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<title>Issue 3250: std::format: # (alternate form) for NaN and inf</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#C++20">C++20</a> status.</em></p>
<h3 id="3250"><a href="lwg-defects.html#3250">3250</a>. <code>std::format</code>: <code>#</code> (alternate form) for NaN and inf</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#C++20">C++20</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2019-08-05 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</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#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
We have:
</p>
<blockquote><p>
"For floating-point numbers, the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it."
</p></blockquote>
<p>
So does that mean that infinity is formatted as <code>"inf."</code> and NaN as <code>"nan."</code>? (Or 
something like that? Where exactly do we add the decimal point in this case?) Or does this 
affect infinity but not NaN (because we can handwave that a NaN value is not a 
"floating-point number")?
<p/>
I suspect that this should only cover finite floating-point numbers.
<p/>
<b>Victor Zverovich:</b>
<p/>
Right. So infinity and NaN should be still formatted as <code>"inf"</code> and <code>"nan"</code> 
without a decimal point.
</p>

<p><i>[2020-02 Status to Immediate on Thursday morning in Prague.]</i></p>



<p id="res-3250"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a> as indicated:</p>

<blockquote>
<p>
-6- [&hellip;] For floating-point types, the alternate form causes the result of the conversion 
<ins>of finite values</ins> to always contain a decimal-point character, even if no digits follow 
it. Normally, a decimal-point character appears in the result of these conversions only if a digit 
follows it. In addition, for <code>g</code> and <code>G</code> conversions, trailing zeros are not removed 
from the result.
</p>
</blockquote>
</li>
</ol>




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