<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Boost.MultiIndex Documentation - Future work</title> <link rel="stylesheet" href="style.css" type="text/css"> <link rel="start" href="index.html"> <link rel="prev" href="tests.html"> <link rel="up" href="index.html"> <link rel="next" href="release_notes.html"> </head> <body> <h1><img src="../../../boost.png" alt="boost.png (6897 bytes)" align= "middle" width="277" height="86">Boost.MultiIndex Future work</h1> <div class="prev_link"><a href="tests.html"><img src="prev.gif" alt="tests" border="0"><br> Tests </a></div> <div class="up_link"><a href="index.html"><img src="up.gif" alt="index" border="0"><br> Index </a></div> <div class="next_link"><a href="release_notes.html"><img src="next.gif" alt="release notes" border="0"><br> Release notes </a></div><br clear="all" style="clear: all;"> <hr> <p> A number of new functionalities are considered for inclusion into future releases of Boost.MultiIndex. Some of them depend on the potential for extensibility of the library, which has been a guiding principle driving the current internal design of <code>multi_index_container</code>. </p> <h2>Contents</h2> <ul> <li><a href="#ranked_indices">Ranked indices</a></li> <li><a href="#notifying">Notifying indices</a></li> <li><a href="#constraints">Constraints</a></li> <li><a href="#user_defined_indices">User-defined indices</a></li> <li><a href="#indexed_maps">Indexed maps</a></li> <li><a href="#move_semantics">Move semantics</a></li> </ul> <h2><a name="ranked_indices">Ranked indices</a></h2> <p> Ordered indices are implemented using red-black trees; these trees can be augmented with additional information to obtain a type of data structure called <a href="http://pine.cs.yale.edu/pinewiki/OrderStatisticsTree"><i>order-statistics trees</i></a>, allowing for logarithmic search of the <i>n</i>-th element. It has been proposed that order-statistics trees be used to devise a new type of <i>ranked indices</i> that support <code>operator[]</code> while retaining the functionality of ordered indices. </p> <h2><a name="notifying">Notifying indices</a></h2> <p> <i>Notifying indices</i> can be implemented as decorators over preexistent index types, with the added functionality that internal events of the index (insertion, erasing, modifying of elements) are signalled to an external entity --for instance, by means of the <a href="../../../doc/html/signals.html">Boost.Signals</a> library. This functionality can have applications for: <ol> <li>Logging,</li> <li>interfacing to GUI-based applications,</li> <li>synchronization between separate data structures.</li> </ol> </p> <p> The following is a sketch of a possible realization of notifying indices: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>insert_log</span> <span class=special>{</span> <span class=keyword>void</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>)</span> <span class=special>{</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>clog</span><span class=special><<</span><span class=string>"insert: "</span><span class=special><<</span><span class=identifier>x</span><span class=special><<</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>endl</span><span class=special>;</span> <span class=special>}</span> <span class=special>};</span> <span class=keyword>int</span> <span class=identifier>main</span><span class=special>()</span> <span class=special>{</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=keyword>int</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>notifying</span><span class=special><</span><span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=keyword>int</span><span class=special>></span> <span class=special>></span> <span class=special>>,</span> <span class=comment>// notifying index</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=keyword>int</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>indexed_t</span><span class=special>;</span> <span class=identifier>indexed_t</span> <span class=identifier>t</span><span class=special>;</span> <span class=comment>// on_insert is the signal associated to insertions</span> <span class=identifier>t</span><span class=special>.</span><span class=identifier>on_insert</span><span class=special>.</span><span class=identifier>connect</span><span class=special>(</span><span class=identifier>insert_log</span><span class=special>());</span> <span class=identifier>t</span><span class=special>.</span><span class=identifier>insert</span><span class=special>(</span><span class=number>0</span><span class=special>);</span> <span class=identifier>t</span><span class=special>.