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<tr><th>Doc. no.:</th>    <td>P0325R4</td></tr>
<tr><th>Date:</th>    <td>2019-07-17</td></tr>
<tr><th>Audience:</th>    <td>LWG</td></tr>
<tr><th>Reply-to:</th>    <td>Zhihao Yuan &lt;zy at miator dot net&gt;</td></tr>
</tbody></table><h1 id="to_array-from-LFTS-with-updates"><a class="anchor hidden-xs" href="#to_array-from-LFTS-with-updates" title="to_array-from-LFTS-with-updates"><span class="octicon octicon-link"></span></a><code>to_array</code> from LFTS with updates</h1><h2 id="Changes-since-R3"><a class="anchor hidden-xs" href="#Changes-since-R3" title="Changes-since-R3"><span class="octicon octicon-link"></span></a>Changes since R3</h2><ul>
<li>simplify returns</li>
<li>mandates and expects</li>
<li>highlight insertion in wording</li>
<li>add feature testing macro</li>
</ul><h2 id="Changes-since-R2"><a class="anchor hidden-xs" href="#Changes-since-R2" title="Changes-since-R2"><span class="octicon octicon-link"></span></a>Changes since R2</h2><ul>
<li>switch back to <code>{{ a[</code><span class="mathjax"><span class="MathJax_Preview" style="color: inherit;"></span><span class="MathJax" id="MathJax-Element-1-Frame" tabindex="0" style="position: relative;" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mi>i</mi></math>" role="presentation"><nobr aria-hidden="true"><span class="math" id="MathJax-Span-1" style="width: 0.38em; display: inline-block;"><span style="display: inline-block; position: relative; width: 0.323em; height: 0px; font-size: 116%;"><span style="position: absolute; clip: rect(1.602em, 1000.28em, 2.597em, -1000em); top: -2.425em; left: 0em;"><span class="mrow" id="MathJax-Span-2"><span class="mi" id="MathJax-Span-3" style="font-family: MathJax_Math; font-style: italic;">i</span></span><span style="display: inline-block; width: 0px; height: 2.425em;"></span></span></span><span style="display: inline-block; overflow: hidden; vertical-align: -0.075em; border-left: 0px solid; width: 0px; height: 0.905em;"></span></span></nobr><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>i</mi></math></span></span><script type="math/tex" id="MathJax-Element-1">i</script></span><code>]... }}</code> in specification</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 to adopt <code>to_array</code> from Library Fundamentals TS along with an rvalue-reference overload based on the feedback from the users.</p><pre><code class="c++ hljs"><span class="hljs-keyword">auto</span> lv = to_array(<span class="hljs-string">"foo"</span>);               <span class="hljs-comment">// array&lt;char, 4&gt;</span>
<span class="hljs-keyword">auto</span> rv = to_array&lt;<span class="hljs-keyword">int</span>&gt;({ <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span> })    <span class="hljs-comment">// array&lt;int, 2&gt;</span>
</code></pre><h2 id="Discussion"><a class="anchor hidden-xs" href="#Discussion" title="Discussion"><span class="octicon octicon-link"></span></a>Discussion</h2><p>In C++17 we introduced deduction guide on <code>std::array</code>:</p><pre><code class="c++ hljs">auto a = array{ <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span> };
</code></pre><p>But it is impossible to create an <code>array&lt;char, 4&gt;</code> from <code>array{ "foo" }</code> because otherwise, one won’t be able to explain why <code>array{ "foo", "bar" }</code> creates <code>array&lt;char const*, 2&gt;</code>.  So one overload set, despite whether it’s <code>std::array</code> deduction guide or <code>make_array</code> from LFTSv2, is not enough to serve the purpose of “creating a <code>std::array</code> from elements” and “creating a <code>std::array</code> by copying from a built-in array” at the same time.  That is why<sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup> we introduced <code>to_array</code> in LFTSv2:</p><pre><code class="c++ hljs"><span class="hljs-built_in">auto</span> a1 = to_array(<span class="hljs-string">"foo"</span>);          // <span class="hljs-built_in">array</span>&lt;<span class="hljs-built_in">char</span>, <span class="hljs-number">4</span>&gt;
<span class="hljs-built_in">auto</span> a2 = <span class="hljs-built_in">array</span>{ <span class="hljs-string">"foo"</span> };           // <span class="hljs-built_in">array</span>&lt;<span class="hljs-built_in">char</span> <span class="hljs-keyword">const</span>*, <span class="hljs-number">1</span>&gt;
</code></pre><p>For the same reason, <code>to_array</code> should be adopted into the IS.</p><p>However, the original version of <code>to_array</code> only takes an lvalue reference-to-array, and users reported<sup class="footnote-ref"><a href="#fn2" id="fnref2">[2]</a></sup> some issues regarding this design.  The first issue is that it becomes very hackish to deduce only the array bound from a <em>braced-init-list</em>:</p><pre><code class="c++ hljs"><span class="hljs-built_in">auto</span> x = to_array&lt;<span class="hljs-built_in">int</span> <span class="hljs-keyword">const</span>&gt;({ <span class="hljs-number">2</span>, <span class="hljs-number">4</span> });     // <span class="hljs-built_in">array</span>&lt;<span class="hljs-built_in">int</span>, <span class="hljs-number">2</span>&gt;
