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<title>Issue 3243: std::format and negative zeroes</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="3243"><a href="lwg-defects.html#3243">3243</a>. <code>std::format</code> and negative zeroes</h3>
<p><b>Section:</b> 28.5.2 <a href="https://wg21.link/format.string">[format.string]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2019-07-31 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.string">issues</a> in [format.string].</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>
What are these:
</p>
<blockquote><pre>
std::format("{}", -0.0);
std::format("{:+}", -0.0);
std::format("{:-}", -0.0);
std::format("{: }", -0.0);
</pre></blockquote>
<p>
with
</p>
<blockquote><pre>
"{:{}} {}"
</pre></blockquote>
<p>
A negative zero is not a negative number, so I <em>think</em> the answer for an implementation that supports 
signed zeroes is <code>"0"</code>, <code>"-0"</code>, <code>"0"</code>, <code>" 0"</code>. Is that the intent? (Note 
that this doesn't satisfy <code>to_chars</code>' round-trip guarantee.)
<p/>
Or should the behavior actually be that <code>"+"</code> means "insert a leading <code>+</code> if <code>to_chars</code>' 
output does not start with <code>-"</code> and <code>" "</code> actually means "insert a leading space if 
<code>to_chars</code>' output does not start with <code>-"</code> (that is, the same as <code>"%+f"</code> and <code>"% f"</code>) 
&mdash; so that the result of all four calls is <code>"-0"</code>?
<p/>
<b>Victor Zverovich:</b>
<p/>
The latter. <code>std::format</code> is specified in terms of <code>to_chars</code> and the intent is to have similar 
round trip guarantee here, so the result should be <code>"-0"</code>, <code>"-0"</code>, <code>"-0"</code>, <code>"-0"</code>. 
This has also been extensively discussed in LEWG and the outcome of that discussion was to print <code>'-'</code> 
for negative zeros.
<p/>
So I think it should be clarified that <code>'-'</code> and space apply to negative numbers and negative zero.
</p>

<p><i>[2019-08-17 Priority set to 2 based on reflector discussion]</i></p>


<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify the <i>sign</i> options Table [tab:format.sign] in 28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a> as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 59: Meaning of <i>sign</i> options [tab:format.sign]</caption>
<tr align="center">
<th>Option</th>
<th>Meaning</th>
</tr> 

<tr>
<td>
<code>'+'</code>
</td>
<td>
Indicates that a sign should be used for both non-negative and negative numbers.
</td>
</tr>

<tr>
<td>
<code>'-'</code>
</td>
<td>
Indicates that a sign should be used only for negative numbers <ins>and negative zero</ins> 
(this is the default behavior).
</td>
</tr>

<tr>
<td>
space
</td>
<td>
Indicates that a leading space should be used for non-negative numbers <ins>other than negative 
zero</ins>, and a minus sign for negative numbers <ins>and negative zero</ins>.
</td>
</tr>

</table>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2020-02-13, Prague]</i></p>

<p>
Rebased and some wording finetuning by LWG.
</p>
<p><i>[2020-02 Status to Immediate on Thursday night in Prague.]</i></p>



<p id="res-3243"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.</p>

<ol>
<li><p>Modify the <i>sign</i> options Table [tab:format.sign] in 28.5.2.2 <a href="https://wg21.link/format.string.std">[format.string.std]</a> as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 58: Meaning of <i>sign</i> options [tab:format.sign]</caption>
<tr align="center">
<th>Option</th>
<th>Meaning</th>
</tr> 

<tr>
<td>
<code>'+'</code>
</td>
<td>
Indicates that a sign should be used for both non-negative and negative numbers.
</td>
</tr>

<tr>
<td>
<code>'-'</code>
</td>
<td>
Indicates that a sign should be used <del>only</del> for negative numbers <ins>and negative zero only</ins> 
(this is the default behavior).
</td>
</tr>

<tr>
<td>
space
</td>
<td>
Indicates that a leading space should be used for non-negative numbers <ins>other than negative 
zero</ins>, and a minus sign for negative numbers <ins>and negative zero</ins>.
</td>
</tr>

</table>
</blockquote>
</li>

</ol>




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