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<title>Issue 4165: Should swapping a built-in array or std::array with itself result in UB?</title>
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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>
<h3 id="4165"><a href="lwg-active.html#4165">4165</a>. Should swapping a built-in array or <code class='backtick'>std::array</code> with itself result in UB?</h3>
<p><b>Section:</b> 18.4.9 <a href="https://wg21.link/concept.swappable">[concept.swappable]</a>, 22.2.2 <a href="https://wg21.link/utility.swap">[utility.swap]</a>, 23.3.3.3 <a href="https://wg21.link/array.members">[array.members]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-10-13 <b>Last modified:</b> 2025-06-13</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#concept.swappable">active issues</a> in [concept.swappable].</p>
<p><b>View all other</b> <a href="lwg-index.html#concept.swappable">issues</a> in [concept.swappable].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, the <code class='backtick'>std::swap</code> overload for built-in arrays, the <code class='backtick'>swap</code> member function of 
<code class='backtick'>std::array</code>, the <code class='backtick'>std::swap</code> overload for <code class='backtick'>std::array</code>, and the expression-equivalent-to 
operation of <code class='backtick'>std::ranges::swap</code> for built-in arrays are specified to be equivalent to performing 
<code class='backtick'>std::(ranges::)swap_ranges</code>. As <code class='backtick'>swap_ranges</code> functions require that the swapped ranges shall 
not overlap (26.7.3 <a href="https://wg21.link/alg.swap">[alg.swap]</a>/2), swapping a built-in array or a nonzero-length 
<code>std::array</code> with itself results in undefined behavior.
<p/>
It seems that no other swap operation in the standard library has precondition that both arguments 
don't refer to the same object. Should we make self-swapping of arrays well-defined? If so, 
perhaps we should change the semantics to plain loops that execute <code class='backtick'>swap</code> or <code class='backtick'>ranges::swap</code> on 
elements.
</p>

<p><i>[2025-06-13; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
Instead of a loop I'd prefer something like:
<blockquote>
<i>Effects</i>:
If <code class='backtick'>addressof(a) == addressof(b)</code>, no effects.
Otherwise, equivalent to <code class='backtick'>swap_ranges(begin(a), end(a), begin(b))</code>.
</blockquote>
</p>



<p id="res-4165"><b>Proposed resolution:</b></p>





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