C++ named requirements: LessThanComparable

< cpp‎ | named req

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

Requirements

The type T satisfies LessThanComparable if

Given

  • a, b, and c, expressions of type T or const T.

The following expressions must be valid and have their specified effects.

Expression Return type Requirements
a < b implicitly convertible to bool Establishes strict weak ordering relation with the following properties:
  • For all a, !(a < a).
  • 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).

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

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