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<title>Issue 3539: format_to must not copy models of output_iterator&lt;const charT&amp;&gt;</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="3539"><a href="lwg-defects.html#3539">3539</a>. <code>format_to</code> must not copy models of <code>output_iterator&lt;const charT&amp;&gt;</code></h3>
<p><b>Section:</b> 28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2021-03-31 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.functions">issues</a> in [format.functions].</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>
28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a> specifies the overloads of <code>format_to</code> as:
</p>
<blockquote>
<pre>
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, string_view fmt, const Args&amp;... args);
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, wstring_view fmt, const Args&amp;... args);
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
using context = basic_format_context&lt;Out, decltype(fmt)::value_type&gt;;
return vformat_to(out, fmt, make_format_args&lt;context&gt;(args...));
</pre>
</blockquote>
</blockquote>
<pre>
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, const locale&amp; loc, string_view fmt, const Args&amp;... args);
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, const locale&amp; loc, wstring_view fmt, const Args&amp;... args);
  Out format_to(Out out, wstring_view fmt, const Args&amp;... args);
</pre>
<blockquote>
<p>
-9- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
using context = basic_format_context&lt;Out, decltype(fmt)::value_type&gt;;
return vformat_to(out, loc, fmt, make_format_args&lt;context&gt;(args...));
</pre>
</blockquote>
</blockquote>
</blockquote>
<p>
but the overloads of <code>vformat_to</code> take their first argument by value (from the same subclause):
</p>
<blockquote>
<pre>
template&lt;class Out&gt;
  Out vformat_to(Out out, string_view fmt,
                 format_args_t&lt;type_identity_t&lt;Out&gt;, char&gt; args);
template&lt;class Out&gt;
  Out vformat_to(Out out, wstring_view fmt,
                 format_args_t&lt;type_identity_t&lt;Out&gt;, wchar_t&gt; args);
template&lt;class Out&gt;
  Out vformat_to(Out out, const locale&amp; loc, string_view fmt,
                 format_args_t&lt;type_identity_t&lt;Out&gt;, char&gt; args);
template&lt;class Out>
  Out vformat_to(Out out, const locale&amp; loc, wstring_view fmt,
                 format_args_t&lt;type_identity_t&lt;Out&gt;, wchar_t&gt; args);
</pre>
<blockquote>
<p>
-10- Let <code>charT</code> be <code>decltype(fmt)::value_type</code>.
<p/>
-11- <i>Constraints:</i> <code>Out</code> satisfies <code>output_iterator&lt;const charT&amp;&gt;</code>.
<p/>
-12- <i>Preconditions:</i> <code>Out</code> models <code>output_iterator&lt;const charT&amp;&gt;</code>.
</p>
</blockquote>
</blockquote>
<p>
and require its type to model <code>output_iterator&lt;const charT&amp;&gt;</code>. <code>output_iterator&lt;T, U&gt;</code> 
refines <code>input_or_output_iterator&lt;T&gt;</code> which refines <code>movable&lt;T&gt;</code>, but it notably does not 
refine <code>copyable&lt;T&gt;</code>. Consequently, the "Equivalent to" code for the <code>format_to</code> overloads is 
copying an iterator that could be move-only. I suspect it is not the intent that calls to <code>format_to</code> with 
move-only iterators be ill-formed, but that it was simply an oversight that this wording needs updating to be 
consistent with the change to allow move-only single-pass iterators in C++20.
</p>

<p><i>[2021-04-20; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven 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-3539"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, string_view fmt, const Args&amp;... args);
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, wstring_view fmt, const Args&amp;... args);
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
using context = basic_format_context&lt;Out, decltype(fmt)::value_type&gt;;
return vformat_to(<ins>std::move(</ins>out<ins>)</ins>, fmt, make_format_args&lt;context&gt;(args...));
</pre>
</blockquote>
</blockquote>
<pre>
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, const locale&amp; loc, string_view fmt, const Args&amp;... args);
template&lt;class Out, class... Args&gt;
  Out format_to(Out out, const locale&amp; loc, wstring_view fmt, const Args&amp;... args);
</pre>
<blockquote>
<p>
-9- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
using context = basic_format_context&lt;Out, decltype(fmt)::value_type&gt;;
return vformat_to(<ins>std::move(</ins>out<ins>)</ins>, loc, fmt, make_format_args&lt;context&gt;(args...));
</pre>
</blockquote>
</blockquote>
</blockquote>
</li>

</ol>




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