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<title>Issue 2345: integer_sequence should have a self-typedef ::type</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#NAD">NAD</a> status.</em></p>
<h3 id="2345"><a href="lwg-closed.html#2345">2345</a>. <code>integer_sequence</code> should have a self-typedef <code>::type</code></h3>
<p><b>Section:</b> 21.2.2 <a href="https://wg21.link/intseq.intseq">[intseq.intseq]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2013-11-01 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>2
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
21.3.4 <a href="https://wg21.link/meta.help">[meta.help]</a> says that <code>integral_constant&lt;T, v&gt;</code> provides <code>::value_type</code> (for <code>T</code>) 
and <code>::type</code> (for itself).
<p/>
21.2.2 <a href="https://wg21.link/intseq.intseq">[intseq.intseq]</a> says that <code>integer_sequence&lt;T, I...&gt;</code> provides <code>::value_type</code> (for <code>T</code>), 
but nothing for itself.
<p/>
Self-typedefs can be useful when users create chains of derived classes, then want to get the Standard base type.  
This is especially relevant to <code>integer_sequence</code>, as variadic templates encourage recursive inheritance.
</p>

<p><i>[2014-02-13 Issaquah: Close as NAD]</i></p>

<p>
AJM: My own implementation used a different alias for types representing parameter packs, and specifically
did <em>not</em> define <code>type</code>.  I tried it both ways, and found bugs more quickly when <code>type</code> was
not defined.
</p>



<p id="res-2345"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3797.</p>

<ol>
<li><p>Edit 21.2.2 <a href="https://wg21.link/intseq.intseq">[intseq.intseq]</a> as indicated:</p>

<blockquote><pre>
namespace std {
  template&lt;class T, T... I&gt;
  struct integer_sequence {
    typedef T value_type;
    <ins>typedef integer_sequence&lt;T, I...&gt; type;</ins>
    static constexpr size_t size() noexcept { return sizeof...(I); }
  };
}
</pre></blockquote>
</li>

</ol>






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