</code></pre><p>The language treats <em>braced-init-list</em> as a prvalue of an array type when deducing against reference-to-array.  The code above adds <code>const</code> to coin a reference that can bind to such a prvalue, and leaves the <code>const</code> to be stripped later.  Anyhow, the code isn’t doing what it says.</p><p>The second issue is that it does not support move-only elements, such as <code>unique_ptr</code>, no matter how you hack.</p><p>So we should add the rvalue-reference overload to fill this hole regarding type system.</p><h2 id="Impact-on-the-Standard"><a class="anchor hidden-xs" href="#Impact-on-the-Standard" title="Impact-on-the-Standard"><span class="octicon octicon-link"></span></a>Impact on the Standard</h2><p>The following table illustrates the uses and the corresponding overload resolution after adopting the proposed addition.  Given</p><pre><code class="c++ hljs"><span class="hljs-keyword">int</span> a[<span class="hljs-number">3</span>] = {};
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">A</span> {</span> <span class="hljs-keyword">int</span> a; <span class="hljs-keyword">double</span> b; };
</code></pre><table>
<thead>
<tr>
<th>use</th>
<th>argument</th>
<th>overload</th>
<th>result</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>to_array("meow")</code></td>
<td>lvalue <code>char const[5]</code></td>
<td>lvalue ref</td>
<td><code>array&lt;char, 5&gt;</code></td>
</tr>
<tr>
<td><code>to_array({1, 2})</code></td>
<td>prvalue <code>int[2]</code></td>
<td>rvalue ref</td>
<td><code>array&lt;int, 2&gt;</code></td>
</tr>
<tr>
<td><code>to_array&lt;long&gt;({1, 2})</code></td>
<td>prvalue <code>long[2]</code></td>
<td>rvalue ref</td>
<td><code>array&lt;long, 2&gt;</code></td>
</tr>
<tr>
<td><code>to_array(a)</code></td>
<td>lvalue <code>int[3]</code></td>
<td>lvalue ref</td>
<td><code>array&lt;int, 3&gt;</code></td>
</tr>
<tr>
<td><code>to_array(std::move(a))</code></td>
<td>xvalue <code>int (&amp;&amp;)[3]</code></td>
<td>rvalue ref</td>
<td><code>array&lt;int, 3&gt;</code></td>
</tr>
<tr>
<td><code>to_array&lt;A&gt;({{3, .1}})</code></td>
<td>prvalue <code>A[1]</code></td>
<td>rvalue ref</td>
<td><code>array&lt;A, 1&gt;</code></td>
</tr>
<tr>
<td><code>to_array((int[]){3, 4})</code></td>
<td>C99 compound literal</td>
<td>see below</td>
<td><code>array&lt;int, 2&gt;</code></td>
</tr>
</tbody>
</table><p>The last entry is a valuable case to study.  Compound literals are lvalues in the C standards, but C does not have rvalue objects of compound types anyway.  Both Clang and GCC implement C99 compound literals as extensions in C++ and treat them as prvalues.  Let’s take a closer look at this feature,</p><pre><code class="c++ hljs">(<span class="hljs-name"><span class="hljs-builtin-name">int</span></span>[]){<span class="hljs-number">3</span>, <span class="hljs-number">4</span>}
</code></pre><p>It is an array literal formed by a C-style cast notation (like <code>(char)</code>) for an array type followed by an initializer list.  In the same way, we can interpret <code>to_array</code> as</p><pre><code class="c++ hljs"><span class="xml">to_array<span class="hljs-tag">&lt;<span class="hljs-name">int</span>&gt;</span>(</span><span class="hljs-template-variable">{3, 4}</span><span class="xml">)
</span></code></pre><p>a <code>std::array</code> literal formed by a C++ style cast notation (like <code>static_cast&lt;char&gt;(...)</code>) for the array element type surrounding an initializer list.</p><p>An interesting consequence of this proposal is that we no longer need <code>make_array</code>:</p><table>
<thead>
<tr>
<th>LFTSv2</th>
<th>C++20</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>make_array(1, 2, 3)</code></td>
<td><code>array{ 1, 2, 3 }</code></td>
</tr>
<tr>
<td><code>make_array&lt;long&gt;(1, 2, 3)</code></td>
<td><code>to_array&lt;long&gt;({ 1, 2, 3 })</code></td>
</tr>
</tbody>
</table><p>In the future, we may have <code>array&lt;long&gt;{ 1, 2, 3 }</code> to replace this specific use of <code>to_array</code>, but that will not defeat the primary purpose of <code>to_array</code> – a facility to create <code>std::array</code> by copying or moving from a built-in array.</p><p>Supplying the bound with <code>to_array&lt;T, N&gt;({ ... })</code> is more strict compared to <code>array&lt;T, N&gt;{ ... }</code>, because brace elision in aggregate initialization can sometimes give unexpected outcomes:</p><pre><code class="c++ hljs">using E = complex&lt;double&gt;;

