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        auto(x): decay-copy in the language - HackMD
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<tr><th>Doc. no.:</th>    <td>P0849R1</td></tr>
<tr><th>Date:</th>    <td>2019-06-03</td></tr>
<tr><th>Audience:</th>    <td>EWG, LEWG</td></tr>
<tr><th>Reply-to:</th>    <td>Zhihao Yuan &lt;zy at miator dot net&gt;</td></tr>
</tbody></table><h1 id="autox-decay-copy-in-the-language"><a class="anchor hidden-xs" href="#autox-decay-copy-in-the-language" title="autox-decay-copy-in-the-language"><span class="octicon octicon-link"></span></a>auto(x): <em>decay-copy</em> in the language</h1><h2 id="Changes-Since-R0"><a class="anchor hidden-xs" href="#Changes-Since-R0" title="Changes-Since-R0"><span class="octicon octicon-link"></span></a>Changes Since R0</h2><ul>
<li>updated examples</li>
<li>discussed <code>decltype(auto)(x)</code></li>
<li>added library wording</li>
</ul><h2 id="Introduction"><a class="anchor hidden-xs" href="#Introduction" title="Introduction"><span class="octicon octicon-link"></span></a>Introduction</h2><p>This paper proposes <code>auto(x)</code> and <code>auto{x}</code> for casting <code>x</code> into a prvalue value as if passing <code>x</code> as a function argument by value.  The functionality is realized as the <em><code>decay-copy</code></em> function in the standard for exposition only.</p><h2 id="Motivation"><a class="anchor hidden-xs" href="#Motivation" title="Motivation"><span class="octicon octicon-link"></span></a>Motivation</h2><h3 id="Obtaining-a-prvalue-copy-is-necessary"><a class="anchor hidden-xs" href="#Obtaining-a-prvalue-copy-is-necessary" title="Obtaining-a-prvalue-copy-is-necessary"><span class="octicon octicon-link"></span></a>Obtaining a prvalue copy is necessary</h3><p>A generic way to obtain a copy of an object in C++ is <code>auto a = x;</code> but such a copy is an lvalue.  We could often convey the purpose in code more accurately if we can obtain a copy as a prvalue.  In the following example, let <code>Container</code> be a concept,</p><pre><code class="c++ hljs"><span class="hljs-selector-tag">void</span> <span class="hljs-selector-tag">pop_front_alike</span>(<span class="hljs-selector-tag">Container</span> <span class="hljs-selector-tag">auto</span>&amp; <span class="hljs-selector-tag">x</span>) {
    <span class="hljs-attribute">std</span>::<span class="hljs-built_in">erase</span>(x.begin(), x.<span class="hljs-built_in">end</span>(), <span class="hljs-built_in">auto</span>(x.front()));
}
</code></pre><p>If we wrote</p><pre><code class="c++ hljs"><span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">pop_front_alike</span><span class="hljs-params">(Container <span class="hljs-keyword">auto</span>&amp; x)</span> </span>{
    <span class="hljs-keyword">auto</span> a = x.front();
    <span class="hljs-built_in">std</span>::erase(x.begin(), x.end(), a);
}
</code></pre><p>, questions arise – why this is not equivalent to</p><pre><code class="c++ hljs"><span class="hljs-selector-tag">void</span> <span class="hljs-selector-tag">pop_front_alike</span>(<span class="hljs-selector-tag">Container</span> <span class="hljs-selector-tag">auto</span>&amp; <span class="hljs-selector-tag">x</span>) {
    <span class="hljs-attribute">std</span>::<span class="hljs-built_in">erase</span>(x.begin(), x.<span class="hljs-built_in">end</span>(), x.<span class="hljs-built_in">front</span>());
}
</code></pre><p>The problem is, the statement to obtain an lvalue copy is a declaration:</p><pre><code class="c++ hljs"><span class="hljs-attribute">    auto a</span> = x.front();
</code></pre><p>Its primary purpose is to declare <code>a</code> while the fact that <code>a</code> ends up being a copy is a property of this declaration.  In contrast, the expression to obtain an rvalue copy is a clear command to perform a copy:</p><pre><code class="c++ hljs">    auto(<span class="hljs-name">x</span>.front())
