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  <title>LessThanComparable</title>
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<body bgcolor="#FFFFFF" link="#0000EE" text="#000000" vlink="#551A8B" alink=
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  <img src="../../boost.png" alt="C++ Boost" width="277" height=
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  <h1>LessThanComparable</h1>

  <h3>Description</h3>

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

  <h3>Refinement of</h3>

  <h3>Associated types</h3>

  <h3>Notation</h3>

  <table summary="">
    <tr>
      <td valign="top"><tt>X</tt></td>

      <td valign="top">A type that is a model of LessThanComparable</td>
    </tr>

    <tr>
      <td valign="top"><tt>x</tt>, <tt>y</tt>, <tt>z</tt></td>

      <td valign="top">Object of type <tt>X</tt></td>
    </tr>
  </table>

  <h3>Definitions</h3>

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

  <p>If <tt>operator&lt;</tt> is a strict weak ordering, and if each
  equivalence class has only a single element, then <tt>operator&lt;</tt> is
  a <i>total ordering</i>.</p>

  <h3>Valid expressions</h3>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Type requirements</th>

      <th>Return type</th>
    </tr>

    <tr>
      <td valign="top">Less</td>

      <td valign="top"><tt>x &lt; y</tt></td>

      <td valign="top">&nbsp;</td>

      <td valign="top">Convertible to <tt>bool</tt></td>
    </tr>
  </table>

  <h3>Expression semantics</h3>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Precondition</th>

      <th>Semantics</th>

      <th>Postcondition</th>
    </tr>

    <tr>
      <td valign="top">Less</td>

      <td valign="top"><tt>x &lt; y</tt></td>

      <td valign="top"><tt>x</tt> and <tt>y</tt> are in the domain of
      <tt>&lt;</tt></td>

      <td valign="top">&nbsp;</td>
    </tr>
  </table>

  <h3>Complexity guarantees</h3>

  <h3>Invariants</h3>

  <table border summary="">
    <tr>
      <td valign="top">Irreflexivity</td>

      <td valign="top"><tt>x &lt; x</tt> must be false.</td>
    </tr>

    <tr>
      <td valign="top">Antisymmetry</td>

      <td valign="top"><tt>x &lt; y</tt> implies !(y &lt; x) <a href=
      "#n2">[2]</a></td>
    </tr>

    <tr>
      <td valign="top">Transitivity</td>

      <td valign="top"><tt>x &lt; y</tt> and <tt>y &lt; z</tt> implies <tt>x
      &lt; z</tt> <a href="#n3">[3]</a></td>
    </tr>
  </table>

  <h3>Models</h3>

  <ul>
    <li>int</li>
  </ul>

  <h3>Notes</h3>

  <p><a name="n1" id="n1">[1]</a> Only <tt>operator&lt;</tt> is fundamental;
  the other inequality operators are essentially syntactic sugar.</p>

  <p><a name="n2" id="n2">[2]</a> Antisymmetry is a theorem, not an axiom: it
  follows from irreflexivity and transitivity.</p>

  <p><a name="n3" id="n3">[3]</a> Because of irreflexivity and transitivity,
  <tt>operator&lt;</tt> always satisfies the definition of a <i>partial
  ordering</i>. The definition of a <i>strict weak ordering</i> is stricter,
  and the definition of a <i>total ordering</i> is stricter still.</p>

  <h3>See also</h3>

  <p><a href=
  "http://www.sgi.com/tech/stl/EqualityComparable.html">EqualityComparable</a>,
  <a href=
  "http://www.sgi.com/tech/stl/StrictWeakOrdering.html">StrictWeakOrdering</a><br>
  </p>
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  <p>Revised 
  <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
  December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>

  <table summary="">
    <tr valign="top">
      <td nowrap><i>Copyright &copy; 2000</i></td>

      <td><i><a href="http://www.lsc.nd.edu/~jsiek">Jeremy Siek</a>, Univ.of
      Notre Dame (<a href=
      "mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</a>)</i></td>
    </tr>
  </table>

  <p><i>Distributed under the Boost Software License, Version 1.0. (See
  accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
  copy at <a href=
  "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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