<span class="hljs-comment">// Nicol's quiz time</span>
auto a = array&lt;E, <span class="hljs-number">10</span>&gt;{{<span class="hljs-number">1</span>, <span class="hljs-number">2</span>}, {<span class="hljs-number">3</span>, <span class="hljs-number">4</span>}};
auto b = array&lt;E, <span class="hljs-number">10</span>&gt;{{{<span class="hljs-number">1</span>, <span class="hljs-number">2</span>}, {<span class="hljs-number">3</span>, <span class="hljs-number">4</span>}}};
auto c = array&lt;E, <span class="hljs-number">10</span>&gt;{{<span class="hljs-number">1</span>, <span class="hljs-number">2</span>}};
</code></pre><h2 id="Implementation"><a class="anchor hidden-xs" href="#Implementation" title="Implementation"><span class="octicon octicon-link"></span></a>Implementation</h2><p>Godbolted: <a href="https://godbolt.org/g/GqbygN" target="_blank" rel="noopener">https://godbolt.org/g/GqbygN</a></p><h2 id="Wording"><a class="anchor hidden-xs" href="#Wording" title="Wording"><span class="octicon octicon-link"></span></a>Wording</h2><p>This wording is relative to N4820.</p><p>New section 22.3.7.6 [array.creation] (between [array.zero] and [array.tuple], which was 22.3.7.6):</p><h3 id="22376-Array-creation-functions-arraycreation"><a class="anchor hidden-xs" href="#22376-Array-creation-functions-arraycreation" title="22376-Array-creation-functions-arraycreation"><span class="octicon octicon-link"></span></a>22.3.7.6 Array creation functions [array.creation]</h3><pre><code class="c++ hljs">template&lt;class <span class="hljs-built_in">T</span>, size_t <span class="hljs-built_in">N</span>&gt;
  constexpr array&lt;remove_cv_t&lt;<span class="hljs-built_in">T</span>&gt;, <span class="hljs-built_in">N</span>&gt; to_array(<span class="hljs-built_in">T</span> (&amp;a)[<span class="hljs-built_in">N</span>]);
</code></pre><blockquote>
<p><em>Mandates:</em> <code>is_array_v&lt;T&gt;</code> is <code>false</code>and <code>is_constructible_v&lt;T, T&amp;&gt;</code> is <code>true</code>.</p>
<p><em>Expects:</em> <code>T</code> meets the <em>Cpp17CopyConstructible</em> requirements.</p>
<p><em>Returns:</em> <code>{{ a[0],</code> <span class="mathjax"><span class="MathJax_Preview" style="color: inherit;"></span><span class="MathJax" id="MathJax-Element-31-Frame" tabindex="0" style="position: relative;" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mo>.</mo><mo>.</mo><mo>.</mo></math>" role="presentation"><nobr aria-hidden="true"><span class="math" id="MathJax-Span-140" style="width: 1.404em; display: inline-block;"><span style="display: inline-block; position: relative; width: 1.185em; height: 0px; font-size: 116%;"><span style="position: absolute; clip: rect(2.089em, 1001.11em, 2.532em, -1000em); top: -2.371em; left: 0em;"><span class="mrow" id="MathJax-Span-141"><span class="mo" id="MathJax-Span-142" style="font-family: MathJax_Main;">.</span><span class="mo" id="MathJax-Span-143" style="font-family: MathJax_Main; padding-left: 0.167em;">.</span><span class="mo" id="MathJax-Span-144" style="font-family: MathJax_Main; padding-left: 0.167em;">.</span></span><span style="display: inline-block; width: 0px; height: 2.371em;"></span></span></span><span style="display: inline-block; overflow: hidden; vertical-align: -0.063em; border-left: 0px solid; width: 0px; height: 0.264em;"></span></span></nobr><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>.</mo><mo>.</mo><mo>.</mo></math></span></span><script type="math/tex" id="MathJax-Element-31">...</script></span><code>, a[N - 1] }}</code>.</p>
</blockquote><pre><code class="c++ hljs">template&lt;class <span class="hljs-built_in">T</span>, size_t <span class="hljs-built_in">N</span>&gt;
  constexpr array&lt;remove_cv_t&lt;<span class="hljs-built_in">T</span>&gt;, <span class="hljs-built_in">N</span>&gt; to_array(<span class="hljs-built_in">T</span> (&amp;&amp;a)[<span class="hljs-built_in">N</span>]);
</code></pre><blockquote>
<p><em>Mandates:</em> <code>is_array_v&lt;T&gt;</code> is <code>false</code>and <code>is_move_constructible_v&lt;T&gt;</code> is <code>true</code>.</p>
<p><em>Expects:</em> <code>T</code> meets the <em>Cpp17MoveConstructible</em> requirements.</p>
<p><em>Returns:</em> <code>{{ std::move(a[0]),</code> <span class="mathjax"><span class="MathJax_Preview" style="color: inherit;"></span><span class="MathJax" id="MathJax-Element-30-Frame" tabindex="0" style="position: relative;" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mo>.</mo><mo>.</mo><mo>.</mo></math>" role="presentation"><nobr aria-hidden="true"><span class="math" id="MathJax-Span-135" style="width: 1.404em; display: inline-block;"><span style="display: inline-block; position: relative; width: 1.185em; height: 0px; font-size: 116%;"><span style="position: absolute; clip: rect(2.089em, 1001.11em, 2.532em, -1000em); top: -2.371em; left: 0em;"><span class="mrow" id="MathJax-Span-136"><span class="mo" id="MathJax-Span-137" style="font-family: MathJax_Main;">.</span><span class="mo" id="MathJax-Span-138" style="font-family: MathJax_Main; padding-left: 0.167em;">.</span><span class="mo" id="MathJax-Span-139" style="font-family: MathJax_Main; padding-left: 0.167em;">.</span></span><span style="display: inline-block; width: 0px; height: 2.371em;"></span></span></span><span style="display: inline-block; overflow: hidden; vertical-align: -0.063em; border-left: 0px solid; width: 0px; height: 0.264em;"></span></span></nobr><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>.</mo><mo>.</mo><mo>.</mo></math></span></span><script type="math/tex" id="MathJax-Element-30">...</script></span><code>, std::move(a[N - 1]) }}</code>.</p>
</blockquote><p>Add those signatures to 22.3.2 [array.syn]:</p><pre><code>
namespace std {
  // 22.3.7, class template array
  template&lt;class T, size_t N&gt; struct array;
  
