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<title>Allowing Virtual Function Calls in Constant Expressions</title>
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<body class="article">
<div id="header">
<h1>Allowing Virtual Function Calls 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>P1064R0</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 paper proposes allowing virtual function calls 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="#_qa">Q&amp;A</a>
<ul class="sectlevel2">
<li><a href="#_can_a_constexpr_virtual_function_override_a_non_constexpr_one">Can a <code>constexpr</code> virtual function override a non-<code>constexpr</code> one?</a></li>
<li><a href="#_can_a_non_constexpr_virtual_function_override_a_constexpr_one">Can a non-<code>constexpr</code> virtual function override a <code>constexpr</code> one?</a></li>
<li><a href="#_what_happens_when_some_overriders_are_constexpr_and_some_are_not">What happens when some overriders are <code>constexpr</code> and some are not?</a></li>
</ul>
</li>
<li><a href="#_motivating_example">Motivating Example</a></li>
<li><a href="#_implementability">Implementability</a></li>
<li><a href="#_further_work">Further Work</a></li>
<li><a href="#_acknowledgments">Acknowledgments</a></li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_summary">Summary</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Virtual function calls are currently prohibited in constant expressions. Since
in a constant expression the dynamic type of the object is required to be known
(in order to, for example, diagnose undefined behavior in casts), the restriction
is unnecessary and artificial. We propose the restriction be removed.</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>Remove the first bullet of 10.1.5 [dcl.constexpr] p3:</em></span></p>
</div>
<div class="paragraph">
<p><span class="red">— it shall not be virtual (13.3);</span></p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_qa">Q&amp;A</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_can_a_constexpr_virtual_function_override_a_non_constexpr_one">Can a <code>constexpr</code> virtual function override a non-<code>constexpr</code> one?</h3>
<div class="paragraph">
<p>Yes. This is required when, for instance, the programmer has no control
over the base class, or when the base virtual function is pure.</p>
</div>
</div>
<div class="sect2">
<h3 id="_can_a_non_constexpr_virtual_function_override_a_constexpr_one">Can a non-<code>constexpr</code> virtual function override a <code>constexpr</code> one?</h3>
<div class="paragraph">
<p>Yes.</p>
</div>
</div>
<div class="sect2">
<h3 id="_what_happens_when_some_overriders_are_constexpr_and_some_are_not">What happens when some overriders are <code>constexpr</code> and some are not?</h3>
<div class="paragraph">
<p>The final overrider is selected, as usual, and if it&#8217;s not <code>constexpr</code>, the
expression is not a constant expression.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_motivating_example">Motivating Example</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The standard library provides, in [syserr], a very well-designed framework for
reporting error codes. This framework is not perfect, and can be enhanced in
several ways, two of which we&#8217;ll use to illustrate the motivation for this
proposal.</p>
</div>
<div class="paragraph">
<p>First, <code>error_code</code> is not <code>constexpr</code>-"enabled", and it&#8217;s useful for it to be
not just because this would make it usable in constant expressions, but because
<code>constexpr</code>-"enabled" types tend to optimize better, and <code>error_code</code> is used
in performance-sensitive contexts and by performance-sensitive audiences.</p>
</div>
<div class="paragraph">
<p>Adding the necessary <code>constexpr</code> qualifiers to <code>error_code</code> and making it a
literal type isn&#8217;t hard, as it&#8217;s basically a pair of an integer and a pointer.
We end up with</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>class error_code
{
private:

    int val_;
    const error_category* cat_;

public:

    constexpr error_code() noexcept;
    constexpr error_code(int val, const error_category&amp; cat) noexcept;
    template&lt;class ErrorCodeEnum&gt;
    constexpr error_code(ErrorCodeEnum e) noexcept;

    constexpr void assign(int val, const error_category&amp; cat) noexcept;
    template&lt;class ErrorCodeEnum&gt;
    constexpr error_code&amp; operator=(ErrorCodeEnum e) noexcept;
    constexpr void clear() noexcept;

    constexpr int value() const noexcept;
    constexpr const error_category&amp; category() const noexcept;
    constexpr explicit operator bool() const noexcept;

    error_condition default_error_condition() const noexcept;
    string message() const;
};</code></pre>
</div>
</div>
<div class="paragraph">
<p>A second enhancement we might wish to pursue is to address the limitation of
<code>error_code</code> of hardcoding zero as the success value. There are error categories
that consider all nonnegative values successful, and there are (admittedly very rare)
others in which zero is a failure. To address this, we might delegate the
responsibility of deciding whether a value represents a success to the error
category, by adding a virtual member function <code>failed</code> to it:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>class error_category
{
public:

