C++ named requirements: EqualityComparable - cppreference.com (original) (raw)
The type must work with == operator and the result should have standard semantics.
[edit] 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) |
[edit] 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).
[edit] 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, therequirements did not apply to const T objects | they apply toconst T instead of T |
| LWG 2114(P2167R3) | C++98 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened |