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 , the following expression is valid and has its specified effects:

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 , the C++ standard library makes a distinction between

• 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 objects	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_comparable equality_comparable_with (C++20)

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