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<title>Issue 2489: mem_fn() should be noexcept</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="2489"><a href="lwg-defects.html#2489">2489</a>. <code>mem_fn()</code> should be <code>noexcept</code></h3>
<p><b>Section:</b> 22.10.16 <a href="https://wg21.link/func.memfn">[func.memfn]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2015-03-27 <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#func.memfn">issues</a> in [func.memfn].</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>mem_fn()</code> is wide contract and doesn't do anything that could throw exceptions, so it should be marked <code>noexcept</code>.
<p/>
Note that <code>mem_fn()</code> is perfectly happy to wrap a null PMF/PMD, it just can't be invoked later. This is exactly like 
<code>std::function</code>, which can be constructed from null PMFs/PMDs. Therefore, <code>mem_fn()</code> will remain wide contract forever.
</p>

<p><i>[2015-05, Lenexa]</i></p>

<p>
Move to Immediate.
</p>



<p id="res-2489"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4296.</p>

<ol>
<li><p>Change 22.10 <a href="https://wg21.link/function.objects">[function.objects]</a> p2 "Header <code>&lt;functional&gt;</code> synopsis" as depicted:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 20.9.11, member function adaptors:</i>
template&lt;class R, class T&gt; <i>unspecified</i> mem_fn(R T::*) <ins>noexcept</ins>;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Change 22.10.16 <a href="https://wg21.link/func.memfn">[func.memfn]</a> as depicted:</p>

<blockquote>
<pre>
template&lt;class R, class T&gt; <i>unspecified</i> mem_fn(R T::* pm) <ins>noexcept</ins>;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
<del>-4- <i>Throws</i>: Nothing.</del>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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