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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN">

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<h1><img src="../../../../boost.png" alt="boost.png (6897 bytes)" align=
"middle" width="277" height="86">Boost.MultiIndex Tutorial: Techniques</h1>

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<hr>

<h2>Contents</h2>

<ul>
  <li><a href="#emulate_std_containers">Emulating standard containers with
    <code>multi_index_container</code></a>
    <ul>
      <li><a href="#emulate_assoc_containers">Emulation of associative
       containers</a></li>
      <li><a href="#emulate_std_list">Emulation of <code>std::list</code></a></li>
    </ul>
  </li>
  <li><a href="#metaprogrammming">Metaprogramming and <code>multi_index_container</code></a>
    <ul>
      <li><a href="#mpl_analysis">MPL analysis</a></li>
      <li><a href="#mpl_synthesis">MPL synthesis</a></li>
    </ul>
  </li>
</ul>

<h2><a name="emulate_std_containers">Emulating standard containers with
  <code>multi_index_container</code></a></h2>

<h3><a name="emulate_assoc_containers">Emulation of associative
containers</a></h3>

<p>
Academic movitations aside, there is a practical interest in emulating standard
associative containers by means of <code>multi_index_container</code>, namely to take
advantage of extended functionalities provided by <code>multi_index_container</code> for
lookup, range querying and updating.
</p>

<p>
In order to emulate a <code>std::set</code> one can follow the substitution
rule:
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>set</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>Compare</span><span class=special>,</span><span class=identifier>Allocator</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Key</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>&gt;,</span><span class=identifier>Compare</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>Allocator</span>
  <span class=special>&gt;</span>
</pre></blockquote>

<p>
In the default case where <code>Compare=std::less&lt;Key></code> and
<code>Allocator=std::allocator&lt;Key></code>, the substitution rule is
simplified as
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>set</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>&gt;</span> <span class=special>-&gt;</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>&gt;</span>
</pre></blockquote>

<p>
The substitution of <code>multi_index_container</code> for <code>std::set</code> keeps
the whole set of functionality provided by <code>std::set</code>, so in
principle it is a drop-in replacement needing no further adjustments.
</p>

<p>
<code>std::multiset</code> can be emulated in a similar manner, according to the
following rule:
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>multiset</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>Compare</span><span class=special>,</span><span class=identifier>Allocator</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Key</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>&gt;,</span><span class=identifier>Compare</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>Allocator</span>
  <span class=special>&gt;</span>
</pre></blockquote>

<p>
When default values are taken into consideration, the rule takes the form
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>multiset</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Key</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;</span>
  <span class=special>&gt;</span>
</pre></blockquote>

<p>
The emulation of <code>std::multiset</code>s with <code>multi_index_container</code>
results in a slight difference with respect to the interface offered: the member
function <code>insert(const value_type&amp;)</code> does not return an
<code>iterator</code> as in <code>std::multiset</code>s, but rather a
<code>std::pair&lt;iterator,bool></code> in the spirit of <code>std::set</code>s.
In this particular case, however, the <code>bool</code> member of the returned
pair is always <code>true</code>.
</p>

<p>
The case of <code>std::map</code>s and <code>std::multimap</code>s does not lend
itself to such a direct emulation by means of <code>multi_index_container</code>. The main
problem lies in the fact that elements of a <code>multi_index_container</code> are treated
as constant, while the <code>std::map</code> and <code>std::multimap</code> handle
objects of type <code>std::pair&lt;const Key,T></code>, thus allowing for free
modification of the value part. To overcome this difficulty we need to create an ad
hoc pair class: 
</p>

<blockquote><pre>
<span class=keyword>template</span> <span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>T1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>T2</span><span class=special>&gt;</span>
<span class=keyword>struct</span> <span class=identifier>mutable_pair</span>
<span class=special>{</span>
  <span class=keyword>typedef</span> <span class=identifier>T1</span> <span class=identifier>first_type</span><span class=special>;</span>
  <span class=keyword>typedef</span> <span class=identifier>T2</span> <span class=identifier>second_type</span><span class=special>;</span>

  <span class=identifier>mutable_pair</span><span class=special>():</span><span class=identifier>first</span><span class=special>(</span><span class=identifier>T1</span><span class=special>()),</span><span class=identifier>second</span><span class=special>(</span><span class=identifier>T2</span><span class=special>()){}</span>
  <span class=identifier>mutable_pair</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>T1</span><span class=special>&amp;</span> <span class=identifier>f</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>T2</span><span class=special>&amp;</span> <span class=identifier>s</span><span class=special>):</span><span class=identifier>first</span><span class=special>(</span><span class=identifier>f</span><span class=special>),</span><span class=identifier>second</span><span class=special>(</span><span class=identifier>s</span><span class=special>){}</span>
  <span class=identifier>mutable_pair</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special>&lt;</span><span class=identifier>T1</span><span class=special>,</span><span class=identifier>T2</span><span class=special>&gt;&amp;</span> <span class=identifier>p</span><span class=special>):</span><span class=identifier>first</span><span class=special>(</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>first</span><span class=special>),</span><span class=identifier>second</span><span class=special>(</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>second</span><span class=special>){}</span>

