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<a name="intrusive.obtaining_iterators_from_values"></a><a class="link" href="obtaining_iterators_from_values.html" title="Obtaining iterators from values"> Obtaining iterators
from values</a>
</h2></div></div></div>
<p>
<span class="bold"><strong>Boost.Intrusive</strong></span> offers another useful feature
that's not present in STL containers: it's possible to obtain an iterator to
a value from the value itself. This feature is implemented in <span class="bold"><strong>Boost.Intrusive</strong></span>
containers by a function called <code class="computeroutput"><span class="identifier">iterator_to</span></code>:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">iterator</span> <span class="identifier">iterator_to</span><span class="special">(</span><span class="identifier">reference</span> <span class="identifier">value</span><span class="special">);</span>
<span class="identifier">const_iterator</span> <span class="identifier">iterator_to</span><span class="special">(</span><span class="identifier">const_reference</span> <span class="identifier">value</span><span class="special">);</span>
</pre>
<p>
For <span class="bold"><strong>Boost.Intrusive</strong></span> containers that have local
iterators, like unordered associative containers, we can also obtain local
iterators:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">local_iterator</span> <span class="identifier">local_iterator_to</span><span class="special">(</span><span class="identifier">reference</span> <span class="identifier">value</span><span class="special">);</span>
<span class="identifier">const_local_iterator</span> <span class="identifier">local_iterator_to</span><span class="special">(</span><span class="identifier">const_reference</span> <span class="identifier">value</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
</pre>
<p>
For most <span class="bold"><strong>Boost.Intrusive</strong></span> containers (<code class="computeroutput"><a class="link" href="../boost/intrusive/list.html" title="Class template list">list</a></code>, <code class="computeroutput"><a class="link" href="../boost/intrusive/slist.html" title="Class template slist">slist</a></code>,
<code class="computeroutput"><a class="link" href="../boost/intrusive/set.html" title="Class template set">set</a></code>, <code class="computeroutput"><a class="link" href="../boost/intrusive/multiset.html" title="Class template multiset">multiset</a></code>)
we have an alternative static <code class="computeroutput"><span class="identifier">s_iterator_to</span></code>
function.
</p>
<p>
For unordered associative containers (<code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_set.html" title="Class template unordered_set">unordered_set</a></code>,
<code class="computeroutput"><a class="link" href="../boost/intrusive/multiset.html" title="Class template multiset">multiset</a></code>), <code class="computeroutput"><span class="identifier">iterator_to</span></code> has no static alternative function.
On the other hand, <code class="computeroutput"><span class="identifier">local_iterator_to</span></code>
functions have their <code class="computeroutput"><span class="identifier">s_local_iterator_to</span></code>
static alternatives.
</p>
<p>
Alternative static functions are available under certain circumstances explained
in the <a class="link" href="value_traits.html#intrusive.value_traits.stateful_value_traits" title="Stateful value traits">Stateful
value traits</a> section; if the programmer uses hooks provided by <span class="bold"><strong>Boost.Intrusive</strong></span>, those functions will be available.
</p>
<p>
Let's see a small function that shows the use of <code class="computeroutput"><span class="identifier">iterator_to</span></code>
and <code class="computeroutput"><span class="identifier">local_iterator_to</span></code>:
</p>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</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">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">hash</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></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="keyword">class</span> <span class="identifier">intrusive_data</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">data_id_</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">void</span> <span class="identifier">set</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">id</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">data_id_</span> <span class="special">=</span> <span class="identifier">id</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//This class can be inserted in an intrusive list
</span> <span class="identifier">list_member_hook</span><span class="special"><></span> <span class="identifier">list_hook_</span><span class="special">;</span>
<span class="comment">//This class can be inserted in an intrusive unordered_set
</span> <span class="identifier">unordered_set_member_hook</span><span class="special"><></span> <span class="identifier">unordered_set_hook_</span><span class="special">;</span>
<span class="comment">//Comparison operators
</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">intrusive_data</span> <span class="special">&</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">intrusive_data</span> <span class="special">&</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">data_id_</span> <span class="special">==</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">data_id_</span><span class="special">;</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">intrusive_data</span> <span class="special">&</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">intrusive_data</span> <span class="special">&</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">data_id_</span> <span class="special">!=</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">data_id_</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//The hash function
</span> <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">intrusive_data</span> <span class="special">&</span><span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash</span><span class="special"><</span><span class="keyword">int</span><span class="special">>()(</span><span class="identifier">i</span><span class="special">.</span><span class="identifier">data_id_</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="comment">//Definition of the intrusive list that will hold intrusive_data
