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<title>Issue 2315: weak_ptr should be movable</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++14">C++14</a> status.</em></p>
<h3 id="2315"><a href="lwg-defects.html#2315">2315</a>. <code>weak_ptr</code> should be movable</h3>
<p><b>Section:</b> 20.3.2.3 <a href="https://wg21.link/util.smartptr.weak">[util.smartptr.weak]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2013-09-21 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.weak">issues</a> in [util.smartptr.weak].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Like <code>shared_ptr</code>, <code>weak_ptr</code> should be movable to avoid unnecessary atomic increments/decrements of the weak refcount.
</p>

<p><i>[2014-02-13 Issaquah: Move to Immediate]</i></p>




<p id="res-2315"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3691.</p>

<ol>
<li><p>Edit 20.3.2.3 <a href="https://wg21.link/util.smartptr.weak">[util.smartptr.weak]</a>/1, class template <code>weak_ptr</code> synopsis, as indicated:</p>

<blockquote><pre>
namespace std {
  template&lt;class T&gt; class weak_ptr {
  public:
    typedef T element_type;

    <i>// 20.9.2.3.1, constructors</i>
    constexpr weak_ptr() noexcept;
    template&lt;class Y&gt; weak_ptr(shared_ptr&lt;Y&gt; const&amp; r) noexcept;
    weak_ptr(weak_ptr const&amp; r) noexcept;
    template&lt;class Y&gt; weak_ptr(weak_ptr&lt;Y&gt; const&amp; r) noexcept;
    <ins>weak_ptr(weak_ptr&amp;&amp; r) noexcept;
    template&lt;class Y&gt; weak_ptr(weak_ptr&lt;Y&gt;&amp;&amp; r) noexcept;</ins>

    [&hellip;]

    <i>// 20.9.2.3.3, assignment</i>
    weak_ptr&amp; operator=(weak_ptr const&amp; r) noexcept;
    template&lt;class Y&gt; weak_ptr&amp; operator=(weak_ptr&lt;Y&gt; const&amp; r) noexcept;
    template&lt;class Y&gt; weak_ptr&amp; operator=(shared_ptr&lt;Y&gt; const&amp; r) noexcept;
    <ins>weak_ptr&amp; operator=(weak_ptr&amp;&amp; r) noexcept;
    template&lt;class Y&gt; weak_ptr&amp; operator=(weak_ptr&lt;Y&gt;&amp;&amp; r) noexcept;</ins>
  };
}
</pre></blockquote>
</li>

<li><p>Add the following new paragraphs at the end of sub-clause 20.3.2.3.2 <a href="https://wg21.link/util.smartptr.weak.const">[util.smartptr.weak.const]</a>:</p>

<blockquote><pre>
<ins>weak_ptr(weak_ptr&amp;&amp; r) noexcept;
template&lt;class Y&gt; weak_ptr(weak_ptr&lt;Y&gt;&amp;&amp; r) noexcept;</ins>
</pre><blockquote>
<p>
<ins>-?- <i>Remark:</i> The second constructor shall not participate in overload resolution unless <code>Y*</code> is implicitly 
convertible to <code>T*</code>.</ins>
<p/>
<ins>-?- <i>Effects:</i> Move-constructs a <code>weak_ptr</code> instance from <code>r</code>.</ins>
<p/>
<ins>-?- <i>Postconditions:</i> <code>*this</code> shall contain the old value of <code>r</code>. <code>r</code> shall be <em>empty</em>. 
<code>r.use_count() == 0</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Edit 20.3.2.3.4 <a href="https://wg21.link/util.smartptr.weak.assign">[util.smartptr.weak.assign]</a> as indicated:</p>

<blockquote><pre>
weak_ptr&amp; operator=(const weak_ptr&amp; r) noexcept;
template&lt;class Y&gt; weak_ptr&amp; operator=(const weak_ptr&lt;Y&gt;&amp; r) noexcept;
template&lt;class Y&gt; weak_ptr&amp; operator=(const shared_ptr&lt;Y&gt;&amp; r) noexcept;
</pre><blockquote>
<p>
-1- <i>Effects:</i> [&hellip;]
<p/>
-2- <i>Remarks:</i> [&hellip;]
<p/>
<ins>-?- <i>Returns:</i> <code>*this</code>.</ins>
</p>
</blockquote>
</blockquote>

<blockquote><pre>
<ins>weak_ptr&amp; operator=(weak_ptr&amp;&amp; r) noexcept;
template&lt;class Y&gt; weak_ptr&amp; operator=(weak_ptr&lt;Y&gt;&amp;&amp; r) noexcept;</ins>
</pre><blockquote>
<p>
<ins>-?- <i>Effects:</i> Equivalent to <code>weak_ptr(std::move(r)).swap(*this)</code>.</ins>
<p/>
<ins>-?- <i>Returns:</i> <code>*this</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

</ol>





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