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  <title>Disallow Binding a Returned glvalue to a Temporary</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Disallow Binding a Returned
glvalue to a Temporary</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2748R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2023-01-12</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      Evolution<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Brian Bi<br>&lt;<a href="mailto:bbi10@bloomberg.net" class="email">bbi10@bloomberg.net</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#introduction" id="toc-introduction"><span class="toc-section-number">1</span> Introduction</a></li>
<li><a href="#background" id="toc-background"><span class="toc-section-number">2</span> Background</a></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">3</span> Proposal</a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">4</span> Wording</a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">5</span> References</a></li>
</ul>
</div>
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>The following code contains a bug: It initializes a reference from an
object of a different type (the programmer has forgotten that the first
element of the pair is <code class="sourceCode default">const</code>),
resulting in the creation of a temporary. As a result, the reference
<code class="sourceCode default">d_first</code> is always dangling:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> X <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>    <span class="kw">const</span> std<span class="op">::</span>map<span class="op">&lt;</span>std<span class="op">::</span>string, <span class="dt">int</span><span class="op">&gt;</span> d_map;</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>    <span class="kw">const</span> std<span class="op">::</span>pair<span class="op">&lt;</span>std<span class="op">::</span>string, <span class="dt">int</span><span class="op">&gt;&amp;</span> d_first;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>    X<span class="op">(</span><span class="kw">const</span> std<span class="op">::</span>map<span class="op">&lt;</span>std<span class="op">::</span>string, <span class="dt">int</span><span class="op">&gt;&amp;</span> map<span class="op">)</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>        <span class="op">:</span> d_map<span class="op">(</span>map<span class="op">)</span>, d_first<span class="op">(*</span>d_map<span class="op">.</span>begin<span class="op">())</span> <span class="op">{}</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>Luckily, the above code is actually ill formed (<span>11.9.3
<a href="https://wg21.link/class.base.init">[class.base.init]</a></span>/8).
But here is a piece of code that contains essentially the same bug:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> Y <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>    std<span class="op">::</span>map<span class="op">&lt;</span>std<span class="op">::</span>string, <span class="dt">int</span><span class="op">&gt;</span> d_map;</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>    <span class="kw">const</span> std<span class="op">::</span>pair<span class="op">&lt;</span>std<span class="op">::</span>string, <span class="dt">int</span><span class="op">&gt;&amp;</span> first<span class="op">()</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a>        <span class="cf">return</span> <span class="op">*</span>d_map<span class="op">.</span>begin<span class="op">()</span>;</span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>This code is valid, although compilers might warn. Like the first
piece of code in this paper, this code snippet always produces a
dangling reference. We should make it likewise ill formed.</p>
<h1 data-number="2" id="background"><span class="header-section-number">2</span> Background<a href="#background" class="self-link"></a></h1>
<p>In <span class="citation" data-cites="CWG1696">[<a href="#ref-CWG1696" role="doc-biblioref">CWG1696</a>]</span>, Richard
Smith pointed out that, while binding a reference member to a temporary
in a <em>mem-initializer</em> was explicitly called out in the Standard
as one of the cases where the lifetime of the temporary is <em>not</em>
extended to the lifetime of the reference, no corresponding wording was
offered for the case in which the expression that produces the temporary
is supplied by a default member initializer.</p>
<p>Initially, the proposed resolution simply resolved the inconsistency
in favor of explicitly specifying that
<em>brace-or-equal-initializers</em> behave the same way as
<em>mem-initializers</em> (i.e., neither extends lifetime). However, at
the Issaquah meeting in 2014, making both ill formed was suggested. CWG
appears to have accepted this suggestion without controversy. (At the
Urbana-Champaign meeting later that year, Issue 1696 was given DR
status).</p>
<p>This change was so uncontroversial because binding a reference to a
