C++ named requirements: LessThanComparable
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C++ named requirements
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
LessThanComparable
(C++11)
(C++11)
(C++11)
(C++17)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++17)
(C++20)
(C++20)
(C++20)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++14)
(C++11)
(C++11)
(C++17)
(C++14)
(C++20)
(C++20)
(C++23)
(C++23)
(C++23)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
The type must work with < operator and the result should have standard semantics.
Contents
[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:
|
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 |