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<title>Allowing Virtual Destructors to be "Trivial"</title>
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</head>
<body class="article">
<div id="header">
<h1>Allowing Virtual Destructors to be "Trivial"</h1>
<div class="details">
<span id="author" class="author">Peter Dimov</span><br>
<span id="email" class="email"><a href="mailto:pdimov@pdimov.com">pdimov@pdimov.com</a></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>P1077R0</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Date
</td>
<td class="hdlist2">
<p>2018-05-04</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 proposal removes the requirement for literal types to not have virtual destructors</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="#_motivation">Motivation</a></li>
<li><a href="#_proposed_changes">Proposed Changes</a></li>
<li><a href="#_example">Example</a></li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_motivation">Motivation</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Virtual destructors are currently non-trivial, by definition ([class.dtor] p6). This is overly
strict.</p>
</div>
<div class="paragraph">
<p>The informal meaning of trivial here is "has no observable effects and can be omitted",
but there are two contexts in which this question can be asked. First, whether the destructor
itself has no observable effects; second, whether an expression invoking the destructor (such as
<code>p-&gt;~X()</code>) has no observable effects.</p>
</div>
<div class="paragraph">
<p>In the first case, it doesn&#8217;t matter whether the destructor is virtual; it either has observable
effects or it doesn&#8217;t, and its virtuality doesn&#8217;t change the outcome.</p>
</div>
<div class="paragraph">
<p>In the second case, it does matter. When <code>~X</code> is virtual, the expression may end up invoking a
derived destructor, and whether <code>~X</code> itself has no effects is irrelevant.</p>
</div>
<div class="paragraph">
<p>Conflating these two meanings creates problems in two places. First, literal types are required
to have trivial destructors. This needlessly prohibits types with virtual destructors from being
literal. For example,</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>struct X1
{
    virtual int f() const;
};</code></pre>
</div>
</div>
<div class="paragraph">
<p>is literal, but</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>struct X2
{
    virtual ~X2() = default;
    virtual int f() const;
};</code></pre>
</div>
</div>
<div class="paragraph">
<p>isn&#8217;t.</p>
</div>
<div class="paragraph">
<p>Second, it classifies certain destructors as non-trivial even when they satisfy the requirement
that an expression invoking them has no effects:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>struct X3
{
    X2 x2;
};</code></pre>
</div>
</div>
<div class="paragraph">
<p><code>~X3</code> in this example is not virtual and has no effects, but the transitive requirement for <code>~X2</code>
to be trivial renders <code>~X3</code> nontrivial as well.</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 N4741.)</em></span></p>
</div>
<div class="paragraph">
<p><span class="navy"><em>Change 15.4 [class.dtor] p6 as follows:</em></span></p>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="openblock">
<div class="content">
<div class="paragraph">
<p>A destructor is <span class="green  underline">quasi-</span> trivial if it is not user-provided and if:</p>
</div>
<div class="paragraph">
<p><span class="red  line-through">— the destructor is not virtual,</span></p>
</div>
<div class="paragraph">
<p>— all of the direct base classes of its class have <span class="green  underline">quasi-</span> trivial destructors, and</p>
</div>
<div class="paragraph">
<p>— for all of the non-static data members of its class that are of class type (or array thereof), each such
class has a <span class="green  underline">quasi-</span> trivial destructor.</p>
</div>
<div class="paragraph">
<p>Otherwise, the destructor is not <span class="green  underline">quasi-</span> trivial.</p>
</div>
</div>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p><span class="navy"><em>Add a new paragraph below the above one:</em></span></p>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="openblock">
<div class="content">
<div class="paragraph">
<p><span class="green">A destructor is trivial if it is quasi-trivial and it is not virtual.</span></p>
</div>
</div>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p><span class="navy"><em>Change 6.7 [basic.type] p10 bullet 10.5.1 as follows:</em></span></p>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="openblock">
<div class="content">
<div class="paragraph">
<p>— it has a <span class="green  underline">quasi-</span> trivial destructor,</p>
</div>
</div>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_example">Example</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The standard library provides, in [syserr], a framework for reporting error codes. This
framework classifies error codes into categories, represented by singleton values of types
derived from the base class <code>error_category</code>.</p>
</div>
<div class="paragraph">
<p><code>error_category</code>, and types derived from it such as <code>generic_category</code>, could easily be made
literal if not for the fact that <code>~error_category</code> is specified as virtual. This precludes
categories to be made <code>constexpr</code> variables, a problem this proposal solves.</p>
</div>
<div class="paragraph">
<p>The reason for the wish to make categories <code>constexpr</code> is subtle. Category equality is
currently specified as identity comparison, <code>this == &amp;rhs</code>. But there are platforms on
which it&#8217;s hard to guarantee that all instances of <code>generic_category</code> and <code>system_category</code>
have the same address. On such a platform it&#8217;s better for equality to be content-based rather
than address-based, for instance by storing an unique identifier in the category, so that all
<code>generic_category</code> instances compare equal.</p>
</div>
</div>
</div>
</div>
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