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  26 <div class="refentry">
  27 <a name="boost.interprocess.weak_ptr"></a><div class="titlepage"></div>
  28 <div class="refnamediv">
  29 <h2><span class="refentrytitle">Class template weak_ptr</span></h2>
  30 <p>boost::interprocess::weak_ptr</p>
  31 </div>
  32 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  33 <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="../../interprocess/indexes_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;
  34
  35 </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>
  36 <span class="keyword">class</span> <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> <span class="special">{</span>
  37 <span class="keyword">public</span><span class="special">:</span>
  38   <span class="comment">// types</span>
  39   <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>
  40   <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>
  41
  42   <span class="comment">// <a class="link" href="weak_ptr.html#boost.interprocess.weak_ptrconstruct-copy-destruct">construct/copy/destruct</a></span>
  43   <a class="link" href="weak_ptr.html#idm45336640913328-bb"><span class="identifier">weak_ptr</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
  44   <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#idm45336640911856-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>
  45   <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#idm45336640907136-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>
  46   <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" title="Class template weak_ptr">weak_ptr</a> <span class="special">&amp;</span> <a class="link" href="weak_ptr.html#idm45336640902416-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>
  47   <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" title="Class template weak_ptr">weak_ptr</a> <span class="special">&amp;</span> <a class="link" href="weak_ptr.html#idm45336640898592-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>
  48
  49   <span class="comment">// <a class="link" href="weak_ptr.html#idm45336640924960-bb">public member functions</a></span>
  50   <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#idm45336640924400-bb"><span class="identifier">lock</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  51   <span class="keyword">long</span> <a class="link" href="weak_ptr.html#idm45336640922240-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>
  52   <span class="keyword">bool</span> <a class="link" href="weak_ptr.html#idm45336640919232-bb"><span class="identifier">expired</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  53   <span class="keyword">void</span> <a class="link" href="weak_ptr.html#idm45336640917072-bb"><span class="identifier">reset</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
  54   <span class="keyword">void</span> <a class="link" href="weak_ptr.html#idm45336640915776-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>
  55 <span class="special">}</span><span class="special">;</span></pre></div>
  56 <div class="refsect1">
  57 <a name="id-1.3.19.17.3.54.5.4"></a><h2>Description</h2>
  58 <p>The <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> class template stores a "weak reference" to an object that's already managed by a <a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a>. To access the object, a <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> can be converted to a <a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a> using the <a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a> constructor or the member function lock. When the last <a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a> to the object goes away and the object is deleted, the attempt to obtain a <a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a> from the <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> 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 <a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a>.</p>
  59 <p>Every <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> 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 <a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> works with the standard library's associative containers.</p>
  60 <p><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> operations never throw exceptions.</p>
  61 <p>The class template is parameterized on T, the type of the object pointed to. </p>
  62 <div class="refsect2">
  63 <a name="id-1.3.19.17.3.54.5.4.6"></a><h3>
  64 <a name="boost.interprocess.weak_ptrconstruct-copy-destruct"></a><code class="computeroutput">weak_ptr</code>
  65         public
  66        construct/copy/destruct</h3>
  67 <div class="orderedlist"><ol class="orderedlist" type="1">
  68 <li class="listitem">
  69 <pre class="literallayout"><a name="idm45336640913328-bb"></a><span class="identifier">weak_ptr</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
  70 <p>Effects: Constructs an empty <code class="computeroutput"><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code>. Postconditions: use_count() == 0. </p>
  71 </li>
  72 <li class="listitem">
  73 <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="idm45336640911856-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>
  74 <p>Effects: If r is empty, constructs an empty <code class="computeroutput"><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code>; otherwise, constructs a <code class="computeroutput"><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code> that shares ownership with r as if by storing a copy of the pointer stored in r.</p>
  75 <p>Postconditions: use_count() == r.use_count().</p>
  76 <p>Throws: nothing. </p>
  77 </li>
  78 <li class="listitem">
  79 <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="idm45336640907136-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>
  80 <p>Effects: If r is empty, constructs an empty <code class="computeroutput"><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code>; otherwise, constructs a <code class="computeroutput"><a class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code> that shares ownership with r as if by storing a copy of the pointer stored in r.</p>
  81 <p>Postconditions: use_count() == r.use_count().</p>
  82 <p>Throws: nothing. </p>
  83 </li>
  84 <li class="listitem">
  85 <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 class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> <span class="special">&amp;</span> <a name="idm45336640902416-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>
  86 <p>Effects: Equivalent to weak_ptr(r).swap(*this).</p>
  87 <p>Throws: nothing.</p>
  88 <p>Notes: The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary. </p>
  89 </li>
  90 <li class="listitem">
  91 <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 class="link" href="weak_ptr.html" title="Class template weak_ptr">weak_ptr</a> <span class="special">&amp;</span> <a name="idm45336640898592-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>
  92 <p>Effects: Equivalent to weak_ptr(r).swap(*this).</p>
  93 <p>Throws: nothing.</p>
  94 <p>Notes: The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary. </p>
  95 </li>
  96 </ol></div>
  97 </div>
  98 <div class="refsect2">
  99 <a name="id-1.3.19.17.3.54.5.4.7"></a><h3>
 100 <a name="idm45336640924960-bb"></a><code class="computeroutput">weak_ptr</code> public member functions</h3>
 101 <div class="orderedlist"><ol class="orderedlist" type="1">
 102 <li class="listitem">
 103 <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="idm45336640924400-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>
 104 <p>Returns: expired()? shared_ptr&lt;T&gt;(): shared_ptr&lt;T&gt;(*this).</p>
 105 <p>Throws: nothing. </p>
 106 </li>
 107 <li class="listitem">
 108 <pre class="literallayout"><span class="keyword">long</span> <a name="idm45336640922240-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>
 109 <p>Returns: 0 if *this is empty; otherwise, the number of <code class="computeroutput"><a class="link" href="shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code> objects that share ownership with *this.</p>
 110 <p>Throws: nothing.</p>
 111 <p>Notes: use_count() is not necessarily efficient. Use only for debugging and testing purposes, not for production code. </p>
 112 </li>
 113 <li class="listitem">
 114 <pre class="literallayout"><span class="keyword">bool</span> <a name="idm45336640919232-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>
 115 <p>Returns: Returns: use_count() == 0.</p>
 116 <p>Throws: nothing.</p>
 117 <p>Notes: expired() may be faster than use_count(). </p>
 118 </li>
 119 <li class="listitem">
 120 <pre class="literallayout"><span class="keyword">void</span> <a name="idm45336640917072-bb"></a><span class="identifier">reset</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
 121 <p>Effects: Equivalent to: weak_ptr().swap(*this). </p>
 122 </li>
 123 <li class="listitem">
 124 <pre class="literallayout"><span class="keyword">void</span> <a name="idm45336640915776-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>
 125 <p>Effects: Exchanges the contents of the two smart pointers.</p>
 126 <p>Throws: nothing. </p>
 127 </li>
 128 </ol></div>
 129 </div>
 130 </div>
 131 </div>
 132 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 133 <td align="left"></td>
 134 <td align="right"><div class="copyright-footer">Copyright &#169; 2005-2015 Ion Gaztanaga<p>
 135         Distributed under the Boost Software License, Version 1.0. (See accompanying
 136         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>)
 137       </p>
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 142 <a accesskey="p" href="managed_weak_ptr.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../interprocess/indexes_reference.html#header.boost.interprocess.smart_ptr.weak_ptr_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="make_managed_weak_ptr.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a>
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