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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="intrusive.clone_from"></a><a class="link" href="clone_from.html" title="Cloning Boost.Intrusive containers">Cloning Boost.Intrusive containers</a>
</h2></div></div></div>
<p>
As previously mentioned, <span class="bold"><strong>Boost.Intrusive</strong></span> containers
are <span class="bold"><strong>non-copyable and non-assignable</strong></span>, because
intrusive containers don't allocate memory at all. To implement a copy-constructor
or assignment operator, the user must clone one by one all the elements of
the container and insert them in another intrusive container. However, cloning
by hand is usually more inefficient than a member cloning function and a specialized
cloning function can offer more guarantees than the manual cloning (better
exception safety guarantees, for example).
</p>
<p>
To ease the implementation of copy constructors and assignment operators of
classes containing <span class="bold"><strong>Boost.Intrusive</strong></span> containers,
all <span class="bold"><strong>Boost.Intrusive</strong></span> containers offer a special
cloning function called <code class="computeroutput"><span class="identifier">clone_from</span></code>.
</p>
<p>
Apart from the container to be cloned, <code class="computeroutput"><span class="identifier">clone_from</span></code>
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>:
</p>
<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>
<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>
</pre>
<p>
This function will make <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
a clone of <code class="computeroutput"><span class="identifier">src</span></code>. Let's explain
the arguments:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The first parameter is the list to be cloned.
</li>
<li class="listitem">
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
to the clone. It must implement the following function: <code class="computeroutput"><span class="identifier">pointer</span>
<span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">value_type</span>
<span class="special">&amp;)</span></code>.
</li>
<li class="listitem">
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
empty the container before cloning and to dispose the elements if an exception
is thrown.
</li>
</ul></div>
<p>
The cloning function works as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
First it clears and disposes all the elements from *this using the disposer
function object.
</li>
<li class="listitem">
After that it starts cloning all the elements of the source container using
the cloner function object.
</li>
<li class="listitem">
If any operation in the cloning function (for example, the cloner function
object) throws, all the constructed elements are disposed using the disposer
function object.
</li>
</ul></div>
<p>
Here is an example of <code class="computeroutput"><span class="identifier">clone_from</span></code>:
</p>
<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>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<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>
<span class="comment">//A class that can be inserted in an intrusive list</span>
<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>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<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>
<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>
<span class="keyword">int</span> <span class="identifier">int_</span><span class="special">;</span>
<span class="comment">//...</span>
<span class="special">};</span>
<span class="comment">//Definition of the intrusive list</span>
<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>
<span class="comment">//Cloner object function</span>
<span class="keyword">struct</span> <span class="identifier">new_cloner</span>
<span class="special">{</span>
<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>
<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>
<span class="special">};</span>
<span class="comment">//The disposer object function</span>
<span class="keyword">struct</span> <span class="identifier">delete_disposer</span>
<span class="special">{</span>
<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>
<span class="special">{</span> <span class="keyword">delete</span> <span class="identifier">delete_this</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">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>
<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>
<span class="comment">//Fill all the nodes and insert them in the list</span>
<span class="identifier">my_class_list</span> <span class="identifier">list</span><span class="special">;</span>
<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>
<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>
<span class="comment">//Now clone "list" using "new" and "delete" object functions</span>
<span class="identifier">my_class_list</span> <span class="identifier">cloned_list</span><span class="special">;</span>
<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>
<span class="comment">//Test that both are equal</span>
<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>
<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>
<span class="keyword">else</span>
<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>
<span class="comment">//Don't forget to free the memory from the second list</span>
<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>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2015 Ion Gaztanaga<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
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