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<title>Issue 2051: Explicit tuple constructors for more than one parameter</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2051"><a href="lwg-defects.html#2051">2051</a>. Explicit <code>tuple</code> constructors for more than one parameter</h3>
<p><b>Section:</b> 22.4.4 <a href="https://wg21.link/tuple.tuple">[tuple.tuple]</a>, 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Ville Voutilainen <b>Opened:</b> 2011-05-01 <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#tuple.tuple">issues</a> in [tuple.tuple].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
One of my constituents wrote the following:
<p/>
-------snip------------
<p/>
So far the only use I've found for <code>std::tuple</code> is as an ad-hoc type to emulate
multiple return values. If the tuple ctor was made non-explicit one could
almost think C++ supported multiple return values especially when combined
with <code>std::tie()</code>.
</p>
<blockquote><pre>
// assume types line_segment and point
// assume function double distance(point const&amp;, point const&amp;)

std::tuple&lt;point, point&gt;
closest_points(line_segment const&amp; a, line_segment const&amp; b) {
 point ax;
 point bx;
 /* some math */

 return {ax, bx};
}


double
distance(line_segment const&amp; a, line_segment const&amp; b) {
 point ax;
 point bx;
 std::tie(ax, bx) = closest_points(a, b);

 return distance(ax, bx);
}
</pre></blockquote>
<p>
-------snap----------
<p/>
See also the messages starting from lib-29330.
<p/>
Some notes:
</p>
<ol>
<li><code>pair</code> allows such a return</li>
<li>a lambda with a deduced return type doesn't allow it for any type</li>
<li><code>decltype</code> refuses <code>{1, 2}</code></li>
</ol>
<p>
I would recommend making non-unary <code>tuple</code> constructors non-explicit.
</p>

<p><i>[Bloomington, 2011]</i></p>

<p>
Move to NAD Future, this would be an extension to existing functionality.
</p>

<p><i>[Portland, 2012]</i></p>

<p>
Move to Open at the request of the Evolution Working Group.
</p>

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

<p>
<p>VV: While in the area of tuples, LWG 2051 should have status of WP, it is resolved by Daniel's "improving pair and tuple" paper.</p>
<p>MC: status Resolved, by N4387</p>
</p>



<p id="res-2051"><b>Proposed resolution:</b></p>
Resolved by the adoption of <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4387.html">N4387</a>.





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