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

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<hr>

<h2>Contents</h2>

<ul>
  <li><a href="#value_semantics">Value semantics</a></li>
  <li><a href="#ctor_args_list">Use of <code>ctor_args_list</code></a></li>
  <li><a href="#special_allocator">Special allocator support</a></li>
  <li><a href="#serialization">Serialization</a></li>
</ul>

<h2><a name="value_semantics">Value semantics</a></h2>

<p>
<code>multi_index_container</code>s have the usual value semantics associated
to copy construction and assignment, i.e. copies of the elements from the source
container are created and inserted into the destination container.
More interestingly, copying also recreates the original order in which
elements are arranged for <i>every index</i> of the container.
This implies that equality of all indices is preserved under copying
or assignment, for those index types where equality is defined. This behavior
can be regarded as a natural extension to the general rule on copy semantics
stating that if <code>y</code> is a copy of <code>x</code>, then
<code>y==x</code>.
</p>

<h2><a name="ctor_args_list">Use of <code>ctor_args_list</code></a></h2>

<p>
Although in most cases <code>multi_index_container</code>s will be default constructed
(or copied from a preexisting <code>multi_index_container</code>), sometimes it is
necessary to specify particular values for the internal objects used (key extractors,
comparison predicates, allocator), for instance if some of these objects do not have
a default constructor. The same situation can arise with standard STL containers,
which allow for the optional specification of such objects:
</p>

<blockquote><pre>
<span class=comment>// example of non-default constructed std::set</span>
<span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>IntegralType</span><span class=special>&gt;</span>
<span class=keyword>struct</span> <span class=identifier>modulo_less</span>
<span class=special>{</span>
  <span class=identifier>modulo_less</span><span class=special>(</span><span class=identifier>IntegralType</span> <span class=identifier>m</span><span class=special>):</span><span class=identifier>modulo</span><span class=special>(</span><span class=identifier>m</span><span class=special>){}</span>

  <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IntegralType</span> <span class=identifier>x</span><span class=special>,</span><span class=identifier>IntegralType</span> <span class=identifier>y</span><span class=special>)</span><span class=keyword>const</span>
  <span class=special>{</span>
    <span class=keyword>return</span> <span class=special>(</span><span class=identifier>x</span><span class=special>%</span><span class=identifier>modulo</span><span class=special>)&lt;(</span><span class=identifier>y</span><span class=special>%</span><span class=identifier>modulo</span><span class=special>);</span>
  <span class=special>}</span>

<span class=keyword>private</span><span class=special>:</span>
  <span class=identifier>IntegralType</span> <span class=identifier>modulo</span><span class=special>;</span>
<span class=special>};</span>

<span class=keyword>typedef</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>set</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>,</span><span class=identifier>modulo_less</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>modulo_set</span><span class=special>;</span>

<span class=identifier>modulo_set</span> <span class=identifier>m</span><span class=special>(</span><span class=identifier>modulo_less</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;(</span><span class=number>10</span><span class=special>));</span>
</pre></blockquote>

<p>
<code>multi_index_container</code> does also provide this functionality, though in a
considerably more complex fashion, due to the fact that the constructor
of a <code>multi_index_container</code> has to accept values for all the internal
objects of its indices. The full form of <code>multi_index_container</code> constructor
is
</p>

<blockquote><pre>
<span class=keyword>explicit</span> <span class=identifier>multi_index_container</span><span class=special>(</span>
    <span class=keyword>const</span> <span class=identifier>ctor_args_list</span><span class=special>&amp;</span> <span class=identifier>args_list</span><span class=special>=</span><span class=identifier>ctor_args_list</span><span class=special>(),</span>
    <span class=keyword>const</span> <span class=identifier>allocator_type</span><span class=special>&amp;</span> <span class=identifier>al</span><span class=special>=</span><span class=identifier>allocator_type</span><span class=special>());</span>
</pre></blockquote>

