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<title>Allowing dynamic_cast, polymorphic typeid in Constant Expressions</title>
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<body class="article">
<div id="header">
<h1>Allowing <code>dynamic_cast</code>, polymorphic <code>typeid</code> in Constant Expressions</h1>
<div class="details">
<span id="author" class="author">Peter Dimov &lt;pdimov@pdimov.com&gt;, Vassil Vassilev &lt;vvasilev@cern.ch&gt;</span><br>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="hdlist">
<table>
<tr>
<td class="hdlist1">
Document Number
</td>
<td class="hdlist2">
<p>P1327R0</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Date
</td>
<td class="hdlist2">
<p>2018-10-28</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Project
</td>
<td class="hdlist2">
<p>Programming Language C++</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Audience
</td>
<td class="hdlist2">
<p>Evolution Working Group</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Summary
</td>
<td class="hdlist2">
<p>This paper proposes allowing <code>dynamic_cast</code> and polymorphic <code>typeid</code> in constant expressions.</p>
</td>
</tr>
</table>
</div>
<div id="toc" class="toc">
<div id="toctitle" class="title">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_summary">Summary</a></li>
<li><a href="#_proposed_changes">Proposed Changes</a></li>
<li><a href="#_motivating_example">Motivating Example</a></li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_summary">Summary</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://wg21.link/p1064r0">P1064</a>, accepted at Rapperswil, relaxed the
restriction of virtual function calls in constant expressions. These are
now allowed, a capability made possible by the fact that compilers are
effectively required to track the dynamic type of constant expressions.</p>
</div>
<div class="paragraph">
<p>It would be natural, for the same underlying reasons, to also drop the
restriction of <code>dynamic_cast</code> and <code>typeid</code> (applied to polymorphic glvalues)
not appearing in constant expressions. P1064 mentions this in its Further
Work section, and it is our impression that the EWG in Rapperswil was
generally approving of the idea.</p>
</div>
<div class="paragraph">
<p>We conservatively did not propose this in P1064 (our mistake) because we did
not have a motivating example, but we do now.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_proposed_changes">Proposed Changes</h2>
<div class="sectionbody">
<div class="paragraph">
<p><span class="navy"><em>(All edits are relative to N4778.)</em></span></p>
</div>
<div class="paragraph">
<p><span class="navy"><em>Remove the 2.15, 2.19 bullets of 7.7 [expr.const] p2:</em></span></p>
</div>
<div class="paragraph">
<p><span class="red  line-through">(2.15)&#8201;&#8212;&#8201;a dynamic cast (7.6.1.7);</span></p>
</div>
<div class="paragraph">
<p><span class="red  line-through">(2.19)&#8201;&#8212;&#8201;a typeid expression (7.6.1.8) whose operand is a glvalue of a polymorphic class type;</span></p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_motivating_example">Motivating Example</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The papers <a href="https://wg21.link/p1196r0">P1196</a>, <a href="https://wg21.link/p1197r0">P1197</a>,
<a href="https://wg21.link/p1198r0">P1198</a> all propose improvements to
<code>std::error_category</code> that add new members (P1196) or new virtual functions
(P1197, P1198) to it.</p>
</div>
<div class="paragraph">
<p>Unfortunately, changing a standard polymorphic type is an ABI break, and
library implementers are increasingly unwilling to break ABI even in small
quantities.</p>
</div>
<div class="paragraph">
<p>This presents a significant and frustrating obstacle to our efforts to evolve
and improve the standard library.</p>
</div>
<div class="paragraph">
<p>One alternative approach is to avoid changing <code>std::error_category</code>, but
instead derive <code>std::error_category_v2</code> from it, which could now hold the
additional members and virtuals.</p>
</div>
<div class="paragraph">
<p>With this done, <code>std::error_code</code> could use <code>dynamic_cast</code> to check whether
it has been constructed from a "new" category, and adjust accordingly.</p>
</div>
<div class="paragraph">
<p>This, however, clashes with the proposal in <a href="https://wg21.link/p1195r0">P1195</a>
to add <code>constexpr</code> to <code>error_code</code> and friends. We&#8217;re again forced to choose
between one desirable improvement and another, and can&#8217;t have nice things.</p>
</div>
<div class="paragraph">
<p>The ability to use <code>dynamic_cast</code> in constant expressions will solve this
problem.</p>
</div>
<div class="paragraph">
<p><em>-- end</em></p>
</div>
</div>
</div>
</div>
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Last updated 2018-11-02 19:17:40 GTBST
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