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        Compound Literals - HackMD
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<tr><th>Doc. no.:</th>    <td>P2174R0</td></tr>
<tr><th>Date:</th>    <td>2020-5-15</td></tr>
<tr><th>Audience:</th>    <td>EWG Incubator</td></tr>
<tr><th>Reply-to:</th>    <td>Zhihao Yuan &lt;zy at miator dot net&gt;</td></tr>
</tbody></table><h1 id="Compound-Literals" style=""><a class="anchor hidden-xs" href="#Compound-Literals" title="Compound-Literals"><span class="octicon octicon-link"></span></a>Compound Literals</h1><p>This paper proposes to standardize an existing practice, namely compound literals, available in GCC and Clang for C++ language modes.  It shares the syntax with C99 compound literals but produces prvalue.</p><h2 id="Motivation" style=""><a class="anchor hidden-xs" href="#Motivation" title="Motivation"><span class="octicon octicon-link"></span></a>Motivation</h2><p>When teaching <em>new-expression</em>s, the author found that it is useful to let a student correlate creating an object of automatic storage and an object of dynamic storage:</p><pre><code class="cpp hljs"><span class="token keyword">struct</span> Point
<span class="token punctuation">{</span>
    <span class="token keyword">int</span> x<span class="token punctuation">,</span> y<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">new</span> <span class="token function">Point</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span>  <span class="token comment">// returns a pointer to a Point object on heap, because</span>
    <span class="token function">Point</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span>  <span class="token comment">// is an anonymous Point object on stack, while</span>
  Point <span class="token function">a</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// can give a name to such an object</span>
</code></pre><p>But this encounters some trouble when applying to dynamic arrays. In hypothesis, the author wants to see</p><pre><code class="cpp hljs"><span class="token keyword">new</span> <span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span><span class="token number">.3</span><span class="token punctuation">,</span> <span class="token number">.2</span><span class="token punctuation">,</span> <span class="token number">.1</span><span class="token punctuation">}</span>  <span class="token comment">// returns a pointer to a dynamic array, because</span>
    <span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span><span class="token number">.3</span><span class="token punctuation">,</span> <span class="token number">.2</span><span class="token punctuation">,</span> <span class="token number">.1</span><span class="token punctuation">}</span>  <span class="token comment">// is an anonymous array on stack, while</span>
  <span class="token keyword">double</span> a<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span><span class="token number">.3</span><span class="token punctuation">,</span> <span class="token number">.2</span><span class="token punctuation">,</span> <span class="token number">.1</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// can give a name to such an array</span>
</code></pre><p>There is no such grammar for the second line, but it is entirely acceptable to spell such an expression as follows.</p><pre><code class="cpp hljs"><span class="token punctuation">(</span><span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token number">.3</span><span class="token punctuation">,</span> <span class="token number">.2</span><span class="token punctuation">,</span> <span class="token number">.1</span><span class="token punctuation">}</span>
</code></pre><p>Because:</p><ol>
<li>It is intuitive to group tokens with parenthesis.</li>
<li>It works already in GCC and Clang.</li>
<li>C has the same syntax.</li>
</ol><p>Some people, including experts, believe that it is a part of C++ because all relevant C++ compilers, except MSVC, support the proposed syntax. We occasionally find open-source contributors reverting uses of compound literals in people’s C++ code.</p><p>However, the compilers powered by the EDG frontend seem to produce lvalues for the proposed syntax. In the following code snippet,</p><pre><code class="cpp hljs"><span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">{</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre><p><code>x</code> is of type <code>double(&amp;&amp;)[3]</code> in GCC and Clang, but is of type <code>double(&amp;)[3]</code> in NVCC and Intel C++ Compiler. Hence, there is a widely spread existing practice, but with a slightly diverging behavior, which can be better-off if standardized.</p><h2 id="Discussion" style=""><a class="anchor hidden-xs" href="#Discussion" title="Discussion"><span class="octicon octicon-link"></span></a>Discussion</h2><h3 id="There-is-a-workaround-for-it…" style=""><a class="anchor hidden-xs" href="#There-is-a-workaround-for-it…" title="There-is-a-workaround-for-it…"><span class="octicon octicon-link"></span></a>There is a workaround for it…</h3><pre><code class="cpp hljs"><span class="token keyword">using</span> arr_t <span class="token operator">=</span> <span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x <span class="token operator">=</span> arr_t<span class="token punctuation">{</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre><p>It’s silly.</p><h3 id="Why-don’t-you-define-an-alias-template" style=""><a class="anchor hidden-xs" href="#Why-don’t-you-define-an-alias-template" title="Why-don’t-you-define-an-alias-template"><span class="octicon octicon-link"></span></a>Why don’t you define an alias template?</h3><pre><code class="cpp hljs"><span class="token keyword">template</span><span class="token operator">&lt;</span><span class="token keyword">class</span> <span class="token class-name">T</span><span class="token operator">&gt;</span> <span class="token keyword">using</span> id <span class="token operator">=</span> T<span class="token punctuation">;</span>