<p>
The specification of the allocator object poses no particular problems;
as for the <code>ctor_args_list</code>, this object is designed so as to hold
the necessary construction values for every index in the <code>multi_index_container</code>.
From the point of view of the user, <code>ctor_args_list</code> is equivalent
to the type
</p>

<blockquote><pre>
<span class=identifier>boost</span><span class=special>::</span><span class=identifier>tuple</span><span class=special>&lt;</span><span class=identifier>C<sub>0</sub></span><span class=special>,...,</span><span class=identifier>C<sub>I-1</sub></span><span class=special>&gt;</span>
</pre></blockquote>

<p>
where <code>I</code> is the number of indices, and <code>C<sub>i</sub></code> is
</p>

<blockquote><pre>
<span class=identifier>nth_index</span><span class=special>&lt;</span><span class=identifier>i</span><span class=special>&gt;::</span><span class=identifier>type</span><span class=special>::</span><span class=identifier>ctor_args</span>
</pre></blockquote>

<p>
that is, the nested type <code>ctor_args</code> of the <code>i</code>-th index. Each
<code>ctor_args</code> type is in turn a tuple holding values for constructor
arguments of the associated index: so, ordered indices demand a key extractor object
and a comparison predicate, hashed indices take an initial number of buckets,
a key extractor, a hash function and an equality predicate; while sequenced 
and random access indices do not need any construction argument. For instance,
given the definition
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>hashed_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;,</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=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;,</span> <span class=identifier>modulo_less</span><span class=special>&lt;</span><span class=keyword>unsigned</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=identifier>random_access</span><span class=special>&lt;&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>modulo_indexed_set</span><span class=special>;</span>
</pre></blockquote>

<p>
the corresponding <code>ctor_args_list</code> type is equivalent to
</p>

<blockquote><pre>
<span class=identifier>boost</span><span class=special>::</span><span class=identifier>tuple</span><span class=special>&lt;</span>
  <span class=comment>// ctr_args of index #0</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>tuple</span><span class=special>&lt;</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span><span class=special>,</span> <span class=comment>// initial number of buckets; 0 if unspecified</span>
    <span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;,</span>
    <span class=identifier>boost</span><span class=special>::</span><span class=identifier>hash</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;,</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>equal_to</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;,</span>    

  <span class=comment>// ctr_args of index #1</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>tuple</span><span class=special>&lt;</span>
    <span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;,</span>
    <span class=identifier>modulo_less</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
  
  <span class=comment>// sequenced indices do not have any construction argument</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>tuple</span><span class=special>&lt;&gt;,</span>

  <span class=comment>// neither do random access indices</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>tuple</span><span class=special>&lt;&gt;</span>
<span class=special>&gt;</span>
</pre></blockquote>

<p>
Such a <code>modulo_indexed_set</code> cannot be default constructed, because
<code>modulo_less</code> does not provide a default constructor. The following shows
how the construction can be done:
</p>

<blockquote><pre>
<span class=identifier>modulo_indexed_set</span><span class=special>::</span><span class=identifier>ctor_args_list</span> <span class=identifier>args_list</span><span class=special>=</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>make_tuple</span><span class=special>(</span>
    <span class=comment>// ctor_args for index #0 is default constructible</span>
    <span class=identifier>modulo_indexed_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;::</span><span class=identifier>type</span><span class=special>::</span><span class=identifier>ctor_args</span><span class=special>(),</span>
    
    <span class=identifier>boost</span><span class=special>::</span><span class=identifier>make_tuple</span><span class=special>(</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;(),</span><span class=identifier>modulo_less</span><span class=special>&lt;</span><span class=keyword>unsigned</span> <span class=keyword>int</span><span class=special>&gt;(</span><span class=number>10</span><span class=special>)),</span>
    
    <span class=comment>// these are also default constructible (actually, empty tuples)</span> 
    <span class=identifier>modulo_indexed_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special>&lt;</span><span class=number>2</span><span class=special>&gt;::</span><span class=identifier>type</span><span class=special>::</span><span class=identifier>ctor_args</span><span class=special>(),</span>
    <span class=identifier>modulo_indexed_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special>&lt;</span><span class=number>3</span><span class=special>&gt;::</span><span class=identifier>type</span><span class=special>::</span><span class=identifier>ctor_args</span><span class=special>()</span>
  <span class=special>);</span>