  <span class=identifier>T1</span>         <span class=identifier>first</span><span class=special>;</span>
  <span class=keyword>mutable</span> <span class=identifier>T2</span> <span class=identifier>second</span><span class=special>;</span>
<span class=special>};</span>
</pre></blockquote>

<p>
and so the substitution rules are:
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>map</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>T</span><span class=special>,</span><span class=identifier>Compare</span><span class=special>,</span><span class=identifier>Allocator</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Element</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span>
      <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>,</span><span class=identifier>Key</span><span class=special>,&amp;</span><span class=identifier>Element</span><span class=special>::</span><span class=identifier>first</span><span class=special>&gt;,</span><span class=identifier>Compare</span><span class=special>&gt;</span>
    <span class=special>&gt;,</span>
    <span class=keyword>typename</span> <span class=identifier>Allocator</span><span class=special>::</span><span class=keyword>template</span> <span class=identifier>rebind</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>&gt;::</span><span class=identifier>other</span>
  <span class=special>&gt;</span>

<span class=identifier>std</span><span class=special>::</span><span class=identifier>multimap</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>T</span><span class=special>,</span><span class=identifier>Compare</span><span class=special>,</span><span class=identifier>Allocator</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Element</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span>
      <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>,</span><span class=identifier>Key</span><span class=special>,&amp;</span><span class=identifier>Element</span><span class=special>::</span><span class=identifier>first</span><span class=special>&gt;,</span><span class=identifier>Compare</span><span class=special>&gt;</span>
    <span class=special>&gt;,</span>
    <span class=keyword>typename</span> <span class=identifier>Allocator</span><span class=special>::</span><span class=keyword>template</span> <span class=identifier>rebind</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>&gt;::</span><span class=identifier>other</span>
  <span class=special>&gt;</span>

(<span class=identifier>with</span> <span class=identifier>Element</span><span class=special>=</span><span class=identifier>mutable_pair</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>T</span><span class=special>&gt;</span>)
</pre></blockquote>

<p>
If default values are considered, the rules take the form:
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>map</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>T</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Element</span><span class=special>,
    </span><span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>,</span><span class=identifier>Key</span><span class=special>,&amp;</span><span class=identifier>Element</span><span class=special>::</span><span class=identifier>first</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;</span>
  <span class=special>&gt;</span>

<span class=identifier>std</span><span class=special>::</span><span class=identifier>multimap</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>T</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Element</span><span class=special>,
    </span><span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>,</span><span class=identifier>Key</span><span class=special>,&amp;</span><span class=identifier>Element</span><span class=special>::</span><span class=identifier>first</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;</span>
  <span class=special>&gt;</span>

(<span class=identifier>with</span> <span class=identifier>Element</span><span class=special>=</span><span class=identifier>mutable_pair</span><span class=special>&lt;</span><span class=identifier>Key</span><span class=special>,</span><span class=identifier>T</span><span class=special>&gt;</span>)
</pre></blockquote>

<p>
Unlike as with standard sets, the interface of these <code>multi_index_container</code>-emulated
maps does not exactly conform to that of <code>std::map</code>s and
<code>std::multimap</code>s. The most obvious difference is the lack of
<code>operator []</code>, either in read or write mode; this, however, can be
emulated with appropriate use of <code>find</code> and <code>insert</code>.
</p>

<p>
These emulations of standard associative containers with <code>multi_index_container</code>
are comparable to the original constructs in terms of space and time efficiency.
See the <a href="../performance.html">performance section</a> for further details.
</p>

<h3><a name="emulate_std_list">Emulation of <code>std::list</code></a></h3>

<p>
Unlike the case of associative containers, emulating <code>std::list</code>
in Boost.MultiIndex does not add any significant functionality, so the following
is presented merely for completeness sake.
</p>

<p>
Much as with standard maps, the main difficulty to overcome when emulating
<code>std::list</code> derives from the constant nature of elements of a
<code>multi_index_container</code>. Again, some sort of adaption class is needed, like
for instance the following:
</p>

<blockquote><pre>
<span class=keyword>template</span> <span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>&gt;</span>
<span class=keyword>struct</span> <span class=identifier>mutable_value</span>
<span class=special>{</span>
  <span class=identifier>mutable_value</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>T</span><span class=special>&amp;</span> <span class=identifier>t</span><span class=special>):</span><span class=identifier>t</span><span class=special>(</span><span class=identifier>t</span><span class=special>){}</span>
  <span class=keyword>operator</span> <span class=identifier>T</span><span class=special>&amp;()</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>t</span><span class=special>;}</span>

<span class=keyword>private</span><span class=special>:</span>
  <span class=keyword>mutable</span> <span class=identifier>T</span> <span class=identifier>t</span><span class=special>;</span>
<span class=special>};</span>
</pre></blockquote>

<p>
which allows us to use the substitution rule:
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>list</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>,</span><span class=identifier>Allocator</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=identifier>Element</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>sequenced</span><span class=special>&lt;&gt;</span> <span class=special>&gt;,</span>
    <span class=keyword>typename</span> <span class=identifier>Allocator</span><span class=special>::</span><span class=keyword>template</span> <span class=identifier>rebind</span><span class=special>&lt;</span><span class=identifier>Element</span><span class=special>&gt;::</span><span class=identifier>other</span>
  <span class=special>&gt;</span>

