields true If a == b, then b == a. If a == b and b == c, then a == c
	models boolean-testable (since C++20)

Notes

To satisfy this requirement, types that do not have built-in comparison operators have to provide a user-defined operator==

For the types that are both EqualityComparable and LessThanComparable

• Equality, which is the value of the expression a == b and
• Equivalence, which is the value of the expression ! (a < b) && ! (b < a)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	even if T is EqualityComparable, the requirements did not apply to const T	they apply to const T instead of T
LWG 2114 (P2167R3)	C++98	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened

See also

equality_comparableequality_comparable_with (C++20)

specifies that operator == is an equivalence relation (concept)