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<title>Issue 4046: Effects of inserting into or erasing from flat container adaptors when an exception is thrown need to be more permissive</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="4046"><a href="lwg-active.html#4046">4046</a>. Effects of inserting into or erasing from flat container adaptors when an exception is thrown need to be more permissive</h3>
<p><b>Section:</b> 23.2.7.2 <a href="https://wg21.link/associative.reqmts.except">[associative.reqmts.except]</a>, 23.6.8.1 <a href="https://wg21.link/flat.map.overview">[flat.map.overview]</a>, 23.6.9.1 <a href="https://wg21.link/flat.multimap.overview">[flat.multimap.overview]</a>, 23.6.11.1 <a href="https://wg21.link/flat.set.overview">[flat.set.overview]</a>, 23.6.12.1 <a href="https://wg21.link/flat.multiset.overview">[flat.multiset.overview]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-01-23 <b>Last modified:</b> 2024-03-15</p>
<p><b>Priority: </b>2
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The issue was originally discovered while implementing <code>flat_(multi_)set</code> in MSVC STL 
(<a href="https://github.com/microsoft/STL/issues/4059">microsoft/STL#4059</a>).
<p/>
Currently, the effects of inserting a single element into or erasing elements from a flat container 
adaptor when an exception is thrown, are implicitly specified in 23.2.7.2 <a href="https://wg21.link/associative.reqmts.except">[associative.reqmts.except]</a>.
<p/>
However, given erasing elements from a <code>vector</code> or a <code>deque</code> generally needs to assign elements, 
which is not guaranteed to be non-throwing, it seems unreasonable to require <code>erase</code> of flat container 
adaptors not to throw in such a case. Likewise, as inserting a single value into <code>deque</code> is not 
guaranteed to be effectless when an exception is thrown, so flat container adaptors are unlike to provide such 
a guarantee.
<p/>
Perhaps we should conditionally relax the requirements when the underlying containers can't provide them.
</p>

<p><i>[2024-03-15; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>
<p> <a href="https://wg21.link/P2767" title=" flat_map/flat_set omnibus">P2767</a> §3.5 is related/conflicting.  </p>



<p id="res-4046"><b>Proposed resolution:</b></p>





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