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
andc
, expressions of typeT
orconst T
(od 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 |