C++ named requirements: LessThanComparable

 

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C++ named requirements

Basic
DefaultConstructible CopyAssignable	CopyConstructible MoveAssignable (C++11)	MoveConstructible (C++11) Destructible
Type properties
ScalarType TriviallyCopyable (C++11)	StandardLayoutType (C++11) PODType	TrivialType (C++11) ImplicitLifetimeType
Library-Wide
EqualityComparable LessThanComparable Swappable ValueSwappable (C++11)	NullablePointer (C++11) Hash (C++11) Allocator FunctionObject	Callable Predicate BinaryPredicate Compare
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Container SequenceContainer ContiguousContainer (C++17)	ReversibleContainer AssociativeContainer AllocatorAwareContainer	UnorderedAssociativeContainer (C++11)
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RangeAdaptorObject (C++20)	RangeAdaptorClosureObject (C++20)
Other
CharTraits RegexTraits (C++11) BitmaskType LiteralType (C++11)	NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11)	Clock (C++11) TrivialClock (C++11)

 

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	implicitly convertible to bool  (until C++23)	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++23)

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).

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)