(<span class=identifier>with</span> <span class=identifier>Element</span><span class=special>=</span><span class=identifier>mutable_value</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;</span>)
</pre></blockquote>

<p>
or, if the default value <code>Allocator=std::allocator&lt;T></code> is used:
</p>

<blockquote><pre>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>list</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;</span> <span class=special>-&gt;</span>
  <span class=identifier>multi_index_container</span><span class=special>&lt;</span><span class=identifier>mutable_value</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;,</span><span class=identifier>indexed_by</span><span class=special>&lt;</span><span class=identifier>sequenced</span><span class=special>&lt;&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;</span>
</pre></blockquote>

<h2><a name="metaprogrammming">Metaprogramming and <code>multi_index_container</code></a></h2>

<p>
Boost.MultiIndex provides a number of facilities intended to allow the analysis and
synthesis of <code>multi_index_container</code> instantiations by
<a href="../../../../libs/mpl/doc/index.html">MPL</a> metaprograms.
</p>

<h3><a name="mpl_analysis">MPL analysis</a></h3>

<p>
Given a <code>multi_index_container</code> instantiation, the following nested types are
provided for compile-time inspection of the various types occurring in the
definition of the <code>multi_index_container</code>:
<ul>
  <li><code>index_specifier_type_list</code>,</li>
  <li><code>index_type_list</code>,</li>
  <li><code>iterator_type_list</code>,</li>
  <li><code>const_iterator_type_list</code>.</li>
</ul>
Each of these types is an MPL sequence with as many elements as indices
comprise the <code>multi_index_container</code>: for instance, the <code>n</code>-th
element of <code>iterator_type_list</code> is the same as
<code>nth_index_iterator&lt;n>::type</code>.
</p>

<p>
A subtle but important distinction exists between
<code>index_specifier_type_list</code> and <code>index_type_list</code>:
the former typelist holds the index <i>specifiers</i>
with which the <code>multi_index_container</code> instantiation was defined,
while the latter gives access to the actual implementation classes
corresponding to each specifier. An example will help to clarify
this distinction. Given the instantiation:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>sequenced</span><span class=special>&lt;&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>indexed_t</span><span class=special>;</span>
</pre></blockquote>

<p>
<code>indexed_t::index_specifier_type_list</code> is a type list with
elements
</p>

<blockquote><pre>
<span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;</span>
<span class=identifier>sequenced</span><span class=special>&lt;&gt;</span>
</pre></blockquote>

<p>
while <code>indexed_t::index_type_list</code> holds the types
</p>

<blockquote><pre>
<span class=identifier>multi_index_container</span><span class=special>::</span><span class=identifier>nth_type</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;::</span><span class=identifier>type</span>
<span class=identifier>multi_index_container</span><span class=special>::</span><span class=identifier>nth_type</span><span class=special>&lt;</span><span class=number>1</span><span class=special>&gt;::</span><span class=identifier>type</span>
</pre></blockquote>

<p>
so the typelists are radically different. Check the
<a href="../reference/multi_index_container.html#types">reference</a>
for the exact MPL sequence concepts modeled by these type lists.
</p>

<h3><a name="mpl_synthesis">MPL synthesis</a></h3>

<p>
Although typically indices are specified by means of the
<code>indexed_by</code> construct, actually any MPL sequence of
index specifiers can be provided instead:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>mpl</span><span class=special>::</span><span class=identifier>vector</span><span class=special>&lt;</span><span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;,</span><span class=identifier>sequenced</span><span class=special>&lt;&gt;</span> <span class=special>&gt;</span> <span class=identifier>index_list_t</span><span class=special>;</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>index_list_t</span>
<span class=special>&gt;</span> <span class=identifier>indexed_t</span><span class=special>;</span>
</pre></blockquote>

<p>
This possibility enables the synthesis of instantiations of
<code>multi_index_container</code> through MPL metaprograms, as the following
example shows:
</p>

<blockquote><pre>
<span class=comment>// original multi_index_container instantiation</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span>                                <span class=identifier>indexed_t1</span><span class=special>;</span>

<span class=comment>// we take its index list and add an index</span>
<span class=keyword>typedef</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>push_front</span><span class=special>&lt;</span>
  <span class=identifier>indexed_t1</span><span class=special>::</span><span class=identifier>index_specifier_type_list</span><span class=special>,</span>
  <span class=identifier>sequenced</span><span class=special>&lt;&gt;</span>
<span class=special>&gt;::</span><span class=identifier>type</span>                          <span class=identifier>index_list_t</span><span class=special>;</span>

<span class=comment>// augmented multi_index_container</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>index_list_t</span>
<span class=special>&gt;</span>                                <span class=identifier>indexed_t2</span><span class=special>;</span>
</pre></blockquote>

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<br>

<p>Revised February 6th 2006</p>

<p>&copy; Copyright 2003-2006 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
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>)
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