<span class=identifier>modulo_indexed_set</span> <span class=identifier>m</span><span class=special>(</span><span class=identifier>args_list</span><span class=special>);</span>
</pre></blockquote>

<p>
A program is provided in the <a href="../examples.html#example3">examples section</a> that
puts in practise these concepts.
</p>

<h2><a name="special_allocator">Special allocator support</a></h2>

<p>
Boost.MultiIndex allows for a slightly more general class of allocators
than strictly required by the C++ standard, as explained in detail in the
<a href="../reference/multi_index_container.html#instantiation_types">reference</a>.
An important type of non-standard allocators supported are those provided by the
<a href="../../../interprocess/index.html">Boost Interprocess Library</a>;
this opens up the possibility of placing <code>multi_index_container</code>s
in shared memory.
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>interprocess</span><span class=special>/</span><span class=identifier>allocators</span><span class=special>/</span><span class=identifier>allocator</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>interprocess</span><span class=special>/</span><span class=identifier>managed_shared_memory</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>

<span class=keyword>namespace</span> <span class=identifier>bip</span><span class=special>=</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>interprocess</span><span class=special>;</span>

<span class=comment>// a shared memory compatible allocator of ints</span>
<span class=keyword>typedef</span> <span class=identifier>bip</span><span class=special>::</span><span class=identifier>allocator</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span><span class=identifier>bip</span><span class=special>::</span><span class=identifier>managed_shared_memory</span><span class=special>::</span><span class=identifier>segment_manager</span>
<span class=special>&gt;</span> <span class=identifier>shared_int_allocator</span><span class=special>;</span>

<span class=comment>// define a shared memory compatible multi_index_container
// using shared_int_allocator</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>sequenced</span><span class=special>&lt;&gt;,</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=identifier>shared_int_allocator</span>
<span class=special>&gt;</span> <span class=identifier>unique_int_list</span><span class=special>;</span>

<span class=special>...</span>

<span class=comment>// create a managed memory segment</span>
<span class=identifier>bip</span><span class=special>::</span><span class=identifier>managed_shared_memory</span> <span class=identifier>seg</span><span class=special>(</span>
  <span class=identifier>bip</span><span class=special>::</span><span class=identifier>create_only</span><span class=special>,</span><span class=string>&quot;SharedMemoryID&quot;</span><span class=special>,</span><span class=number>65536</span><span class=special>);</span>

<span class=comment>// construct a unique_int_list into the segment</span>
<span class=identifier>unique_int_list</span><span class=special>*</span> <span class=identifier>puil</span><span class=special>=</span><span class=identifier>seg</span><span class=special>.</span><span class=identifier>construct</span><span class=special>&lt;</span><span class=identifier>unique_int_list</span><span class=special>&gt;</span>
  <span class=special>(</span><span class=string>&quot;UniqueIntListID&quot;</span><span class=special>)</span> <span class=comment>// object identifier within the segment
  // Construction args: first a ctor arg list, then a
  // shared memory allocator obtained from the segment object.</span>
  <span class=special>(</span><span class=identifier>unique_int_list</span><span class=special>::</span><span class=identifier>ctor_args_list</span><span class=special>(),</span>
   <span class=identifier>unique_int_list</span><span class=special>::</span><span class=identifier>allocator_type</span><span class=special>(</span><span class=identifier>seg</span><span class=special>.</span><span class=identifier>get_segment_manager</span><span class=special>()));</span>
   </pre></blockquote>

<p>
The examples section includes a <a href="../examples.html#example12">program</a>
that further explores this capability.
</p>

