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<title>Issue 720: Omissions in constexpr usages</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#CD1">CD1</a> status.</em></p>
<h3 id="720"><a href="lwg-defects.html#720">720</a>. Omissions in constexpr usages</h3>
<p><b>Section:</b> 23.3.3 <a href="https://wg21.link/array">[array]</a>, 22.9.2 <a href="https://wg21.link/template.bitset">[template.bitset]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2007-08-25 <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#array">issues</a> in [array].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<ol>
<li>
The member function <code>bool array&lt;T,N&gt;::empty() const</code> should be a
<code>constexpr</code> because this is easily to proof and to implement following it's operational
semantics defined by Table 87 (Container requirements) which says: <code>a.size() == 0</code>.
</li>
<li>
The member function <code>bool bitset&lt;N&gt;::test() const</code> must be a
<code>constexpr</code> (otherwise it would violate the specification of <code>constexpr
bitset&lt;N&gt;::operator[](size_t) const</code>, because it's return clause delegates to <code>test()</code>).
</li>
<li>
I wonder how the constructor <code>bitset&lt;N&gt;::bitset(unsigned long)</code> can
be declared as a <code>constexpr</code>. Current implementations usually have no such <code>bitset</code>
c'tor which would fulfill the requirements of a <code>constexpr</code> c'tor because they have a
non-empty c'tor body that typically contains for-loops or <code>memcpy</code> to compute the
initialisation. What have I overlooked here?
</li>
</ol>

<p><i>[
Sophia Antipolis:
]</i></p>


<blockquote>
<p>
We handle this as two parts
</p>
<ol>
<li>
The proposed resolution is correct; move to ready.
</li>
<li>
The issue points out a real problem, but the issue is larger than just
this solution. We believe a paper is needed, applying the full new
features of C++ (including extensible literals) to update <code>std::bitset</code>.
We note that we do not consider this new work, and that is should be
handled by the Library Working Group.
</li>
</ol>
<p>
In order to have a consistent working paper, Alisdair and Daniel produced a new wording for the resolution.
</p>
</blockquote>



<p id="res-720"><b>Proposed resolution:</b></p>
<ol>
<li>
<p>In the class template definition of 23.3.3 <a href="https://wg21.link/array">[array]</a>/p. 3 change</p>
<blockquote><pre>
<ins>constexpr</ins> bool empty() const;
</pre></blockquote>
</li>

<li>
<p>In the class template definition of 22.9.2 <a href="https://wg21.link/template.bitset">[template.bitset]</a>/p. 1 change</p>
<blockquote><pre>
<ins>constexpr</ins> bool test(size_t pos ) const;
</pre></blockquote>

<p>
and in 22.9.2.3 <a href="https://wg21.link/bitset.members">[bitset.members]</a> change
</p>

<blockquote><pre>
<ins>constexpr</ins> bool test(size_t pos ) const;
</pre></blockquote>

</li>
</ol>





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