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‎issue4250.html‎

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<!DOCTYPE html>
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<title>Issue 4250: swap overloads for indirect and polymorphic only found by ADL</title>
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<meta property="og:title" content="Issue 4250: swap overloads for indirect and polymorphic only found by ADL">
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<meta property="og:description" content="C++ library issue. Status: New">
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<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#New">New</a> status.</em></p>
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<h3 id="4250"><a href="lwg-active.html#4250">4250</a>. <code class='backtick'>swap</code> overloads for <code class='backtick'>indirect</code> and <code class='backtick'>polymorphic</code> only found by ADL</h3>
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<p><b>Section:</b> 20.4.1.7 <a href="https://wg21.link/indirect.swap">[indirect.swap]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-05-01 <b>Last modified:</b> 2025-05-01</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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The non-member <code class='backtick'>swap</code> overloads for <code class='backtick'>std::indirect</code> and <code class='backtick'>std::polymorphic</code>
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are defined as hidden friends, so are only available via ADL.
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This means that calling <code class='backtick'>std::swap(i1, i2)</code> will always use the generic
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<code class='backtick'>std::swap</code> instead of the custom overload for <code class='backtick'>std::indirect</code>.
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</p>
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<p id="res-4250"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 20.2.2 <a href="https://wg21.link/memory.syn">[memory.syn]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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// 20.4.1, class template indirect
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template&lt;class T, class Allocator = allocator&lt;T&gt;&gt;
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class indirect;
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<ins>
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template&lt;class T, class Allocator&gt;
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constexpr void swap(indirect&lt;T, Allocator&gt;&amp; lhs, indirect&lt;T, Allocator&gt;&amp; rhs) noexcept(<i>see below</i>);
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</ins>
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// 20.4.1.10, hash support
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template&lt;class T, class Alloc&gt; struct hash&lt;indirect&lt;T, Alloc&gt;&gt;;
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// 20.4.2, class template polymorphic
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template&lt;class T, class Allocator = allocator&lt;T&gt;&gt;
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class polymorphic;
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<ins>
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template&lt;class T, class Allocator&gt;
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constexpr void swap(polymorphic&lt;T, Allocator&gt;&amp; lhs, polymorphic&lt;T, Allocator&gt;&amp; rhs) noexcept(<i>see below</i>);
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</ins>
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</code></pre>
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</blockquote>
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</li>
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<li><p>Modify 20.4.1.2 <a href="https://wg21.link/indirect.syn">[indirect.syn]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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// 20.4.1.7, swap
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constexpr void swap(indirect&amp; other) noexcept(<i>see below</i>);
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<del>friend constexpr void swap(indirect&amp; lhs, indirect&amp; rhs) noexcept(<i>see below</i>);</del>
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</code></pre>
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</blockquote>
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</li>
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<li><p>Modify 20.4.1.7 <a href="https://wg21.link/indirect.swap">[indirect.swap]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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<ins>template&lt;class T, class Allocator&gt;</ins>
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constexpr void swap(indirect<ins>&lt;T, Allocator&gt;</ins>&amp; lhs, indirect<ins>&lt;T, Allocator&gt;</ins>&amp; rhs) noexcept(noexcept(lhs.swap(rhs)));
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</code></pre>
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<blockquote>-3- <i>Effects</i>: Equivalent to <code class='backtick'>lhs.swap(rhs)</code>.</blockquote>
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</blockquote>
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</li>
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<li><p>Modify 20.4.2.2 <a href="https://wg21.link/polymorphic.syn">[polymorphic.syn]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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// 20.4.2.7, swap
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constexpr void swap(polymorphic&amp; other) noexcept(<i>see below</i>);
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<del>friend constexpr void swap(polymorphic&amp; lhs, polymorphic&amp; rhs) noexcept(<i>see below</i>);</del>
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</code></pre>
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</blockquote>
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</li>
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<li><p>Modify 20.4.2.7 <a href="https://wg21.link/polymorphic.swap">[polymorphic.swap]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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<ins>template&lt;class T, class Allocator&gt;</ins>
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constexpr void swap(polymorphic<ins>&lt;T, Allocator&gt;</ins>&amp; lhs, polymorphic<ins>&lt;T, Allocator&gt;</ins>&amp; rhs) noexcept(noexcept(lhs.swap(rhs)));
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</code></pre>
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<blockquote>-3- <i>Effects</i>: Equivalent to <code class='backtick'>lhs.swap(rhs)</code>.</blockquote>
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</blockquote>
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‎issue4251.html‎

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<!DOCTYPE html>
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<title>Issue 4251: Move assignment for indirect unnecessarily requires copy construction</title>
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<meta property="og:title" content="Issue 4251: Move assignment for indirect unnecessarily requires copy construction">
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<meta property="og:description" content="C++ library issue. Status: New">
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<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4251.html">
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<meta property="og:type" content="website">
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<hr>
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<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#New">New</a> status.</em></p>
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<h3 id="4251"><a href="lwg-active.html#4251">4251</a>. Move assignment for <code class='backtick'>indirect</code> unnecessarily requires copy construction</h3>
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<p><b>Section:</b> 20.4.1.5 <a href="https://wg21.link/indirect.asgn">[indirect.asgn]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-05-01 <b>Last modified:</b> 2025-05-01</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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The move assignment operator for <code class='backtick'>indirect</code> says:
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<blockquote>
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<i>Mandates</i>: <code>is_copy_constructible_t&lt;T&gt;</code> is <code class='backtick'>true</code>.
