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<title>Class template weak_ptr</title>
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<a name="boost.interprocess.weak_ptr"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Class template weak_ptr</span></h2>
<p>boost::interprocess::weak_ptr</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../boost_interprocess_reference.html#header.boost.interprocess.smart_ptr.weak_ptr_hpp" title="Header &lt;boost/interprocess/smart_ptr/weak_ptr.hpp&gt;">boost/interprocess/smart_ptr/weak_ptr.hpp</a>&gt;
</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> A<span class="special">,</span> <span class="keyword">typename</span> D<span class="special">&gt;</span>
<span class="keyword">class</span> <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// types</span>
<span class="keyword">typedef</span> <span class="identifier">T</span> <a name="boost.interprocess.weak_ptr.element_type"></a><span class="identifier">element_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">T</span> <a name="boost.interprocess.weak_ptr.value_type"></a><span class="identifier">value_type</span><span class="special">;</span>
<span class="comment">// <a class="link" href="weak_ptr.html#boost.interprocess.weak_ptrconstruct-copy-destruct">construct/copy/destruct</a></span>
<a class="link" href="weak_ptr.html#id826798-bb"><span class="identifier">weak_ptr</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> <a class="link" href="weak_ptr.html#id826803-bb"><span class="identifier">weak_ptr</span></a><span class="special">(</span><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> <a class="link" href="weak_ptr.html#id826831-bb"><span class="identifier">weak_ptr</span></a><span class="special">(</span><a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> weak_ptr&amp; <a class="link" href="weak_ptr.html#id826859-bb"><span class="keyword">operator</span><span class="special">=</span></a><span class="special">(</span><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> weak_ptr&amp; <a class="link" href="weak_ptr.html#id826887-bb"><span class="keyword">operator</span><span class="special">=</span></a><span class="special">(</span><a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
<span class="comment">// <a class="link" href="weak_ptr.html#id826708-bb">public member functions</a></span>
<a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a><span class="special">&lt;</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <a class="link" href="weak_ptr.html#id826711-bb"><span class="identifier">lock</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">long</span> <a class="link" href="weak_ptr.html#id826728-bb"><span class="identifier">use_count</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">bool</span> <a class="link" href="weak_ptr.html#id826748-bb"><span class="identifier">expired</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">void</span> <a class="link" href="weak_ptr.html#id826765-bb"><span class="identifier">reset</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
<span class="keyword">void</span> <a class="link" href="weak_ptr.html#id826777-bb"><span class="identifier">swap</span></a><span class="special">(</span><span class="identifier">this_type</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id1520617"></a><h2>Description</h2>
<p>The weak_ptr class template stores a "weak reference" to an object that's already managed by a shared_ptr. To access the object, a weak_ptr can be converted to a shared_ptr using the shared_ptr constructor or the member function lock. When the last shared_ptr to the object goes away and the object is deleted, the attempt to obtain a shared_ptr from the weak_ptr instances that refer to the deleted object will fail: the constructor will throw an exception of type bad_weak_ptr, and weak_ptr::lock will return an empty shared_ptr.</p>
<p>Every weak_ptr meets the CopyConstructible and Assignable requirements of the C++ Standard Library, and so can be used in standard library containers. Comparison operators are supplied so that weak_ptr works with the standard library's associative containers.</p>
<p>weak_ptr operations never throw exceptions.</p>
<p>The class template is parameterized on T, the type of the object pointed to. </p>
<div class="refsect2">
<a name="id1520635"></a><h3>
<a name="boost.interprocess.weak_ptrconstruct-copy-destruct"></a><code class="computeroutput">weak_ptr</code>
public
construct/copy/destruct</h3>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<pre class="literallayout"><a name="id826798-bb"></a><span class="identifier">weak_ptr</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
<p>Effects: Constructs an empty weak_ptr. Postconditions: use_count() == 0. </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> <a name="id826803-bb"></a><span class="identifier">weak_ptr</span><span class="special">(</span><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> r<span class="special">)</span><span class="special">;</span></pre>
<p>Effects: If r is empty, constructs an empty weak_ptr; otherwise, constructs a weak_ptr that shares ownership with r as if by storing a copy of the pointer stored in r.</p>
<p>Postconditions: use_count() == r.use_count().</p>
<p>Throws: nothing. </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> <a name="id826831-bb"></a><span class="identifier">weak_ptr</span><span class="special">(</span><a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> r<span class="special">)</span><span class="special">;</span></pre>
<p>Effects: If r is empty, constructs an empty weak_ptr; otherwise, constructs a weak_ptr that shares ownership with r as if by storing a copy of the pointer stored in r.</p>
<p>Postconditions: use_count() == r.use_count().</p>
<p>Throws: nothing. </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> weak_ptr&amp; <a name="id826859-bb"></a><span class="keyword">operator</span><span class="special">=</span><span class="special">(</span><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> r<span class="special">)</span><span class="special">;</span></pre>
<p>Effects: Equivalent to weak_ptr(r).swap(*this).</p>
<p>Throws: nothing.</p>
<p>Notes: The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary. </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Y<span class="special">&gt;</span> weak_ptr&amp; <a name="id826887-bb"></a><span class="keyword">operator</span><span class="special">=</span><span class="special">(</span><a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a><span class="special">&lt;</span> <span class="identifier">Y</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> r<span class="special">)</span><span class="special">;</span></pre>
<p>Effects: Equivalent to weak_ptr(r).swap(*this).</p>
<p>Throws: nothing.</p>
<p>Notes: The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary. </p>
</li>
</ol></div>
</div>
<div class="refsect2">
<a name="id1521150"></a><h3>
<a name="id826708-bb"></a><code class="computeroutput">weak_ptr</code> public member functions</h3>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<pre class="literallayout"><a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a><span class="special">&lt;</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span> <a name="id826711-bb"></a><span class="identifier">lock</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
<p>Returns: expired()? shared_ptr&lt;T&gt;(): shared_ptr&lt;T&gt;(*this).</p>
<p>Throws: nothing. </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">long</span> <a name="id826728-bb"></a><span class="identifier">use_count</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
<p>Returns: 0 if *this is empty; otherwise, the number of shared_ptr objects that share ownership with *this.</p>
<p>Throws: nothing.</p>
<p>Notes: use_count() is not necessarily efficient. Use only for debugging and testing purposes, not for production code. </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">bool</span> <a name="id826748-bb"></a><span class="identifier">expired</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
<p>Returns: Returns: use_count() == 0.</p>
<p>Throws: nothing.</p>
<p>Notes: expired() may be faster than use_count(). </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">void</span> <a name="id826765-bb"></a><span class="identifier">reset</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
<p>Effects: Equivalent to: weak_ptr().swap(*this). </p>
</li>
<li class="listitem">
<pre class="literallayout"><span class="keyword">void</span> <a name="id826777-bb"></a><span class="identifier">swap</span><span class="special">(</span><span class="identifier">this_type</span> <span class="special">&amp;</span> other<span class="special">)</span><span class="special">;</span></pre>
<p>Effects: Exchanges the contents of the two smart pointers.</p>
<p>Throws: nothing. </p>
</li>
</ol></div>
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