C++ named requirements: LessThanComparable - cppreference.com (original) (raw)
The type must work with **<** operator and the result should have standard semantics.
[edit] Requirements
The type T satisfies LessThanComparable if given expressions a, b and c of type T or const T(since C++11), the following expression is valid and has its specified effects:
| Expression | Type | Effects |
|---|---|---|
| a < b | meets BooleanTestable(until C++20) | Establishes strict weak ordering relation with the following properties: For all a, !(a < a) yields true. If a < b, then !(b < a). If a < b and b < c, then a < c. Defining equiv(a, b) as !(a < b) && !(b < a), if equiv(a, b) and equiv(b, c), then equiv(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 2114(P2167R3) | C++98 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened |