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<title>Issue 2434: shared_ptr::use_count() is efficient</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++17">C++17</a> status.</em></p>
<h3 id="2434"><a href="lwg-defects.html#2434">2434</a>. <code>shared_ptr::use_count()</code> is efficient</h3>
<p><b>Section:</b> 20.3.2.2.6 <a href="https://wg21.link/util.smartptr.shared.obs">[util.smartptr.shared.obs]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2014-10-01 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.obs">issues</a> in [util.smartptr.shared.obs].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>shared_ptr</code> and <code>weak_ptr</code> have Notes that their <code>use_count()</code> might be inefficient. 
This is an attempt to acknowledge reflinked implementations (which can be used by Loki smart pointers, for 
example). However, there aren't any <code>shared_ptr</code> implementations that use reflinking, especially 
after C++11 recognized the existence of multithreading. Everyone uses atomic refcounts, so <code>use_count()</code> 
is just an atomic load.
</p>

<p><i>[Urbana 2014-11-07: Move to Ready]</i></p>




<p id="res-2434"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N3936.
</p>

<ol>
<li><p>Change 20.3.2.2.6 <a href="https://wg21.link/util.smartptr.shared.obs">[util.smartptr.shared.obs]</a> p7-p10 as depicted:</p>

<blockquote>
<pre>
long use_count() const noexcept;
</pre>
<blockquote>
<p>
-7- <i>Returns</i>: the number of <code>shared_ptr</code> objects, <code>*this</code> included, that <em>share ownership</em> with 
<code>*this</code>, or 0 when <code>*this</code> is <em>empty</em>.
<p/>
<del>-8- [<i>Note</i>: <code>use_count()</code> is not necessarily efficient. &mdash; <i>end note</i>]</del>
</p>
</blockquote>
<pre>
bool unique() const noexcept;
</pre>
<blockquote>
<p>
-9- <i>Returns</i>: <code>use_count() == 1</code>.
<p/>
-10- [<i>Note</i>: <del><code>unique()</code> may be faster than <code>use_count()</code>.</del> If you are using <code>unique()</code> 
to implement copy on write, do not rely on a specific value when <code>get() == 0</code>. &mdash; <i>end note</i>]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Change 20.3.2.3.6 <a href="https://wg21.link/util.smartptr.weak.obs">[util.smartptr.weak.obs]</a> p1-p4 as depicted:</p>

<blockquote>
<pre>
long use_count() const noexcept;
</pre>
<blockquote>
<p>
-1- <i>Returns</i>: 0 if <code>*this</code> is <em>empty</em>; otherwise, the number of <code>shared_ptr</code> instances 
that <em>share ownership</em> with <code>*this</code>.
<p/>
<del>-2- [<i>Note</i>: <code>use_count()</code> is not necessarily efficient. &mdash; <i>end note</i>]</del>
</p>
</blockquote>
<pre>
bool expired() const noexcept;
</pre>
<blockquote>
<p>
-3- <i>Returns</i>: <code>use_count() == 0</code>.
<p/>
<del>-4- [<i>Note</i>: <code>expired()</code> may be faster than <code>use_count()</code>. &mdash; <i>end note</i>]</del>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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