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LessThanComparable

Description

A type is LessThanComparable if it is ordered: it must be possible to compare two objects of that type using operator<, and operator< must be a strict weak ordering relation.

Refinement of

Associated types

Notation

X A type that is a model of LessThanComparable
x, y, z Object of type X

Definitions

Consider the relation !(x < y) && !(y < x). If this relation is transitive (that is, if !(x < y) && !(y < x) && !(y < z) && !(z < y) implies !(x < z) && !(z < x)), then it satisfies the mathematical definition of an equivalence relation. In this case, operator< is a strict weak ordering.

If operator< is a strict weak ordering, and if each equivalence class has only a single element, then operator< is a total ordering.

Valid expressions

Name Expression Type requirements Return type
Less x < y   Convertible to bool

Expression semantics

Name Expression Precondition Semantics Postcondition
Less x < y x and y are in the domain of <  

Complexity guarantees

Invariants

Irreflexivity x < x must be false.
Antisymmetry x < y implies !(y < x) [2]
Transitivity x < y and y < z implies x < z [3]

Models

Notes

[1] Only operator< is fundamental; the other inequality operators are essentially syntactic sugar.

[2] Antisymmetry is a theorem, not an axiom: it follows from irreflexivity and transitivity.

[3] Because of irreflexivity and transitivity, operator< always satisfies the definition of a partial ordering. The definition of a strict weak ordering is stricter, and the definition of a total ordering is stricter still.

See also

EqualityComparable, StrictWeakOrdering


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Revised 05 December, 2006

Copyright © 2000 Jeremy Siek, Univ.of Notre Dame (jsiek@lsc.nd.edu)

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)