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<title>Issue 1116: Literal constructors for tuple</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="1116"><a href="lwg-defects.html#1116">1116</a>. Literal constructors for tuple</h3>
<p><b>Section:</b> 22.4.4 <a href="https://wg21.link/tuple.tuple">[tuple.tuple]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-05-23 <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#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>
It is not currently possible to construct <code>tuple</code> literal values,
even if the elements are all literal types.  This is because parameters
are passed to constructor by reference.
</p>
<p>
An alternative would be to pass all constructor arguments by value, where it
is known that *all* elements are literal types.  This can be determined with
concepts, although note that the negative constraint really requires
factoring out a separate concept, as there is no way to provide an 'any of
these fails' constraint inline.
</p>
<p>
Note that we will have similar issues with <code>pair</code> (and
<code>tuple</code> constructors from <code>pair</code>) although I am steering
clear of that class while other constructor-related issues settle.
</p>

<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
<del>NAD Editorial</del><ins>Resolved</ins>.  Solved by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2994.htm">N2994</a>.
</p></blockquote>


<p id="res-1116"><b>Proposed resolution:</b></p>
<p>
Ammend the tuple class template declaration in 22.4.4 <a href="https://wg21.link/tuple.tuple">[tuple.tuple]</a> as
follows
</p>

<blockquote>
<p>
Add the following concept:
</p>

<blockquote><pre>
auto concept AllLiteral&lt; typename ... Types &gt; {
  requires LiteralType&lt;Types&gt;...;
}
</pre></blockquote>

<p>
ammend the constructor 
</p>

<blockquote><pre>
<ins>template &lt;class... UTypes&gt;
  requires AllLiteral&lt;Types...&gt;
        &amp;&amp; Constructible&lt;Types, UTypes&gt;...
  explicit tuple(UTypes...);</ins>

template &lt;class... UTypes&gt;
  requires <ins>!AllLiteral&lt;Types...&gt;</ins>
        <ins>&amp;&amp;</ins> Constructible&lt;Types, UTypes&amp;&amp;&gt;...
  explicit tuple(UTypes&amp;&amp;...);
</pre></blockquote>

<p>
ammend the constructor
</p>

<blockquote><pre>
<ins>template &lt;class... UTypes&gt;
  requires AllLiteral&lt;Types...&gt;
        &amp;&amp; Constructible&lt;Types, UTypes&gt;...
  tuple(tuple&lt;UTypes...&gt;);</ins>

template &lt;class... UTypes&gt;
  requires <ins>!AllLiteral&lt;Types...&gt;</ins>
        <ins>&amp;&amp;</ins> Constructible&lt;Types, const UTypes&amp;&gt;...
  tuple(const tuple&lt;UTypes...&gt;&amp;);
</pre></blockquote>

</blockquote>

<p>
Update the same signatures in 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a>, paras 3 and 5.
</p>





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