</span><span class="keyword">typedef</span> <span class="identifier">member_hook</span><span class="special"><</span><span class="identifier">intrusive_data</span><span class="special">,</span> <span class="identifier">list_member_hook</span><span class="special"><></span>
<span class="special">,</span> <span class="special">&</span><span class="identifier">intrusive_data</span><span class="special">::</span><span class="identifier">list_hook_</span><span class="special">></span> <span class="identifier">MemberListOption</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="identifier">intrusive_data</span><span class="special">,</span> <span class="identifier">MemberListOption</span><span class="special">></span> <span class="identifier">list_t</span><span class="special">;</span>
<span class="comment">//Definition of the intrusive unordered_set that will hold intrusive_data
</span><span class="keyword">typedef</span> <span class="identifier">member_hook</span>
<span class="special"><</span> <span class="identifier">intrusive_data</span><span class="special">,</span> <span class="identifier">unordered_set_member_hook</span><span class="special"><></span>
<span class="special">,</span> <span class="special">&</span><span class="identifier">intrusive_data</span><span class="special">::</span><span class="identifier">unordered_set_hook_</span><span class="special">></span> <span class="identifier">MemberUsetOption</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">::</span><span class="identifier">unordered_set</span>
<span class="special"><</span> <span class="identifier">intrusive_data</span><span class="special">,</span> <span class="identifier">MemberUsetOption</span><span class="special">></span> <span class="identifier">unordered_set_t</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="comment">//Create MaxElem objects
</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"><</span><span class="identifier">intrusive_data</span><span class="special">></span> <span class="identifier">nodes</span><span class="special">(</span><span class="identifier">MaxElem</span><span class="special">);</span>
<span class="comment">//Declare the intrusive containers
</span> <span class="identifier">list_t</span> <span class="identifier">list</span><span class="special">;</span>
<span class="identifier">unordered_set_t</span><span class="special">::</span><span class="identifier">bucket_type</span> <span class="identifier">buckets</span><span class="special">[</span><span class="identifier">MaxElem</span><span class="special">];</span>
<span class="identifier">unordered_set_t</span> <span class="identifier">unordered_set</span>
<span class="special">(</span><span class="identifier">unordered_set_t</span><span class="special">::</span><span class="identifier">bucket_traits</span><span class="special">(</span><span class="identifier">buckets</span><span class="special">,</span> <span class="identifier">MaxElem</span><span class="special">));</span>
<span class="comment">//Initialize all the nodes
</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"><</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">set</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="comment">//Now insert them in both intrusive containers
</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="identifier">unordered_set</span><span class="special">.</span><span class="identifier">insert</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 check the iterator_to function
</span> <span class="identifier">list_t</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">list_it</span><span class="special">(</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">begin</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"><</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="special">++</span><span class="identifier">list_it</span><span class="special">)</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">iterator_to</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="special">!=</span> <span class="identifier">list_it</span> <span class="special">||</span>
<span class="identifier">list_t</span><span class="special">::</span><span class="identifier">s_iterator_to</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="special">!=</span> <span class="identifier">list_it</span><span class="special">)</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="comment">//Now check unordered_set::s_iterator_to (which is a member function)
</span> <span class="comment">//and unordered_set::s_local_iterator_to (which is an static member function)
</span> <span class="identifier">unordered_set_t</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">unordered_set_it</span><span class="special">(</span><span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">begin</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"><</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">unordered_set_it</span> <span class="special">=</span> <span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">find</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="keyword">if</span><span class="special">(</span><span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">iterator_to</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="special">!=</span> <span class="identifier">unordered_set_it</span><span class="special">)</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="keyword">if</span><span class="special">(*</span><span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">local_iterator_to</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="special">!=</span> <span class="special">*</span><span class="identifier">unordered_set_it</span> <span class="special">||</span>
<span class="special">*</span><span class="identifier">unordered_set_t</span><span class="special">::</span><span class="identifier">s_local_iterator_to</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="special">!=</span> <span class="special">*</span><span class="identifier">unordered_set_it</span> <span class="special">)</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</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, 2006-2009 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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