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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="intrusive.clone_from"></a><a class="link" href="clone_from.html" title="Cloning Boost.Intrusive containers">Cloning Boost.Intrusive containers</a>
  28 </h2></div></div></div>
  29 <p>
  30       As previously mentioned, <span class="bold"><strong>Boost.Intrusive</strong></span> containers
  31       are <span class="bold"><strong>non-copyable and non-assignable</strong></span>, because
  32       intrusive containers don't allocate memory at all. To implement a copy-constructor
  33       or assignment operator, the user must clone one by one all the elements of
  34       the container and insert them in another intrusive container. However, cloning
  35       by hand is usually more inefficient than a member cloning function and a specialized
  36       cloning function can offer more guarantees than the manual cloning (better
  37       exception safety guarantees, for example).
  38     </p>
  39 <p>
  40       To ease the implementation of copy constructors and assignment operators of
  41       classes containing <span class="bold"><strong>Boost.Intrusive</strong></span> containers,
  42       all <span class="bold"><strong>Boost.Intrusive</strong></span> containers offer a special
  43       cloning function called <code class="computeroutput"><span class="identifier">clone_from</span></code>.
  44     </p>
  45 <p>
  46       Apart from the container to be cloned, <code class="computeroutput"><span class="identifier">clone_from</span></code>
  47       takes two function objects as arguments. For example, consider the <code class="computeroutput"><span class="identifier">clone_from</span></code> member function of <code class="computeroutput"><a class="link" href="../boost/intrusive/list.html" title="Class template list">list</a></code>:
  48     </p>
  49 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Cloner</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Disposer</span><span class="special">&gt;</span>
  50 <span class="keyword">void</span> <span class="identifier">clone_from</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">list</span> <span class="special">&amp;</span><span class="identifier">src</span><span class="special">,</span> <span class="identifier">Cloner</span> <span class="identifier">cloner</span><span class="special">,</span> <span class="identifier">Disposer</span> <span class="identifier">disposer</span><span class="special">);</span>
  51 </pre>
  52 <p>
  53       This function will make <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
  54       a clone of <code class="computeroutput"><span class="identifier">src</span></code>. Let's explain
  55       the arguments:
  56     </p>
  57 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  58 <li class="listitem">
  59           The first parameter is the list to be cloned.
  60         </li>
  61 <li class="listitem">
  62           The second parameter is a function object that will clone <code class="computeroutput"><span class="identifier">value_type</span></code> objects and return a pointer
  63           to the clone. It must implement the following function: <code class="computeroutput"><span class="identifier">pointer</span>
  64           <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">value_type</span>
  65           <span class="special">&amp;)</span></code>.
  66         </li>
  67 <li class="listitem">
  68           The second parameter is a function object that will dispose <code class="computeroutput"><span class="identifier">value_type</span></code> objects. It's used first to
  69           empty the container before cloning and to dispose the elements if an exception
  70           is thrown.
  71         </li>
  72 </ul></div>
  73 <p>
  74       The cloning function works as follows:
  75     </p>
  76 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  77 <li class="listitem">
  78           First it clears and disposes all the elements from *this using the disposer
  79           function object.
  80         </li>
  81 <li class="listitem">
  82           After that it starts cloning all the elements of the source container using
  83           the cloner function object.
  84         </li>
  85 <li class="listitem">
  86           If any operation in the cloning function (for example, the cloner function
  87           object) throws, all the constructed elements are disposed using the disposer
  88           function object.