</code></pre><p>One might argue that the above is indifferent from</p><pre><code class="c++ hljs">    T(<span class="hljs-name">x</span>.front())
</code></pre><p>However, there are plenty of situations that the <code>T</code> is nontrivial to get.  We probably don’t want to write the original example as</p><pre><code class="c++ hljs"><span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">pop_front_alike</span><span class="hljs-params">(Container <span class="hljs-keyword">auto</span>&amp; x)</span> </span>{
    <span class="hljs-keyword">using</span> T = <span class="hljs-built_in">std</span>::<span class="hljs-keyword">decay_t</span>&lt;<span class="hljs-keyword">decltype</span>(x.front())&gt;;
    <span class="hljs-built_in">std</span>::erase(x.begin(), x.end(), T(x.front()));
}
</code></pre><h3 id="Obtaining-a-prvalue-copy-with-autox-works-always"><a class="anchor hidden-xs" href="#Obtaining-a-prvalue-copy-with-autox-works-always" title="Obtaining-a-prvalue-copy-with-autox-works-always"><span class="octicon octicon-link"></span></a>Obtaining a prvalue copy with <code>auto(x)</code> works always</h3><p>In standard library specification, we use the following exposition only function to fulfill the role of <code>auto(x)</code>:</p><pre><code class="c++ hljs"><span class="hljs-keyword">template</span>&lt;<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">T</span>&gt;
<span class="hljs-title">constexpr</span> <span class="hljs-title">decay_t</span>&lt;T&gt; <span class="hljs-title">decay_copy</span>(<span class="hljs-title">T</span>&amp;&amp; <span class="hljs-title">v</span>) <span class="hljs-title">noexcept</span>(
    <span class="hljs-title">is_nothrow_convertible_v</span>&lt;T, decay_t&lt;T&gt;&gt;) {</span>
    <span class="hljs-keyword">return</span> <span class="hljs-built_in">std</span>::forward&lt;T&gt;(v);
}
</code></pre><p>This definition involves templates, dependent <code>constexpr</code> , forwarding reference, <code>noexcept</code>, and two traits, and still has caveats if people want to use it in practice.  An obvious issue is that <code>decay_copy(x.front())</code> creates a copy of <code>x.front()</code> even if <code>x.front()</code> is already a prvalue (thus, already a copy).</p><p>There is a less obvious issue which needs a minimal reproduce:</p><pre><code class="c++ hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">A</span> {</span>
    <span class="hljs-keyword">int</span> x;

<span class="hljs-keyword">public</span>:
    A();

    <span class="hljs-function"><span class="hljs-keyword">auto</span> <span class="hljs-title">run</span><span class="hljs-params">()</span> </span>{
        f(A(*<span class="hljs-keyword">this</span>));           <span class="hljs-comment">// ok</span>
        f(<span class="hljs-keyword">auto</span>(*<span class="hljs-keyword">this</span>));        <span class="hljs-comment">// ok as proposed</span>
        f(decay_copy(*<span class="hljs-keyword">this</span>));  <span class="hljs-comment">// ill-formed</span>
    }

<span class="hljs-keyword">protected</span>:
    A(<span class="hljs-keyword">const</span> A&amp;);
};
</code></pre><p>The problem is that <code>decay_copy</code> is nobody’s friend.  We can use <code>A</code> directly in this specific example, but in a more general setting, where a type <code>A</code> has access to a set of type <code>T</code>'s private or protected copy/move constructors, <em><code>decay-copy</code></em> an object of <code>T</code> fails inside <code>A</code>'s class scope, but <code>auto(x)</code> continues to work.</p><h2 id="Discussion"><a class="anchor hidden-xs" href="#Discussion" title="Discussion"><span class="octicon octicon-link"></span></a>Discussion</h2><h3 id="autox-is-a-missing-piece"><a class="anchor hidden-xs" href="#autox-is-a-missing-piece" title="autox-is-a-missing-piece"><span class="octicon octicon-link"></span></a><code>auto(x)</code> is a missing piece</h3><p>Replacing the <code>char</code> in <code>char('a')</code> with <code>auto</code>, we obtain <code>auto('a')</code>, which is a function-style cast.  Such a formula also supports <em>injected-class-names</em> and class template argument deduction in C++17.  Introducing <code>auto(x)</code> and <code>auto{x}</code> significantly improves the language consistency:</p><table>
