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<title>Issue 4276: front() and back() are not hardened for zero-length std::arrays</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#Ready">Tentatively Ready</a> status.</em></p>
<h3 id="4276"><a href="lwg-active.html#4276">4276</a>. <code class='backtick'>front()</code> and <code class='backtick'>back()</code> are not hardened for zero-length <code class='backtick'>std::array</code>s</h3>
<p><b>Section:</b> 23.3.3.5 <a href="https://wg21.link/array.zero">[array.zero]</a> <b>Status:</b> <a href="lwg-active.html#Ready">Tentatively Ready</a>
 <b>Submitter:</b> Jan Schultke <b>Opened:</b> 2025-06-08 <b>Last modified:</b> 2025-08-26</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#array.zero">active issues</a> in [array.zero].</p>
<p><b>View all other</b> <a href="lwg-index.html#array.zero">issues</a> in [array.zero].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively Ready">Tentatively Ready</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The intent of <a href="https://wg21.link/P3471" title=" Standard Library Hardening">P3471</a> "Standard library hardening" is clearly to provide hardened preconditions 
for members of sequence containers, including <code class='backtick'>std::array</code>. However, a zero-length <code class='backtick'>std::array</code> dodges this 
hardening by having undefined behavior for <code class='backtick'>front()</code> and <code class='backtick'>back()</code> explicitly specified in 
23.3.3.5 <a href="https://wg21.link/array.zero">[array.zero]</a> paragraph 3.
<p/>
Without this paragraph, <code class='backtick'>front()</code> and <code class='backtick'>back()</code> would be hardened as well, as specified in 23.2.4 <a href="https://wg21.link/sequence.reqmts">[sequence.reqmts]</a>.
</p>

<p><i>[2025-08-21; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after eight votes in favour during reflector poll.
</p>



<p id="res-4276"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>
<li><p>Modify 23.3.3.5 <a href="https://wg21.link/array.zero">[array.zero]</a> as indicated:</p>

<blockquote>
<p>
<del>-3- The effect of calling <code class='backtick'>front()</code> or <code class='backtick'>back()</code> for a zero-sized array is undefined.</del>
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