temporary, when the reference will outlive the temporary and become
dangling as soon as the full-expression completes, is always a bug. In
some simple cases, a novice programmer might not understand that a
temporary must be materialized when binding a reference to a prvalue. On
the other hand, the examples given in the introduction represent code
that experienced C++ developers can easily write.</p>
<h1 data-number="3" id="proposal"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Just as the dangling reference created by
<code class="sourceCode default">X</code>’s constructor is always a bug,
the same is true for the dangling reference created by
<code class="sourceCode default">Y::first</code>. In fact, I can imagine
some obscure situations in which binding a reference member to a
temporary in a <em>mem-initializer</em> could be useful to cache the
result of an expensive computation, which could then be used by later
<em>mem-initializers</em> and within the <em>compound-statement</em> of
the constructor. In contrast, when binding a returned glvalue to a
temporary, even such obscure, limited applications seem nonexistent.</p>
<p>I propose, therefore, to make binding a returned glvalue to a
temporary ill formed, and I submit that the case for making this change
is as strong as — if not stronger than — the case for making binding a
reference member to a temporary in a <em>mem-initializer</em> ill
formed, a decision that apparently did not engender any recorded
controversy.</p>
<h1 data-number="4" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>The proposed wording is relative to <span class="citation" data-cites="N4917">[<a href="#ref-N4917" role="doc-biblioref">N4917</a>]</span>.</p>
<p>Strike bullet (6.11) in section <span>6.7.7
<a href="https://wg21.link/class.temporary">[class.temporary]</a></span>:</p>
<blockquote>
<ul>
<li><span class="rm" style="color: #bf0303"><del>The lifetime of a
temporary bound to the returned value in a function
<span><code class="sourceCode default">return</code></span> statement
(8.7.4) is not extended; the temporary is destroyed at the end of the
full-expression in the
<span><code class="sourceCode default">return</code></span>
statement.</del></span></li>
</ul>
</blockquote>
<p>Insert a new paragraph, 6, at the end of section <span>8.7.4
<a href="https://wg21.link/stmt.return">[stmt.return]</a></span>:</p>
<blockquote>
<div class="add" style="color: #006e28">
<p>In a function whose return type is a reference, a
<code class="sourceCode default">return</code> statement that binds the
returned reference to a temporary expression (<span>6.7.7
<a href="https://wg21.link/class.temporary">[class.temporary]</a></span>)
is ill-formed.<br />
[<em>Example 2:</em></p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>auto&amp;&amp; f1() {</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>    return 42;  // ill-formed</span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>const double&amp; f2() {</span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>    static int x = 42;</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a>    return x;   // ill-formed</span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a>auto&amp;&amp; id(auto&amp;&amp; r) {</span>
<span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a>    return static_cast&lt;decltype(r)&amp;&amp;&gt;(r);</span>
<span id="cb3-10"><a href="#cb3-10" aria-hidden="true" tabindex="-1"></a>}</span>
<span id="cb3-11"><a href="#cb3-11" aria-hidden="true" tabindex="-1"></a>auto&amp;&amp; f3() {</span>
<span id="cb3-12"><a href="#cb3-12" aria-hidden="true" tabindex="-1"></a>    return id(42);  // OK, but probably a bug</span>
<span id="cb3-13"><a href="#cb3-13" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
<p>— <em>end example</em>]</p>
</div>
</blockquote>
<p>(<em>Note:</em> See <span class="citation" data-cites="CWG_GH200">[<a href="#ref-CWG_GH200" role="doc-biblioref">CWG GitHub issue
200</a>]</span> regarding a possible issue with the above wording.)</p>
<h1 data-number="5" id="bibliography"><span class="header-section-number">5</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" role="doc-bibliography">
<div id="ref-CWG_GH200" class="csl-entry" role="doc-biblioentry">
[CWG GitHub issue 200] Brian Bi. 2022-12-16. Missing definition of
<span>“temporary expression.”</span> <a href="https://github.com/cplusplus/CWG/issues/200"><div class="csl-block">https://github.com/cplusplus/CWG/issues/200</div></a>
</div>
<div id="ref-CWG1696" class="csl-entry" role="doc-biblioentry">
[CWG1696] Richard Smith. 2013-05-31. Temporary lifetime and non-static
data member initializers. <a href="http://wg21.link/CWG1696"><div class="csl-block">http://wg21.link/CWG1696</div></a>
</div>
<div id="ref-N4917" class="csl-entry" role="doc-biblioentry">
[N4917] Thomas Köppe. 2022-09-05. Working Draft, Standard for
Programming Language C++. <a href="http://wg21.link/N4917"><div class="csl-block">http://wg21.link/N4917</div></a>
</div>
</div>
</div>
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