<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2857: {variant,optional,any}::emplace should return the constructed value</title>
<meta property="og:title" content="Issue 2857: {variant,optional,any}::emplace should return the constructed value">
<meta property="og:description" content="C++ library issue. Status: C++17">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2857.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++17">C++17</a> status.</em></p>
<h3 id="2857"><a href="lwg-defects.html#2857">2857</a>. <code>{variant,optional,any}::emplace</code> should return the constructed value</h3>
<p><b>Section:</b> 22.5 <a href="https://wg21.link/optional">[optional]</a>, 22.6 <a href="https://wg21.link/variant">[variant]</a>, 22.7 <a href="https://wg21.link/any">[any]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Zhihao Yuan <b>Opened:</b> 2017-01-25 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#optional">issues</a> in [optional].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When you want to continue operate on the new contained object constructed by <code>emplace</code>, you probably 
don't want to go through the type-safe machinery again.
</p>

<p><i>[2017-01-27 Telecon]</i></p>

<p>Priority 1; send to LEWG.</p>

<p><i>[Kona 2017-03-02]</i></p>

<p>Accepted as Immediate because P1.</p>


<p id="res-2857"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4618">N4618</a>.</p>

<ol>
<li><p>Update the following signatures in 22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a>, class template <code>optional</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 22.5.3.4 <a href="https://wg21.link/optional.assign">[optional.assign]</a>, assignment</i>
[&hellip;]
template &lt;class... Args&gt; <del>void</del><ins>T&amp;</ins> emplace(Args&amp;&amp;...);
template &lt;class U, class... Args&gt;
  <del>void</del><ins>T&amp;</ins> emplace(initializer_list&lt;U&gt;, Args&amp;&amp;...);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 22.5.3.4 <a href="https://wg21.link/optional.assign">[optional.assign]</a> as indicated:</p>
<blockquote>
<pre>
template &lt;class... Args&gt; <del>void</del><ins>T&amp;</ins> emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-27- <i>Postconditions:</i> <code>*this</code> contains a value.
<p/>
<ins>-?- <i>Returns:</i> A reference to the new contained value.</ins>
<p/>
-28- <i>Throws:</i> Any exception thrown by the selected constructor of <code>T</code>.
</p>
</blockquote>
<pre>
template &lt;class U, class... Args&gt; <del>void</del><ins>T&amp;</ins> emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-31- <i>Postconditions:</i> <code>*this</code> contains a value.
<p/>
<ins>-?- <i>Returns:</i> A reference to the new contained value.</ins>
<p/>
-32- <i>Throws:</i> Any exception thrown by the selected constructor of <code>T</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify the following signatures in 22.6.3 <a href="https://wg21.link/variant.variant">[variant.variant]</a>, class template <code>variant</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 22.6.3.5 <a href="https://wg21.link/variant.mod">[variant.mod]</a>, modifiers</i>
template &lt;class T, class... Args&gt; <del>void</del><ins>T&amp;</ins> emplace(Args&amp;&amp;...);
template &lt;class T, class U, class... Args&gt;
  <del>void</del><ins>T&amp;</ins> emplace(initializer_list&lt;U&gt;, Args&amp;&amp;...);
template &lt;size_t I, class... Args&gt; <del>void</del><ins>variant_alternative_t&lt;I, variant&lt;Types...&gt;&gt;&amp;</ins> emplace(Args&amp;&amp;...);
template &lt;size_t I, class U, class... Args&gt;
  <del>void</del><ins>variant_alternative_t&lt;I, variant&lt;Types...&gt;&gt;&amp;</ins> emplace(initializer_list&lt;U&gt;, Args&amp;&amp;...);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 22.6.3.5 <a href="https://wg21.link/variant.mod">[variant.mod]</a> as indicated:</p>

<blockquote>
<pre>
template &lt;class T, class... Args&gt; <del>void</del><ins>T&amp;</ins> emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> Equivalent to <code><ins>return</ins> emplace&lt;<i>I</i>&gt;(std::forward&lt;Args&gt;(args)...)<ins>;</ins></code> 
where <code><i>I</i></code> is the zero-based index of <code>T</code> in <code>Types...</code>.
<p/>
[&hellip;]
</p>
</blockquote>

<pre>
template &lt;class T, class U, class... Args&gt;
  <del>void</del><ins>T&amp;</ins> emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-3- <i>Effects:</i> Equivalent to <code><ins>return</ins> emplace&lt;<i>I</i>&gt;(il, std::forward&lt;Args&gt;(args)...)<ins>;</ins></code> 
where <code><i>I</i></code> is the zero-based index of <code>T</code> in <code>Types...</code>.
<p/>
[&hellip;]
</p>
</blockquote>

<pre>
template &lt;size_t I, class... Args&gt; <del>void</del><ins>variant_alternative_t&lt;I, variant&lt;Types...&gt;&gt;&amp;</ins> emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-7- <i>Postconditions:</i> <code>index()</code> is <code>I</code>.
<p/>
<ins>-?- <i>Returns:</i> A reference to the new contained value.</ins>
<p/>
-8- <i>Throws:</i> Any exception thrown during the initialization of the contained value.
<p/>
[&hellip;]
</p>
</blockquote>

<pre>
template &lt;size_t I, class U, class... Args&gt;
  <del>void</del><ins>variant_alternative_t&lt;I, variant&lt;Types...&gt;&gt;&amp;</ins> emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-12- <i>Postconditions:</i> <code>index()</code> is <code>I</code>.
<p/>
<ins>-?- <i>Returns:</i> A reference to the new contained value.</ins>
<p/>
-13- <i>Throws:</i> Any exception thrown during the initialization of the contained value.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify the following signatures in 22.7.4 <a href="https://wg21.link/any.class">[any.class]</a>, class <code>any</code> synopsis, as indicated:</p>
<blockquote>
<pre>
[&hellip;]
<i>// 22.7.4.4 <a href="https://wg21.link/any.modifiers">[any.modifiers]</a>, modifiers</i>
template &lt;class ValueType, class... Args&gt;
  <del>void</del><ins>decay_t&lt;ValueType&gt;&amp;</ins> emplace(Args&amp;&amp; ...);
template &lt;class ValueType, class U, class... Args&gt;
  <del>void</del><ins>decay_t&lt;ValueType&gt;&amp;</ins> emplace(initializer_list&lt;U&gt;, Args&amp;&amp;...);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 22.7.4.4 <a href="https://wg21.link/any.modifiers">[any.modifiers]</a> as indicated:</p>
<blockquote>
<pre>
template &lt;class ValueType, class... Args&gt;
  <del>void</del><ins>decay_t&lt;ValueType&gt;&amp;</ins> emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-4- <i>Postconditions:</i> <code>*this</code> contains a value.
<p/>
<ins>-?- <i>Returns:</i> A reference to the new contained value.</ins>
<p/>
-5- <i>Throws:</i> Any exception thrown by the selected constructor of <code>T</code>.
<p/>
[&hellip;]
</p>
</blockquote>

<pre>
template &lt;class ValueType, class U, class... Args&gt;
  <del>void</del><ins>decay_t&lt;ValueType&gt;&amp;</ins> emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-10- <i>Postconditions:</i> <code>*this</code> contains a value.
<p/>
<ins>-?- <i>Returns:</i> A reference to the new contained value.</ins>
<p/>
-11- <i>Throws:</i> Any exception thrown by the selected constructor of <code>T</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

</ol>





</body>
</html>
