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) , 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)

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

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)