<span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x <span class="token operator">=</span> id<span class="token operator">&lt;</span><span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token operator">&gt;</span><span class="token punctuation">{</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre><p>It’s not only silly but also expert only. The author has never seen the code outside <a href="http://eel.is/c++draft/class.temporary#6.example-1" target="_blank" rel="noopener">[class.temporary]/6.8</a>.</p><p>Let’s focus more on the portability aspect of our situation.</p><h3 id="Why-producing-prvalue-rather-than-closely-matching-C99" style=""><a class="anchor hidden-xs" href="#Why-producing-prvalue-rather-than-closely-matching-C99" title="Why-producing-prvalue-rather-than-closely-matching-C99"><span class="octicon octicon-link"></span></a>Why producing prvalue rather than closely matching C99?</h3><p>Given the fact that a typedef followed by <em>braced-init-list</em> produces prvalue in C++,</p><pre><code class="cpp hljs"><span class="token keyword">using</span> arr_t <span class="token operator">=</span> <span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x <span class="token operator">=</span> arr_t<span class="token punctuation">{</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>  <span class="token comment">// double(&amp;&amp;)[3]</span>
</code></pre><p>it would be a <a href="https://www.destroyallsoftware.com/talks/wat" target="_blank" rel="noopener">wat</a> for a parentasised typedef followed by <em>braced-init-list</em> to produce lvalue:</p><pre><code class="cpp hljs"><span class="token keyword">using</span> arr_t <span class="token operator">=</span> <span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x <span class="token operator">=</span> <span class="token punctuation">(</span>arr_t<span class="token punctuation">)</span><span class="token punctuation">{</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// double(&amp;)[3] ??</span>
</code></pre><p>To quote Richard,</p><blockquote>
<p>If we allow this syntax, I think the only consistent and rational semantic model is for <code>T{inits}</code> to mean the same thing as <code>(T){inits}</code>, and we should be cognizant that we are somewhat diverging from C’s semantics in adopting that model.</p>
</blockquote><p>The “Initializer lists”<sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup> paper discussed an additional issue in ch. 5.5.</p><h2 id="Wording" style=""><a class="anchor hidden-xs" href="#Wording" title="Wording"><span class="octicon octicon-link"></span></a>Wording</h2><p>The wording is relative to N4861.</p><p>Extend the grammar:</p><blockquote>
<p><em>cast-expression</em>:<br>
&nbsp;&nbsp;<em>unary-expression</em><br>
&nbsp;&nbsp;<code>(</code> <em>type-id</em> <code>)</code> <em>cast-expression</em><br>
&nbsp;&nbsp;<ins><code>(</code> <em>type-id</em> <code>)</code> <em>braced-init-list</em></ins></p>
</blockquote><p>Insert a new paragraph after <a href="http://eel.is/c++draft/expr.cast#1" target="_blank" rel="noopener">[expr.cast]/1</a>:</p><blockquote>
<p>The result of the expression <code>(T)</code> <em>cast-expression</em> is of type <code>T</code>. The result is an lvalue if <code>T</code> is an lvalue reference type or an rvalue reference to function type and an xvalue if <code>T</code> is an rvalue reference to object type; otherwise the result is a prvalue. <em>[Note</em>: […] — <em>end note]</em><br>
<ins>If an expression is of form <code>(</code> <em>type-id</em> <code>)</code> <em>braced-init-list</em>, let <span class="mathjax"><span class="MathJax_Preview" style="color: inherit;"></span><span id="MathJax-Element-9-Frame" class="mjx-chtml MathJax_CHTML" tabindex="0" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mi>i</mi><mi>n</mi><mi>i</mi><mi>t</mi></math>" role="presentation" style="font-size: 113%; position: relative;"><span id="MJXc-Node-37" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-38" class="mjx-mrow"><span id="MJXc-Node-39" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.279em;">i</span></span><span id="MJXc-Node-40" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.224em; padding-bottom: 0.279em;">n</span></span><span id="MJXc-Node-41" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.279em;">i</span></span><span id="MJXc-Node-42" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.39em; padding-bottom: 0.279em;">t</span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>i</mi><mi>n</mi><mi>i</mi><mi>t</mi></math></span></span><script type="math/tex" id="MathJax-Element-9">init</script></span> be the <em>braced-init-list</em> and <code>T</code> be the <em>type-id</em>. The expression is equivalent to <code>T</code> <span class="mathjax"><span class="MathJax_Preview" style="color: inherit;"></span><span id="MathJax-Element-10-Frame" class="mjx-chtml MathJax_CHTML" tabindex="0" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mi>i</mi><mi>n</mi><mi>i</mi><mi>t</mi></math>" role="presentation" style="font-size: 113%; position: relative;"><span id="MJXc-Node-43" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-44" class="mjx-mrow"><span id="MJXc-Node-45" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.279em;">i</span></span><span id="MJXc-Node-46" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.224em; padding-bottom: 0.279em;">n</span></span><span id="MJXc-Node-47" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.279em;">i</span></span><span id="MJXc-Node-48" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.39em; padding-bottom: 0.279em;">t</span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>i</mi><mi>n</mi><mi>i</mi><mi>t</mi></math></span></span><script type="math/tex" id="MathJax-Element-10">init</script></span> (<a href="http://eel.is/c++draft/expr.type.conv" target="_blank" rel="noopener">[expr.type.conv]</a>).</ins></p>
</blockquote><h2 id="References" style=""><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>Stroustrup, Bjarne and Gabriel Dos Reis. N2215 <em>Initializer lists (Rev. 3)</em>.
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2215.pdf" target="_blank" rel="noopener">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2215.pdf</a> <a href="#fnref1" class="footnote-backref">↩︎</a></p>
</li>
</ol>
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