<h2><a name="serialization">Serialization</a></h2>

<p>
<code>multi_index_container</code>s can be archived and retrieved by means of the
<a href="../../../serialization/index.html">Boost Serialization Library</a>. Both regular
and XML archives are supported. The usage is straightforward and does not
differ from that of any other serializable type. For instance:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>archive</span><span class=special>/</span><span class=identifier>text_oarchive</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>archive</span><span class=special>/</span><span class=identifier>text_iarchive</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>fstream</span><span class=special>&gt;</span>

<span class=special>...</span>

<span class=keyword>void</span> <span class=identifier>save</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>employee_set</span><span class=special>&amp;</span> <span class=identifier>es</span><span class=special>)</span>
<span class=special>{</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>ofstream</span> <span class=identifier>ofs</span><span class=special>(</span><span class=string>&quot;data&quot;</span><span class=special>);</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>archive</span><span class=special>::</span><span class=identifier>text_oarchive</span> <span class=identifier>oa</span><span class=special>(</span><span class=identifier>ofs</span><span class=special>);</span>
  <span class=identifier>oa</span><span class=special>&lt;&lt;</span><span class=identifier>es</span><span class=special>;</span>
<span class=special>}</span>

<span class=keyword>void</span> <span class=identifier>load</span><span class=special>(</span><span class=identifier>employee_set</span><span class=special>&amp;</span> <span class=identifier>es</span><span class=special>)</span>
<span class=special>{</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>ifstream</span> <span class=identifier>ifs</span><span class=special>(</span><span class=string>&quot;data&quot;</span><span class=special>);</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>archive</span><span class=special>::</span><span class=identifier>text_iarchive</span> <span class=identifier>ia</span><span class=special>(</span><span class=identifier>ifs</span><span class=special>);</span>
  <span class=identifier>ia</span><span class=special>&gt;&gt;</span><span class=identifier>es</span><span class=special>;</span>
<span class=special>}</span>

<span class=special>...</span>

<span class=identifier>employee_set</span> <span class=identifier>es</span><span class=special>;</span>
<span class=special>...</span> <span class=comment>// fill it with data</span>
<span class=identifier>save</span><span class=special>(</span><span class=identifier>es</span><span class=special>);</span>

<span class=special>...</span>

<span class=identifier>employee_set</span> <span class=identifier>restored_es</span><span class=special>;</span>
<span class=identifier>load</span><span class=special>(</span><span class=identifier>restored_es</span><span class=special>);</span>
</pre></blockquote>

<p>
Serialization capabilities are automatically provided by just linking with
the appropriate Boost.Serialization library module: it is not necessary
to explicitly include any header from Boost.Serialization,
apart from those declaring the type of archive used in the process. If not used,
however, serialization support can be disabled by globally defining the macro
<code>BOOST_MULTI_INDEX_DISABLE_SERIALIZATION</code>. Disabling serialization
for Boost.MultiIndex can yield a small improvement in build times, and may
be necessary in those defective compilers that fail to correctly process
Boost.Serialization headers.
</p>

<p>
In accordance with Boost.MultiIndex
<a href="#value_semantics">value semantics</a>, retrieving an
archived <code>multi_index_container</code> restores not only
the elements, but also the order they were arranged into for
every index of the container. There is an exception to this rule,
though: for <a href="indices.html#hashed_indices">hashed
indices</a>, no guarantee is made about the order in which elements will
be iterated in the restored container; in general, it is unwise to rely on
the ordering of elements of a hashed index, since it can change in arbitrary
ways during insertion or rehashing --this is precisely the reason why
hashed indices and TR1 unordered associative containers do not define
an equality operator.
</p>

<p>
Iterators to indices of a <code>multi_index_container</code> can also be
serialized. Serialization of iterators must be done only after serializing
their corresponding container.
</p>

<p>
<a href="../examples.html#example9">Example 9</a> in the examples section shows
the serialization capabilities of Boost.MultiIndex.
</p>

<hr>

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</a></div><br clear="all" style="clear: all;">

<br>

<p>Revised July 17th 2007</p>

<p>&copy; Copyright 2003-2007 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>)
</p>

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