</span><span class=identifier>insert</span><span class=special>(</span><span class=number>1</span><span class=special>);</span> <span class=keyword>return</span> <span class=number>0</span><span class=special>;</span> <span class=special>}</span> <span class=comment>// output: // insert: 0 // insert: 1</span> </pre></blockquote> <h2><a name="constraints">Constraints</a></h2> <p> The notifying indices functionality described above exploits a powerful design pattern based on <i>index adaptors</i>, decorators over preexistent indices which add some functionality or somehow change the semantics of the underlying index. This pattern can be used for the implementation of <i>constraints</i>, adaptors that restrict the elements accepted by an index according to some validation predicate. The following is a possible realization of how constraints syntax may look like: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>is_even</span> <span class=special>{</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>x</span><span class=special>%</span><span class=number>2</span><span class=special>==</span><span class=number>0</span><span class=special>;}</span> <span class=special>};</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=keyword>int</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>constrained</span><span class=special><</span><span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=keyword>int</span><span class=special>></span> <span class=special>>,</span><span class=identifier>is_even</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>indexed_t</span><span class=special>;</span> </pre></blockquote> <h2><a name="user_defined_indices">User-defined indices</a></h2> <p> The mechanisms by which Boost.MultiIndex orchestrates the operations of the indices held by a <code>multi_index_container</code> are simple enough to make them worth documenting so that the (bold) user can write implementations for her own indices. </p> <h2><a name="indexed_maps">Indexed maps</a></h2> <p> <code>multi_index_container</code> is rich enough to provide the basis for implementation of <i>indexed maps</i>, i.e. maps which can be looked upon several different keys. The motivation for having such a container is mainly aesthetic convenience, since it would not provide any additional feature to similar constructs based directly on <code>multi_index_container</code>. </p> <p> The main challenge in writing an indexed map lies in the design of a reasonable interface that resembles that of <code>std::map</code> as much as possible. There seem to be fundamental difficulties in extending the syntax of a <code>std::map</code> to multiple keys. For one example, consider the situation: </p> <blockquote><pre> <span class=identifier>indexed_map</span><span class=special><</span><span class=keyword>int</span><span class=special>,</span><span class=identifier>string</span><span class=special>,</span><span class=keyword>double</span><span class=special>></span> <span class=identifier>m</span><span class=special>;</span> <span class=comment>// keys are int and string, double is the mapped to value</span> <span class=special>...</span> <span class=identifier>cout</span><span class=special><<</span><span class=identifier>m</span><span class=special>[</span><span class=number>0</span><span class=special>]<<</span><span class=identifier>endl</span><span class=special>;</span> <span class=comment>// OK</span> <span class=identifier>cout</span><span class=special><<</span><span class=identifier>m</span><span class=special>[</span><span class=string>"zero"</span><span class=special>]<<</span><span class=identifier>endl</span><span class=special>;</span> <span class=comment>// OK</span> <span class=identifier>m</span><span class=special>[</span><span class=number>1</span><span class=special>]=</span><span class=number>1.0</span><span class=special>;</span> <span class=comment>// !!</span> </pre></blockquote> <p> In the last sentence of the example, the user has no way of providing the <code>string</code> key mapping to the same value as <code>m[1]</code>. This and similar problems have to be devoted a careful study when designing the interface of a potential indexed map. </p> <h2><a name="move_semantics">Move semantics</a></h2> <p> Andrei Alexandrescu introduced a technique for simulating move constructors called Mojo (see his article in C/C++ User Journal <a href="http://www.cuj.com/documents/s=8246/cujcexp2102alexandr/"> "Generic<Programming>: Move Constructors"</a>.) Move semantics alleviates the computational load involved in the creation and copying of temporary objects, specially for heavy classes as <code>multi_index_container</code>s are. David Abrahams and Gary Powell provide an alternative implementation of move semantics in their paper <a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2004/n1610.html"> "Clarification of Initialization of Class Objects by rvalues"</a> for the C++ Evolution Working Group. </p> <p> Adding move semantics to <code>multi_index_container</code> is particularly beneficial when the container is used as an internal building block in other libraries (vg. relational database frameworks), enabling the efficient development of functions returning <code>multi_index_container</code>s. Without support for move semantics, this scheme is impractical and less elegant syntaxes should be resorted to. </p> <hr> <div class="prev_link"><a href="tests.html"><img src="prev.gif" alt="tests" border="0"><br> Tests </a></div> <div class="up_link"><a href="index.html"><img src="up.gif" alt="index" border="0"><br> Index </a></div> <div class="next_link"><a href="release_notes.html"><img src="next.gif" alt="release notes" border="0"><br> Release notes </a></div><br clear="all" style="clear: all;"> <br> <p>Revised July 5th 2007</p> <p>© Copyright 2003-2007 Joaquín M López Muñoz. 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>) </p> </body> </html>