C++ named requirements: LessThanComparable

The type must work with < operator and the result should have standard semantics.

Requirements

The type T satisfies LessThanComparable if given expressions a, b and c of type T or constT(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)`.
`a < b`	models `boolean-testable` (since C++20)

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 2114 (P2167R3)	C++98	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened

Compare	a BinaryPredicate that establishes an ordering relation (named requirement)
strict_weak_order (C++20)	specifies that a `relation` imposes a strict weak ordering (concept)

C++ named requirements: LessThanComparable

Requirements

Notes

Defect reports

See also