C++ named requirements: LessThanComparable

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


The type T satisfies LessThanComparable if.


  • 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)


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)
specifies that a relation imposes a strict weak ordering

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