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 const T (since C++11)

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)

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

• 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

See also

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)