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<title>Issue 3462: &sect;[formatter.requirements]: Formatter requirements forbid use of fc.arg()</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++23">C++23</a> status.</em></p>
<h3 id="3462"><a href="lwg-defects.html#3462">3462</a>. &sect;[formatter.requirements]: Formatter requirements forbid use of <code>fc.arg()</code></h3>
<p><b>Section:</b> 28.5.6.1 <a href="https://wg21.link/formatter.requirements">[formatter.requirements]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Alberto Barbati <b>Opened:</b> 2020-06-30 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#formatter.requirements">issues</a> in [formatter.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The requirements on the expression <code>f.format(t, fc)</code> in [tab:formatter] say
</p>
<blockquote><p>
Formats <code>t</code> according to the specifiers stored in <code>*this</code>, writes the output to <code>fc.out()</code>
and returns an iterator past the end of the output range. The output shall only depend on <code>t</code>,
<code>fc.locale()</code>, and the range <code>[pc.begin(), pc.end())</code> from the last call to <code>f.parse(pc)</code>.
</p></blockquote>
<p>
Strictly speaking, this wording effectively forbids <code>f.format(t, fc)</code> from calling <code>fc.arg(n)</code>,
whose motivation is precisely to allow a formatter to rely on arguments different from <code>t</code>. According to this
interpretation, there's no conforming way to implement the <code>"{ <i>arg-id</i> }"</code> form of the <i>width</i> and
<i>precision</i> fields of standard format specifiers. Moreover, the formatter described in the example if paragraph
28.5.6.7 <a href="https://wg21.link/format.context">[format.context]</a>/8 would also be non-conforming.
</p>

<p><i>[2020-07-12; Reflector prioritization]</i></p>

<p>
Set priority to 3 after reflector discussions.
</p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 28.5.6.1 <a href="https://wg21.link/formatter.requirements">[formatter.requirements]</a>, Table [tab:formatter], as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 67: <i>Formatter</i> requirements [tab:formatter]</caption>
<tr>
<th align="center">Expression</th>
<th align="center">Return type</th>
<th align="center">Requirement</th>
</tr>

<tr>
<td colspan="3" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>f.format(t, fc)</code>
</td>
<td>
<code>FC::iterator</code>
</td>
<td>
Formats <code>t</code> according to the specifiers stored in
<code>*this</code>, writes the output to <code>fc.out()</code> and returns
an iterator past the end of the output range. The
output shall only depend on <code>t</code>, <code>fc.locale()</code>,
<del>and</del> the range <code>[pc.begin(), pc.end())</code> from
the last call to <code>f.parse(pc)</code><ins>, and <code>fc.arg(n)</code>,
where <code>n</code> is a <code>size_t</code> index value that has been
validated with a call to <code>pc.check_arg_id(n)</code> in the last
call to <code>f.parse(pc)</code></ins>.
</td>
</tr>

<tr>
<td colspan="3" align="center">
<code>&hellip;</code>
</td>
</tr>
</table>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2021-05-20 Tim comments and updates wording]</i></p>

<p>During reflector discussion Victor said that the formatter requirements should
allow dependency on any of the format arguments in the context. The wording below
reflects that.
</p>

<p><i>[2021-05-24; Reflector poll]</i></p>

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

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3462"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 28.5.6.1 <a href="https://wg21.link/formatter.requirements">[formatter.requirements]</a>, Table [tab:formatter], as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 67: <i>Formatter</i> requirements [tab:formatter]</caption>
<tr>
<th align="center">Expression</th>
<th align="center">Return type</th>
<th align="center">Requirement</th>
</tr>

<tr>
<td colspan="3" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>f.format(t, fc)</code>
</td>
<td>
<code>FC::iterator</code>
</td>
<td>
Formats <code>t</code> according to the specifiers stored in
<code>*this</code>, writes the output to <code>fc.out()</code> and returns
an iterator past the end of the output range. The
output shall only depend on <code>t</code>, <code>fc.locale()</code>,
<ins><code>fc.arg(n)</code> for any value <code>n</code> of type <code>size_t</code>,</ins>
and the range <code>[pc.begin(), pc.end())</code> from
the last call to <code>f.parse(pc)</code>.
</td>
</tr>

<tr>
<td colspan="3" align="center">
<code>&hellip;</code>
</td>
</tr>
</table>
</blockquote>
</li>
</ol>





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