  …
  template&lt;class T, size_t N&gt;
    void swap(array&lt;T, N&gt;&amp; x, array&lt;T, N&gt;&amp; y) noexcept(noexcept(x.swap(y)));

  <ins>// 22.3.7.6, Array creation functions</ins>
  <ins>template&lt;class T, size_t N&gt;</ins>
    <ins>constexpr array&lt;remove_cv_t&lt;T&gt;, N&gt; to_array(T (&amp;a)[N]);</ins>
  <ins>template&lt;class T, size_t N&gt;</ins>
    <ins>constexpr array&lt;remove_cv_t&lt;T&gt;, N&gt; to_array(T (&amp;&amp;a)[N]);</ins>
  …
}
</code></pre><p>Add the following entry to Table 36 [tab:support.ft]:</p><table>
<thead>
<tr>
<th>Macro name</th>
<th>Value</th>
<th>Header(s)</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>__cpp_lib_to_array</code></td>
<td><code>xxxxxxL</code></td>
<td><code>&lt;array&gt;</code></td>
</tr>
</tbody>
</table><h2 id="Acknowledgements"><a class="anchor hidden-xs" href="#Acknowledgements" title="Acknowledgements"><span class="octicon octicon-link"></span></a>Acknowledgements</h2><p>Thank Peter Sommerlad for the inputs on 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>N4391 <em>make_array, revision 4</em>.
<a href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4391.html" target="_blank" rel="noopener">http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4391.html</a> <a href="#fnref1" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn2" class="footnote-item"><p>LWG 2814 <em><code>to_array</code> should take rvalue reference as well</em>.
<a href="https://cplusplus.github.io/LWG/issue2814" target="_blank" rel="noopener">https://cplusplus.github.io/LWG/issue2814</a> <a href="#fnref2" class="footnote-backref">↩︎</a></p>
</li>
</ol>
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        function windowResize() {
            //toc right
            var paddingRight = parseFloat(markdown.css('padding-right'));
            var right = ($(window).width() - (markdown.offset().left + markdown.outerWidth() - paddingRight));
            toc.css('right', right + 'px');
            //affix toc left
            var newbool;
            var rightMargin = (markdown.parent().outerWidth() - markdown.outerWidth()) / 2;
            //for ipad or wider device
            if (rightMargin >= 133) {
                newbool = true;
                var affixLeftMargin = (tocAffix.outerWidth() - tocAffix.width()) / 2;
                var left = markdown.offset().left + markdown.outerWidth() - affixLeftMargin;
                tocAffix.css('left', left + 'px');
            } else {
                newbool = false;
            }
            if (newbool != enoughForAffixToc) {
                enoughForAffixToc = newbool;
                generateScrollspy();
            }
        }
        $(window).resize(function () {
            windowResize();
        });
        $(document).ready(function () {
            windowResize();
            generateScrollspy();
        });

