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<title>Boost.Flyweight Documentation - Tracking policies reference</title>
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<h1><img src="../../../../boost.png" alt="Boost logo" align=
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"middle" width="277" height="86">Boost.Flyweight
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Tracking policies reference</h1>
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<div class="prev_link"><a href="locking.html"><img src="../prev.gif" alt="locking policies" border="0"><br>
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Locking policies
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</a></div>
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<div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight reference" border="0"><br>
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Boost.Flyweight reference
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<div class="next_link"><a href="../performance.html"><img src="../next.gif" alt="performance" border="0"><br>
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Performance
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</a></div><br clear="all" style="clear: all;">
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<hr>
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<h2>Contents</h2>
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<ul>
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<li><a href="#preliminary">Preliminary concepts</a></li>
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<li><a href="#tracking">Tracking policies</a></li>
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<li><a href="#tracking_tag_synopsis">Header
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<code>"boost/flyweight/tracking_tag.hpp"</code> synopsis</a>
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<ul>
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<li><a href="#is_tracking">Class template <code>is_tracking</code></a></li>
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<li><a href="#tracking_construct">Class template <code>tracking</code></a></li>
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</ul>
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</li>
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<li><a href="#refcounted_fwd_synopsis">Header
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<code>"boost/flyweight/refcounted_fwd.hpp"</code> synopsis</a>
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</li>
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<li><a href="#refcounted_synopsis">Header
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<code>"boost/flyweight/refcounted.hpp"</code> synopsis</a>
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<ul>
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<li><a href="#refcounted">Class <code>refcounted</code></a></li>
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</ul>
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</li>
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<li><a href="#no_tracking_fwd_synopsis">Header
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<code>"boost/flyweight/no_tracking_fwd.hpp"</code> synopsis</a>
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</li>
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<li><a href="#no_tracking_synopsis">Header
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<code>"boost/flyweight/no_tracking.hpp"</code> synopsis</a>
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<ul>
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<li><a href="#no_tracking">Class <code>no_tracking</code></a></li>
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</ul>
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</li>
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</ul>
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<h2><a name="preliminary">Preliminary concepts</a></h2>
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<p>
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A <i>tracking policy helper</i> provides access to some of the functionality
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of a <a href="factories.html#factory">factory</a> so as to be used
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in the implementation of an associated <a href="#tracking"><code>Tracking Policy</code></a>.
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In the following table, <code>TrackingHelper</code> is a tracking policy
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helper associated to a <a href="factories.html#factory">factory</a> type
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<code>Factory</code> of elements of type <code>Entry</code>, <code>h</code>
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is a value of
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<code>Factory::handle_type</code> associated to a <code>Factory</code> <code>f</code>
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and <code>check</code> is a value of
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a <a href="https://boost.org/sgi/stl/Predicate.html"><code>Predicate</code></a>
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type <code>Checker</code> with argument of type <code>Factory::handle_type</code>.
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</p>
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<p align="center">
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<table cellspacing="0">
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<caption><b>Tracking Policy Helper requirements.</b></caption>
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<tr>
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<th align="center">expression</th>
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<th align="center">return type</th>
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<th align="center">assertion/note<br>pre/post-condition</th>
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</tr>
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<tr>
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<td><code>TrackingHelper::entry(h);</code></td>
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<td><code>const Entry&</code></td>
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<td>Returns <code>f.entry(h)</code>.</td>
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</tr>
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<tr class="odd_tr">
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<td><code>TrackingHelper::erase(h,check);</code></td>
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<td><code>void</code></td>
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<td>If <code>check(h)</code>, invokes <code>f.erase(h)</code>.</td>
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</tr>
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</table>
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</p>
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<p>
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The execution of <code>TrackingHelper::erase</code> (including the
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invocation of <code>check(h)</code>) is done in a
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synchronized manner so as to prevent any other thread of execution from
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simultaneously accessing the factory's insertion or deletion facilities.
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</p>
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<h2><a name="tracking">Tracking policies</a></h2>
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<p>
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A <i>tracking policy</i> defines the strategy to be followed by a
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<a href="flyweight.html#flyweight"><code>flyweight</code></a> instantiation when
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all the flyweight objects associated to a given value are destroyed.
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The tracking policy contributes some type information necessary for the
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definition of the <code>flyweight</code> internal
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<a href="factories.html#factory">factory</a>.
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</p>
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<p>
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A type <code>Tracking</code> is a tracking policy if:
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<ul>
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<li>One of the following conditions is satisfied:
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<ol type="a">
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<li><a href="#is_tracking"><code>is_tracking<Tracking>::type</code></a> is
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<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::true_</code></a>,</li>
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<li><code>Tracking</code> is of the form
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<a href="#tracking_construct"><code>tracking<Tracking'></code></a>.</li>
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</ol>
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</li>
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<li>The expression <code>Tracking::entry_type</code> (or
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<code>Tracking'::entry_type</code> if (b) applies) is an
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<a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>MPL Lambda
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Expression</code></a> that resolves, when invoked with different types
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(<code>Value</code>,<code>Key</code>) such that <code>Value</code> is
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<a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
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and implicitly convertible to <code>const Key&</code>, to an
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<a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
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type <code>Entry</code> implicitly convertible to both <code>const Value&</code>
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and <code>const Key&</code>.
