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<title>Issue 3806: Should concept formattable&lt;T, charT&gt; default to char?</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#NAD">NAD</a> status.</em></p>
<h3 id="3806"><a href="lwg-closed.html#3806">3806</a>. Should concept <code>formattable&lt;T, charT&gt;</code> default to <code>char</code>?</h3>
<p><b>Section:</b> 28.5.6.3 <a href="https://wg21.link/format.formattable">[format.formattable]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-10-28 <b>Last modified:</b> 2023-01-24</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#format.formattable">active issues</a> in [format.formattable].</p>
<p><b>View all other</b> <a href="lwg-index.html#format.formattable">issues</a> in [format.formattable].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
For many uses of <code>&lt;format&gt;</code> there's no need to explicitly mention the output type as 
<code>char</code>. There are either typedefs for <code>char</code> specializations (<code>format_context</code>, 
<code>format_parse_context</code>, <code>format_args</code>, etc.) or a default template argument 
(<code>formatter</code>, <code>range_formatter</code>). But for the new <code>formattable</code> concept you 
always need to specify the character type:
</p>
<blockquote><pre>
static_assert( std::formattable&lt;int&gt; ); // <span style="color:red;font-weight:bolder">ill-formed</span>
static_assert( std::formattable&lt;int, char&gt; ); // OK
</pre></blockquote>
<p>
Should the concept have a default template argument for the second parameter, to make it easier to 
check whether something is <code>formattable</code> as <code>char</code>?
</p>

<p><i>[2022-11-01; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
Two votes for NAD, the convenience makes it easier to misuse.
</p>

<p><i>[2022-11-30; Reflector poll]</i></p>

<p>
Set status to "Tentatively NAD" after ten votes in reflector poll.
</p>

<p><i>[2022-11-30 LWG telecon. Status changed: Tentatively NAD &rarr; NAD.]</i></p>


<p><i>[2023-01-24 LEWG electronic poll; weak consensus to reject the propsed change.]</i></p>



<p id="res-3806"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 28.5.1 <a href="https://wg21.link/format.syn">[format.syn]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 28.5.6 <a href="https://wg21.link/format.formatter">[format.formatter]</a>, formatter</i>
template&lt;class T, class charT = char&gt; struct formatter;

<i>// 28.5.6.3 <a href="https://wg21.link/format.formattable">[format.formattable]</a>, concept formattable</i>
template&lt;class T, class charT <ins>= char</ins>&gt;
  concept formattable = <i>see below</i>;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 28.5.6.3 <a href="https://wg21.link/format.formattable">[format.formattable]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> This repeats the default template argument already shown in the synopsis, 
which would not be valid in C++ code. That is consistent with our presentation style though, 
as this is not C++ code, it's a specification. See e.g. <code>indirect_strict_weak_order</code> and 
<code>subrange</code>.]
</p>
</blockquote>

<blockquote>
<p>
-1- Let <code><i>fmt-iter-for</i>&lt;charT&gt;</code> be an unspecified type that models 
<code>output_iterator&lt;const charT&amp;&gt;</code> (24.3.4.10 <a href="https://wg21.link/iterator.concept.output">[iterator.concept.output]</a>).
</p>
<blockquote>
<pre>
template&lt;class T, class charT <ins>= char</ins>&gt;
  concept formattable =
    semiregular&lt;formatter&lt;remove_cvref_t&lt;T&gt;, charT&gt;&gt; &amp;&amp;
    requires(formatter&lt;remove_cvref_t&lt;T&gt;, charT&gt; f,
             const formatter&lt;remove_cvref_t&lt;T&gt;, charT&gt; cf,
             T t,
             basic_format_context&lt;<i>fmt-iter-for</i>&lt;charT&gt;, charT&gt; fc,
             basic_format_parse_context&lt;charT&gt; pc) {
      { f.parse(pc) } -&gt; same_as&lt;basic_format_parse_context&lt;charT&gt;::iterator&gt;;
      { cf.format(t, fc) } -&gt; same_as&lt;<i>fmt-iter-for</i>&lt;charT&gt;&gt;;
    };
</pre>
</blockquote>
</blockquote>
</li>

</ol>





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