<thead>
<tr>
<th>variable definition</th>
<th>function-style cast</th>
<th>new expression</th>
</tr>
</thead>
<tbody>
<tr>
<td>auto v(x);</td>
<td><mark>auto(x)</mark></td>
<td>new auto(x)</td>
</tr>
<tr>
<td>auto v{x};</td>
<td><mark>auto{x}</mark></td>
<td>new auto{x}</td>
</tr>
<tr>
<td>ClassTemplate v(x);</td>
<td>ClassTemplate(x)</td>
<td>new ClassTemplate(x)</td>
</tr>
<tr>
<td>ClassTemplate v{x};</td>
<td>ClassTemplate{x}</td>
<td>new ClassTemplate{x}</td>
</tr>
</tbody>
</table><p>** <em>The type of <code>x</code> is a specialization of <code>ClassTemplate</code>.</em></p><p>With this proposal, all the cells in the table copy construct form <code>x</code> (given CTAD’s default behavior) to obtain lvalues, prvalues, and pointers to objects, categorized by their columns.  Defining <code>auto(x)</code> as a library<sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup> facility loses orthogonality.</p><p>Introducing <code>auto(x)</code> into the language even improves the library consistency:</p><table>
<thead>
<tr>
<th>type function style</th>
<th>expression style</th>
</tr>
</thead>
<tbody>
<tr>
<td>void_t&lt;decltype(<em>expr</em>)&gt;</td>
<td>decltype(void(<em>expr</em>))</td>
</tr>
<tr>
<td>decay_t&lt;decltype(<em>expr</em>)&gt;</td>
<td><mark>decltype(auto(<em>expr</em>))</mark></td>
</tr>
</tbody>
</table><h3 id="decltypeautox-has-a-different-motivation"><a class="anchor hidden-xs" href="#decltypeautox-has-a-different-motivation" title="decltypeautox-has-a-different-motivation"><span class="octicon octicon-link"></span></a><code>decltype(auto){x}</code> has a different motivation</h3><p>Should we also allow <code>decltype(auto){x}</code>?  Interestingly, this is not motivated only for consistency.  <code>decltype(auto){x}</code> is equivalent to <code>static_cast&lt;decltype(x)&gt;(x)</code>.  If <code>x</code> is a variable of type <code>T&amp;&amp;</code>, <code>x</code> is an lvalue but <code>static_cast&lt;T&amp;&amp;&gt;(x)</code> is an xvalue.  Yes, this is a <code>std::forward</code>, arguably better, because 1. it does not repeat a type parameter; 2. it can properly “forward” a prvalue if that is accidentally requested.  However, these properties all have pros and cons.</p><p>In a word, <code>decltype(auto){x}</code> is motivated differently from <code>auto(x)</code>.  A different paper, such as P0644<sup class="footnote-ref"><a href="#fn2" id="fnref2">[2]</a></sup>, may be a better place to discuss it.</p><h2 id="Implementation"><a class="anchor hidden-xs" href="#Implementation" title="Implementation"><span class="octicon octicon-link"></span></a>Implementation</h2><p>Try it out: <a href="https://godbolt.miator.net/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAKxAEZSAbAQwDtRkBSAJgCFufSAZ1QBXYskwgA5NwDMeFsgYisAag6yAwi0wB3Ddg4AGAILGTBTAFsADs0vqtBAJ43MLJlcyqAKgfMAbqh46KoAZhB%2BXABs3NEAlBp8puYKBKpWTAoQiSkA7MkmqsWqyAhMxKpMHACsfDUAIo5N3FwseBKtSeYlqmmlqCyCBHGqlRpNACxc3aa9EUwiRBBM8YmyhfMrS6gcBcR7DeuFPSWLywAe8c2qYFJMd7NFJSKCCsCq6ROqWEoubttlqtjuZQVx5GFVEZTsUFjsciC5iU4UQ9nw8kcnltzqgIFxSLI1ljkYDdgV6DMMYizEjijpdFUdmjDklejDVPTGUDqb1OTi0ZSGsS6XoubiBYceSU%2BfD8YThRzRfyCvjJiyNmDZO58GFQRipPFGNIalJSCxpEZTahpJp%2BPxVMIxBJ1ODaKaCBaDYaANYgGpGI1SSam81SS2ka1SU2CEABj1hg2kOCwJBoWx4BiYMgUCBpmwZrMgKzOJSsYCkMIZyzEGMQABGntIdYUFWc0jdpDTXhYBAA8iwGG2E6QsJk2JnG/hiJhkAQ8AFMDHh5gLjOlpIpB20pgGI2CMQ8FZPYbmGwUHbeIw8HWY5BDagbHPBkuALQAdSYDAYqhfw3QE04Xh%2BC4Jho1EcRJFoE9jRDRtI2LUs2FUCBcEIEgXVkehVE0VB00zcZXWuW0gN4d1jx9P0A13INYOHSNo1jUh40tNZAy4Wjw3opjyNIBcazwQYQEmIA" target="_blank" rel="noopener">Godbolt</a></p><h2 id="Wording"><a class="anchor hidden-xs" href="#Wording" title="Wording"><span class="octicon octicon-link"></span></a>Wording</h2><p>The wording is relative to N4810.</p><p>Modify 7.6.1.3 [expr.type.conv]/1 as indicated:</p><blockquote>