    // ...

    virtual bool failed(int ev) const noexcept;

    // ...
};</code></pre>
</div>
</div>
<div class="paragraph">
<p>Then we add a member function <code>failed</code> to <code>error_code</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>class error_code
{
public:

    // ...

    bool failed() const noexcept { return cat_-&gt;failed(val_); }

    // ...
};</code></pre>
</div>
</div>
<div class="paragraph">
<p>Unfortunately, we can&#8217;t pursue both of these at the same time. Since <code>error_code::failed</code>
calls the virtual <code>error_category::failed</code>, it can&#8217;t be <code>constexpr</code>. We are forced to
choose, and there is no inherent reason for this choice being forced on us.</p>
</div>
<div class="paragraph">
<p>With this proposal, we just declare both <code>failed</code> functions as <code>constexpr</code> and everything
sorts itself out.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_implementability">Implementability</h2>
<div class="sectionbody">
<div class="paragraph">
<p>We have produced a fairly trivial patch for Clang that is successfully able
to compile the following example:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>struct X1
{
    virtual int f() const = 0;
};

struct X2: public X1
{
    constexpr virtual int f() const { return 2; }
};

struct X3: public X2
{
    virtual int f() const { return 3; }
};

struct X4: public X3
{
    constexpr virtual int f() const { return 4; }
};

constexpr int (X1::*pf)() const = &amp;X1::f;

constexpr X2 x2;
static_assert( x2.f() == 2 );
static_assert( (x2.*pf)() == 2 );

constexpr X1 const&amp; r2 = x2;
static_assert( r2.f() == 2 );
static_assert( (r2.*pf)() == 2 );

constexpr X1 const* p2 = &amp;x2;
static_assert( p2-&gt;f() == 2 );
static_assert( (p2-&gt;*pf)() == 2 );

constexpr X4 x4;
static_assert( x4.f() == 4 );
static_assert( (x4.*pf)() == 4 );

constexpr X1 const&amp; r4 = x4;
static_assert( r4.f() == 4 );
static_assert( (r4.*pf)() == 4 );

constexpr X1 const* p4 = &amp;x4;
static_assert( p4-&gt;f() == 4 );
static_assert( (p4-&gt;*pf)() == 4 );</code></pre>
</div>
</div>
<div class="paragraph">
<p>At the time of writing, our proof of concept does not yet handle virtual calls
in constructors and destructors properly, but we expect extending it to be
correct in this area to not pose any significant problems.</p>
</div>
<div class="paragraph">
<p>On the other hand, covariant returns appear to work under our implementation,
even though we have made no specific effort to handle them, as evidenced by
the following example compiling without errors:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>struct X1
{
    constexpr virtual X1 const* f() const { return this; }
};

struct Y
{
    int m = 0;
};

struct X2: public Y, public X1
{
    constexpr virtual X2 const* f() const { return this; }
};

constexpr X1 x1;
static_assert( x1.f() == &amp;x1 );

constexpr X2 x2;
constexpr X1 const&amp; r2 = x2;
static_assert( r2.f() == &amp;r2 );</code></pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_further_work">Further Work</h2>
<div class="sectionbody">
<div class="paragraph">
<p>[expr.const] p2 disallows <code>dynamic_cast</code> and <code>typeid</code> on polymorphic types.
These restrictions are also unnecessary for the same reason; compilers already
maintain the dynamic type information required to resolve them. It would be
a natural extension of this proposal to eliminate those two restrictions as well.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_acknowledgments">Acknowledgments</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The authors thank Richard Smith for his help.</p>
</div>
</div>
</div>
</div>
<div id="footer">
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Last updated 2018-05-05 18:23:16 GTBDT
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