1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN"> 2 3<html> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 6<title>Boost.MultiIndex Documentation - Future work</title> 7<link rel="stylesheet" href="style.css" type="text/css"> 8<link rel="start" href="index.html"> 9<link rel="prev" href="tests.html"> 10<link rel="up" href="index.html"> 11<link rel="next" href="release_notes.html"> 12</head> 13 14<body> 15<h1><img src="../../../boost.png" alt="boost.png (6897 bytes)" align= 16"middle" width="277" height="86">Boost.MultiIndex Future work</h1> 17 18<div class="prev_link"><a href="tests.html"><img src="prev.gif" alt="tests" border="0"><br> 19Tests 20</a></div> 21<div class="up_link"><a href="index.html"><img src="up.gif" alt="index" border="0"><br> 22Index 23</a></div> 24<div class="next_link"><a href="release_notes.html"><img src="next.gif" alt="release notes" border="0"><br> 25Release notes 26</a></div><br clear="all" style="clear: all;"> 27 28<hr> 29 30<p> 31A number of new functionalities are considered for inclusion into 32future releases of Boost.MultiIndex. Some of them depend on the 33potential for extensibility of the library, which has been a guiding 34principle driving the current internal design of <code>multi_index_container</code>. 35</p> 36 37<h2>Contents</h2> 38 39<ul> 40 <li><a href="#notifying">Notifying indices</a></li> 41 <li><a href="#constraints">Constraints</a></li> 42 <li><a href="#user_defined_indices">User-defined indices</a></li> 43 <li><a href="#indexed_maps">Indexed maps</a></li> 44</ul> 45 46<h2><a name="notifying">Notifying indices</a></h2> 47 48<p> 49<i>Notifying indices</i> can be implemented as decorators over 50preexistent index types, with the added functionality that internal 51events of the index (insertion, erasing, modifying of elements) are 52signalled to an external entity --for instance, by means of the 53<a href="../../../doc/html/signals.html">Boost.Signals</a> 54library. This functionality can have applications for: 55<ol> 56 <li>Logging,</li> 57 <li>interfacing to GUI-based applications,</li> 58 <li>synchronization between separate data structures.</li> 59</ol> 60</p> 61 62<p> 63The following is a sketch of a possible realization of notifying 64indices: 65</p> 66 67<blockquote><pre> 68<span class=keyword>struct</span> <span class=identifier>insert_log</span> 69<span class=special>{</span> 70 <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> 71 <span class=special>{</span> 72 <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> 73 <span class=special>}</span> 74<span class=special>};</span> 75 76<span class=keyword>int</span> <span class=identifier>main</span><span class=special>()</span> 77<span class=special>{</span> 78 <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> 79 <span class=keyword>int</span><span class=special>,</span> 80 <span class=identifier>indexed_by</span><span class=special><</span> 81 <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> 82 <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> 83 <span class=special>></span> 84 <span class=special>></span> <span class=identifier>indexed_t</span><span class=special>;</span> 85 86 <span class=identifier>indexed_t</span> <span class=identifier>t</span><span class=special>;</span> 87 88 <span class=comment>// on_insert is the signal associated to insertions</span> 89 <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> 90 91 <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> 92 <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> 93 94 <span class=keyword>return</span> <span class=number>0</span><span class=special>;</span> 95<span class=special>}</span> 96 97<span class=comment>// output: 98// insert: 0 99// insert: 1</span> 100</pre></blockquote> 101 102<h2><a name="constraints">Constraints</a></h2> 103 104<p> 105The notifying indices functionality described above exploits a powerful 106design pattern based on <i>index adaptors</i>, decorators over preexistent 107indices which add some functionality or somehow change the semantics of 108the underlying index. This pattern can be used for the implementation 109of <i>constraints</i>, adaptors that restrict the elements accepted by an 110index according to some validation predicate. The following is a possible 111realization of how constraints syntax may look like: 112</p> 113 114<blockquote><pre> 115<span class=keyword>struct</span> <span class=identifier>is_even</span> 116<span class=special>{</span> 117 <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> 118<span class=special>};</span> 119 120<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> 121 <span class=keyword>int</span><span class=special>,</span> 122 <span class=identifier>indexed_by</span><span class=special><</span> 123 <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> 124 <span class=special>></span> 125<span class=special>></span> <span class=identifier>indexed_t</span><span class=special>;</span> 126</pre></blockquote> 127 128<h2><a name="user_defined_indices">User-defined indices</a></h2> 129 130<p> 131The mechanisms by which Boost.MultiIndex orchestrates the 132operations of the indices held by a <code>multi_index_container</code> are 133simple enough to make them worth documenting so that the (bold) 134user can write implementations for her own indices. 135</p> 136 137<h2><a name="indexed_maps">Indexed maps</a></h2> 138 139<p> 140<code>multi_index_container</code> is rich enough to provide the basis 141for implementation of <i>indexed maps</i>, i.e. maps which 142can be looked upon several different keys. The motivation for having 143such a container is mainly aesthetic convenience, since it 144would not provide any additional feature to similar constructs 145based directly on <code>multi_index_container</code>. 146</p> 147 148<p> 149The main challenge in writing an indexed map lies in the design of a 150reasonable interface that resembles that of <code>std::map</code> as 151much as possible. There seem to be fundamental difficulties in extending 152the syntax of a <code>std::map</code> to multiple keys. For one example, 153consider the situation: 154</p> 155 156<blockquote><pre> 157<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> 158<span class=comment>// keys are int and string, double is the mapped to value</span> 159 160<span class=special>...</span> 161 162<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> 163<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> 164<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> 165</pre></blockquote> 166 167<p> 168In the last sentence of the example, the user has no way of 169providing the <code>string</code> key mapping to the same value 170as <code>m[1]</code>. This and similar problems have to be devoted 171a careful study when designing the interface of a potential 172indexed map. 173</p> 174 175<hr> 176 177<div class="prev_link"><a href="tests.html"><img src="prev.gif" alt="tests" border="0"><br> 178Tests 179</a></div> 180<div class="up_link"><a href="index.html"><img src="up.gif" alt="index" border="0"><br> 181Index 182</a></div> 183<div class="next_link"><a href="release_notes.html"><img src="next.gif" alt="release notes" border="0"><br> 184Release notes 185</a></div><br clear="all" style="clear: all;"> 186 187<br> 188 189<p>Revised April 19th 2015</p> 190 191<p>© Copyright 2003-2015 Joaquín M López Muñoz. 192Distributed under the Boost Software 193License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt"> 194LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> 195http://www.boost.org/LICENSE_1_0.txt</a>) 196</p> 197 198</body> 199</html> 200