<p>A <em>simple-type-specifier</em> (9.1.7.2) or <em>typename-specifier</em> (13.7) followed by a parenthesized optional <em>expression-list</em> or by a <em>braced-init-list</em> (the initializer) constructs a value of the specified type given the initializer. If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by
overload resolution for class template deduction (12.3.1.8) for the remainder of this section. <ins>Otherwise, if the type is <code>auto</code>, it is replaced by the type deduced for the variable <code>x</code> in the invented
declaration (9.1.7.5):</ins></p>
</blockquote><p><ins></ins></p><pre>auto x <em>init</em>;</pre><p></p><blockquote>
<p><ins>, where  <em>init</em> is the initializer.</ins></p>
</blockquote><p>Modify 9.1.7.5 [dcl.spec.auto]/5 as indicated:</p><blockquote>
<p>A placeholder type can also be used in the <em>type-specifier-seq</em> in the <em>new-type-id</em> or <em>type-id</em> of a <em>new-expression</em>
(7.6.2.5) and as a <em>decl-specifier</em> of the <em>parameter-declaration</em>’s <em>decl-specifier-seq</em> in a <em>template-parameter</em>
(13.1).  <ins>The <code>auto</code> <em>type-specifier</em> can also be used as the <em>simple-type-specifier</em> in an explicit type conversion (functional notation) (7.6.1.3).</ins></p>
</blockquote><p>Remove 16.4.2.1 [expos.only.func]/2:</p><blockquote>
<p><s>The following function is defined for exposition only to aid in the specification of the library:</s></p>
</blockquote><pre><code><s>template&lt;class T&gt; constexpr decay_t&lt;T&gt; <i>decay-copy</i>(T&amp;&amp; v)
    noexcept(is_nothrow_convertible_v&lt;T, decay_t&lt;T&gt;&gt;)     // <i>exposition only</i>
  { return std::forward&lt;T&gt;(v); }
</s></code></pre><p>Search and replace “<s>calls to <em><code>decay-copy</code></em> being evaluated in</s>” (the thread/the current thread) with “<ins>where the values produced by <code>auto</code> are materialized in</ins>”.</p><p>Search and replace “<s><em><code>decay-copy</code></em></s>” with “<ins><code>auto</code></ins>”.</p><h2 id="Acknowledgments"><a class="anchor hidden-xs" href="#Acknowledgments" title="Acknowledgments"><span class="octicon octicon-link"></span></a>Acknowledgments</h2><p>Thank Alisdair Meredith, Arthur O’Dwyer, and Billy O’Neal for providing examples and feedback for this paper.</p><h2 id="References"><a class="anchor hidden-xs" href="#References" title="References"><span class="octicon octicon-link"></span></a>References</h2><hr class="footnotes-sep"><section class="footnotes">
<ol class="footnotes-list">
<li id="fn1" class="footnote-item"><p>Krügler, Daniel. P0758R0 <em>Implicit conversion traits and utility functions</em>.
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0758r0.html" target="_blank" rel="noopener">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0758r0.html</a> <a href="#fnref1" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn2" class="footnote-item"><p>Revzin, Barry. P0644R1 <em>Forward without <code>forward</code></em>.
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0644r1.html" target="_blank" rel="noopener">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0644r1.html</a> <a href="#fnref2" class="footnote-backref">↩︎</a></p>
</li>
</ol>
</section></div>
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