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</blockquote>
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However, the only way it ever construct an object is:
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<blockquote>
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constructs a new owned object with the owned object of <code class='backtick'>other</code> as the argument
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as an rvalue
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</blockquote>
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and that only ever happens when <code><i>alloc</i> == other.<i>alloc</i></code>
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is false.
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</p>
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<p>
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It seems like we should require <code class='backtick'>is_move_constructible_v</code> instead,
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and only if the allocator traits mean we need to construct an object.
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</p>
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<p>
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Additionally, the noexcept-specifier for the move assignment doesn't match
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the effects. The noexcept-specifier says it can't throw if POCMA is true,
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but nothing in the effects says that ownership can be transferred in that case.
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We only do a non-throwing transfer when the allocators are equal.
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I think we should also transfer ownership when
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</p>
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<p id="res-4251"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 20.4.1.5 <a href="https://wg21.link/indirect.asgn">[indirect.asgn]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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constexpr indirect&amp; operator=(indirect&amp;&amp; other)
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noexcept(allocator_traits&lt;Allocator&gt;::propagate_on_container_move_assignment::value ||
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allocator_traits&lt;Allocator&gt;::is_always_equal::value);
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</code></pre>
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<blockquote>
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<p>
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-5- <i>Mandates</i>:
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<ins>
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If
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<code>allocator_traits&lt;Allocator&gt;::propagate_on_container_move_assignment::value</code>
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is <code class='backtick'>false</code>
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and
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<code>allocator_traits&lt;Allocator&gt;::is_always_equal::value</code>
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is <code class='backtick'>false</code>,
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</ins>
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<code>is_<del>copy</del><ins>move</ins>_constructible_t&lt;T&gt;</code> is <code class='backtick'>true</code>.
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</p>
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<p>
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-6- <i>Effects</i>:
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If <code class='backtick'>addressof(other) == this</code> is <code class='backtick'>true</code>, there are no effects.
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Otherwise:
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<ol style="list-style-type:none">
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<li>(6.1) &mdash;
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The allocator needs updating if
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<code>allocator_traits&lt;Allocator&gt;::propagate_on_container_move_assignment::value</code>
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is <code class='backtick'>true</code>.
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</li>
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<li>(6.2) &mdash;
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If <code class='backtick'>other</code> is valueless, <code class='backtick'>*this</code> becomes valueless<del> and the owned object
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in <code class='backtick'>*this</code>, if any, is destroyed using
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<code>allocator_traits&lt;Allocator&gt;::destroy</code>
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and then the storage is deallocated</del>.
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</li>
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<li>(6.3) &mdash;
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Otherwise,
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<ins>if the allocator needs updating or</ins>
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if <code><i>alloc</i> == other.<i>alloc</i></code> is <code class='backtick'>true</code>,
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<del>
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swaps the owned objects in <code class='backtick'>*this</code> and <code class='backtick'>other</code>;
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the owned object in <code class='backtick'>other</code>, if any, is then destroyed using
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<code>allocator_traits&lt;Allocator&gt;::destroy</code>
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and then the storage is deallocated
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</del>
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<ins><code class='backtick'>*this</code> takes ownership of the owned object of <code class='backtick'>other</code></ins>.
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</li>
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<li>(6.4) &mdash;
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Otherwise, constructs a new owned object with the owned object of <code class='backtick'>other</code>
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as the argument as an rvalue, using either the allocator in <code class='backtick'>*this</code>
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or the allocator in <code class='backtick'>other</code> if the allocator needs updating.
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</li>
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<li>(6.5) &mdash;
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The previously owned object in <code class='backtick'>*this</code>, if any, is destroyed using
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<code>allocator_traits&lt;Allocator&gt;::destroy</code>
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and then the storage is deallocated.
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</li>
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<li>(6.6) &mdash;
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If the allocator needs updating,
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the allocator in <code class='backtick'>*this</code> is replaced with a copy of the allocator in <code class='backtick'>other</code>.
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</li>
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</ol>
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</p>
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<p>-7- <i>Postcondition</i>: <code class='backtick'>other</code> is valueless.</p>
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