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

• a, b and c, expressions of type T or const T (since C++17)

The following expressions must be valid and have their specified effects:

Expression	Return type	Requirements
a == b	implicitly convertible to bool	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

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