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</li>
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<li>The expression <code>Tracking::handle_type</code> (or
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<code>Tracking'::handle_type</code> if (b) applies) is an
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<a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>MPL Lambda
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Expression</code></a>; this expression, when invoked with types
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(<code>InternalHandle</code>,<a href="#preliminary"><code>TrackingHelper</code></a>),
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with <code>InternalHandle</code> being
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<a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
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and providing the nothrow guarantee for copy and assignment,
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resolves to an
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<a href="https://boost.org/sgi/stl/Assignable.html"><code>Assignable</code></a>
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type <code>Handle</code> which also provides the nothrow guarantee for
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copy and assignment and is constructible from and implicitly
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convertible to <code>InternalHandle</code>.
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<code>TrackingHelper</code> is an incomplete type at the time of
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invocation of <code>Tracking::handle_type</code>.
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</li>
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</ul>
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<code>Tracking::handle_type</code> is parameterized by a helper that provides
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access to some of the functionality of the factory associated to the
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tracking policy. This factory's associated entry and handle types are the types
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<code>Entry</code> and <code>Handle</code> defined above, respectively.
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</p>
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<h2><a name="tracking_tag_synopsis">Header
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<a href="../../../../boost/flyweight/tracking_tag.hpp"><code>"boost/flyweight/tracking_tag.hpp"</code></a> synopsis</a></h2>
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<blockquote><pre>
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<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>
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<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>
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<span class=keyword>struct</span> <span class=identifier>tracking_marker</span><span class=special>;</span>
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<span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>></span>
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<span class=keyword>struct</span> <span class=identifier>is_tracking</span>
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<span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>></span>
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<span class=keyword>struct</span> <span class=identifier>tracking</span><span class=special>;</span>
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<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>
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<span class=special>}</span> <span class=comment>// namespace boost</span>
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</pre></blockquote>
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<h3><a name="is_tracking">Class template <code>is_tracking</code></a></h3>
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<p>
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Unless specialized by the user, <code>is_tracking<T>::type</code> is
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<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::true_</code></a>
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if <code>T</code> is derived from <code>tracking_marker</code>, and it is
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<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::false_</code></a>
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otherwise.
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</p>
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<h3><a name="tracking_construct">Class template <code>tracking</code></a></h3>
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<p>
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<code>tracking<T></code> is a syntactic construct meant to indicate
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that <code>T</code> is a tracking policy without resorting to the
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mechanisms provided by the <code>is_tracking</code> class template.
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</p>
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<h2><a name="refcounted_fwd_synopsis">Header
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<a href="../../../../boost/flyweight/refcounted_fwd.hpp"><code>"boost/flyweight/refcounted_fwd.hpp"</code></a> synopsis</a></h2>
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<blockquote><pre>
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<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>
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<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>
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<span class=keyword>struct</span> <span class=identifier>refcounted</span><span class=special>;</span>
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<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>
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<span class=special>}</span> <span class=comment>// namespace boost</span>
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</pre></blockquote>
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<p>
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<code>refcounted_fwd.hpp</code> forward declares the class
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<a href="#refcounted"><code>refcounted</code></a>.
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</p>
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<h2><a name="refcounted_synopsis">Header
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<a href="../../../../boost/flyweight/refcounted.hpp"><code>"boost/flyweight/refcounted.hpp"</code></a> synopsis</a></h2>
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<h3><a name="refcounted">Class <code>refcounted</code></a></h3>
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<p>
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<a href="#tracking"><code>Tracking Policy</code></a> providing
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<a href="flyweight.html#flyweight"><code>flyweight</code></a>
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instantiations with reference counting semantics: when all the flyweight objects
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associated to a given value are destroyed, the corresponding entry is
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erased from <code>flyweight</code>'s internal
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<a href="factories.html#factory">factory</a>.
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</p>
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<h2><a name="no_tracking_fwd_synopsis">Header
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<a href="../../../../boost/flyweight/no_tracking_fwd.hpp"><code>"boost/flyweight/no_tracking_fwd.hpp"</code></a> synopsis</a></h2>
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<blockquote><pre>
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<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>
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<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>
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<span class=keyword>struct</span> <span class=identifier>no_tracking</span><span class=special>;</span>
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<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>
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<span class=special>}</span> <span class=comment>// namespace boost</span>
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</pre></blockquote>
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<p>
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<code>no_tracking_fwd.hpp</code> forward declares the class
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<a href="#no_tracking"><code>no_tracking</code></a>.
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</p>
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<h2><a name="no_tracking_synopsis">Header
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<a href="../../../../boost/flyweight/no_tracking.hpp"><code>"boost/flyweight/no_tracking.hpp"</code></a> synopsis</a></h2>
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<h3><a name="no_tracking">Class <code>no_tracking</code></a></h3>
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<p>
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Null <a href="#tracking"><code>Tracking Policy</code></a>: elements inserted
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into <code>flyweight</code>'s internal <a href="factories.html#factory">factory</a>
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are not erased until program termination.
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</p>
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<hr>
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<div class="prev_link"><a href="locking.html"><img src="../prev.gif" alt="locking policies" border="0"><br>
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Locking policies
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</a></div>
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<div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight reference" border="0"><br>
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Boost.Flyweight reference
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</a></div>
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<div class="next_link"><a href="../performance.html"><img src="../next.gif" alt="performance" border="0"><br>
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Performance
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</a></div><br clear="all" style="clear: all;">
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<br>
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<p>Revised April 24th 2019</p>
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<p>© Copyright 2006-2019 Joaquín M López Muñoz.
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Distributed under the Boost Software
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License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
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LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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