  89         </li>
  90 </ul></div>
  91 <p>
  92       Here is an example of <code class="computeroutput"><span class="identifier">clone_from</span></code>:
  93     </p>
  94 <p>
  95 </p>
  96 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  97 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
  98 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
  99
 100 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
 101
 102 <span class="comment">//A class that can be inserted in an intrusive list</span>
 103 <span class="keyword">class</span> <span class="identifier">my_class</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">list_base_hook</span><span class="special">&lt;&gt;</span>
 104 <span class="special">{</span>
 105    <span class="keyword">public</span><span class="special">:</span>
 106    <span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="keyword">const</span> <span class="identifier">my_class</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">my_class</span> <span class="special">&amp;</span><span class="identifier">b</span><span class="special">)</span>
 107    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">int_</span> <span class="special">==</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">int_</span><span class="special">;</span>   <span class="special">}</span>
 108
 109    <span class="keyword">int</span> <span class="identifier">int_</span><span class="special">;</span>
 110
 111    <span class="comment">//...</span>
 112 <span class="special">};</span>
 113
 114 <span class="comment">//Definition of the intrusive list</span>
 115 <span class="keyword">typedef</span> <span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">my_class</span><span class="special">&gt;</span> <span class="identifier">my_class_list</span><span class="special">;</span>
 116
 117 <span class="comment">//Cloner object function</span>
 118 <span class="keyword">struct</span> <span class="identifier">new_cloner</span>
 119 <span class="special">{</span>
 120    <span class="identifier">my_class</span> <span class="special">*</span><span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">my_class</span> <span class="special">&amp;</span><span class="identifier">clone_this</span><span class="special">)</span>
 121    <span class="special">{</span>  <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">my_class</span><span class="special">(</span><span class="identifier">clone_this</span><span class="special">);</span>  <span class="special">}</span>
 122 <span class="special">};</span>
 123
 124 <span class="comment">//The disposer object function</span>
 125 <span class="keyword">struct</span> <span class="identifier">delete_disposer</span>
 126 <span class="special">{</span>
 127    <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">my_class</span> <span class="special">*</span><span class="identifier">delete_this</span><span class="special">)</span>
 128    <span class="special">{</span>  <span class="keyword">delete</span> <span class="identifier">delete_this</span><span class="special">;</span>  <span class="special">}</span>
 129 <span class="special">};</span>
 130
 131 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 132 <span class="special">{</span>
 133    <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">MaxElem</span> <span class="special">=</span> <span class="number">100</span><span class="special">;</span>
 134    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">my_class</span><span class="special">&gt;</span> <span class="identifier">nodes</span><span class="special">(</span><span class="identifier">MaxElem</span><span class="special">);</span>
 135
 136    <span class="comment">//Fill all the nodes and insert them in the list</span>
 137    <span class="identifier">my_class_list</span> <span class="identifier">list</span><span class="special">;</span>
 138
 139    <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">MaxElem</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">nodes</span><span class="special">[</span><span class="identifier">i</span><span class="special">].</span><span class="identifier">int_</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
 140
 141    <span class="identifier">list</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">nodes</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">nodes</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
 142
 143    <span class="comment">//Now clone "list" using "new" and "delete" object functions</span>
 144    <span class="identifier">my_class_list</span> <span class="identifier">cloned_list</span><span class="special">;</span>
 145    <span class="identifier">cloned_list</span><span class="special">.</span><span class="identifier">clone_from</span><span class="special">(</span><span class="identifier">list</span><span class="special">,</span> <span class="identifier">new_cloner</span><span class="special">(),</span> <span class="identifier">delete_disposer</span><span class="special">());</span>
 146
 147    <span class="comment">//Test that both are equal</span>
 148    <span class="keyword">if</span><span class="special">(</span><span class="identifier">cloned_list</span> <span class="special">!=</span> <span class="identifier">list</span><span class="special">)</span>
 149       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Both lists are different"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 150    <span class="keyword">else</span>
 151       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Both lists are equal"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 152
 153    <span class="comment">//Don't forget to free the memory from the second list</span>
 154    <span class="identifier">cloned_list</span><span class="special">.</span><span class="identifier">clear_and_dispose</span><span class="special">(</span><span class="identifier">delete_disposer</span><span class="special">());</span>
 155    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 156 <span class="special">}</span>
 157 </pre>
 158 <p>
 159     </p>
 160 </div>
 161 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 162 <td align="left"></td>
 163 <td align="right"><div class="copyright-footer">Copyright &#169; 2005 Olaf Krzikalla<br>Copyright &#169; 2006-2013 Ion Gaztanaga<p>
 164         Distributed under the Boost Software License, Version 1.0. (See accompanying
 165         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 166       </p>
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