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<title>Issue 181: make_pair() unintended behavior</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#TC1">TC1</a> status.</em></p>
<h3 id="181"><a href="lwg-defects.html#181">181</a>. make_pair() unintended behavior</h3>
<p><b>Section:</b> 22.3 <a href="https://wg21.link/pairs">[pairs]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Andrew Koenig <b>Opened:</b> 1999-08-03 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#pairs">issues</a> in [pairs].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>The claim has surfaced in Usenet that expressions such as<br/>
<br/>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <code>make_pair(&quot;abc&quot;, 3)</code><br/>
<br/>
are illegal, notwithstanding their use in examples, because template instantiation tries to bind the first template
parameter to <code> const char (&amp;)[4]</code>, which type is uncopyable.<br/>
<br/>
I doubt anyone intended that behavior...
</p>


<p id="res-181"><b>Proposed resolution:</b></p>
<p>In 22.2 <a href="https://wg21.link/utility">[utility]</a>, paragraph 1 change the following
declaration of make_pair():</p>
<blockquote>
  <pre>template &lt;class T1, class T2&gt; pair&lt;T1,T2&gt; make_pair(const T1&amp;, const T2&amp;);</pre>
</blockquote>
<p>to:</p>
<blockquote>
  <pre>template &lt;class T1, class T2&gt; pair&lt;T1,T2&gt; make_pair(T1, T2);</pre>
</blockquote>
<p>  In 22.3 <a href="https://wg21.link/pairs">[pairs]</a> paragraph 7 and the line before, change:</p>
<blockquote>
<pre>template &lt;class T1, class T2&gt;
pair&lt;T1, T2&gt; make_pair(const T1&amp; x, const T2&amp; y);</pre>
</blockquote>
<p>to:</p>
<blockquote>
<pre>template &lt;class T1, class T2&gt;
pair&lt;T1, T2&gt; make_pair(T1 x, T2 y);</pre>
</blockquote>
<p>and add the following footnote to the effects clause:</p>
<blockquote>
  <p> According to 12.8 [class.copy], an implementation is permitted
  to not perform a copy of an argument, thus avoiding unnecessary
  copies.</p>
</blockquote>


<p><b>Rationale:</b></p>
<p>Two potential fixes were suggested by Matt Austern and Dietmar
K&uuml;hl, respectively, 1) overloading with array arguments, and 2) use of
a reference_traits class with a specialization for arrays.  Andy
Koenig suggested changing to pass by value. In discussion, it appeared
that this was a much smaller change to the standard that the other two
suggestions, and any efficiency concerns were more than offset by the
advantages of the solution. Two implementors reported that the
proposed resolution passed their test suites.</p>




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