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<title>Issue 3639: Handling of fill character width is underspecified in std::format</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#Resolved">Resolved</a> status.</em></p>
<h3 id="3639"><a href="lwg-defects.html#3639">3639</a>. Handling of fill character width is underspecified in <code>std::format</code></h3>
<p><b>Section:</b> 28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Victor Zverovich <b>Opened:</b> 2021-11-13 <b>Last modified:</b> 2023-03-23</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#format.string.std">active issues</a> in [format.string.std].</p>
<p><b>View all other</b> <a href="lwg-index.html#format.string.std">issues</a> in [format.string.std].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a> doesn't specify if implementations should consider the estimated width
of the fill character when substituting it into the formatted results.
<p/>
For example:
</p>
<blockquote><pre>
auto s = std::format("{:&#x1F921;&gt;10}", 42);
</pre></blockquote>
<p>
<code>"&#x1F921;"</code> (U+1F921) is a single code point but its estimated display width is two.
<p/>
There are at least three possible resolutions:
</p>
<ol>
<li><p><code>s == "&#x1F921;&#x1F921;&#x1F921;&#x1F921;42"</code>: use the estimated display width,
correctly displayed on compatible terminals.</p></li>
<li><p><code>s == "&#x1F921;&#x1F921;&#x1F921;&#x1F921;&#x1F921;&#x1F921;&#x1F921;&#x1F921;42"</code>:
assume the display width of 1, incorrectly displayed.</p></li>
<li><p>Require the fill character to have the estimated width of 1.</p></li>
</ol>

<p><i>[2021-11-14; Daniel comments]</i></p>

<p>
Resolving this issue should be harmonized with resolving LWG <a href="lwg-defects.html#3576" title="Clarifying fill character in std::format (Status: Resolved)">3576</a><sup><a href="https://cplusplus.github.io/LWG/issue3576" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll. Sent to SG16.
</p>

<p><i>[2023-01-11; LWG telecon]</i></p>

<p>
<a href="https://wg21.link/P2572" title=" std::format() fill character allowances">P2572</a> would resolve this issue and LWG <a href="lwg-defects.html#3576" title="Clarifying fill character in std::format (Status: Resolved)">3576</a><sup><a href="https://cplusplus.github.io/LWG/issue3576" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2023-03-22 Resolved by the adoption of <a href="https://wg21.link/P2572R1" title=" std::format() fill character allowances">P2572R1</a> in Issaquah. Status changed: SG16 &rarr; Resolved.]</i></p>



<p id="res-3639"><b>Proposed resolution:</b></p>





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