        //remove hash
        function removeHash() {
            window.location.hash = '';
        }

        var backtotop = $('.back-to-top');
        var gotobottom = $('.go-to-bottom');

        backtotop.click(function (e) {
            e.preventDefault();
            e.stopPropagation();
            if (scrollToTop)
                scrollToTop();
            removeHash();
        });
        gotobottom.click(function (e) {
            e.preventDefault();
            e.stopPropagation();
            if (scrollToBottom)
                scrollToBottom();
            removeHash();
        });

        var toggle = $('.expand-toggle');
        var tocExpand = false;

        checkExpandToggle();
        toggle.click(function (e) {
            e.preventDefault();
            e.stopPropagation();
            tocExpand = !tocExpand;
            checkExpandToggle();
        })

        function checkExpandToggle () {
            var toc = $('.ui-toc-dropdown .toc');
            var toggle = $('.expand-toggle');
            if (!tocExpand) {
                toc.removeClass('expand');
                toggle.text('Expand all');
            } else {
                toc.addClass('expand');
                toggle.text('Collapse all');
            }
        }

        function scrollToTop() {
            $('body, html').stop(true, true).animate({
                scrollTop: 0
            }, 100, "linear");
        }

        function scrollToBottom() {
            $('body, html').stop(true, true).animate({
                scrollTop: $(document.body)[0].scrollHeight
            }, 100, "linear");
        }
    </script>
</body>

</html>
