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<title>Issue 4061: Should std::basic_format_context be default-constructible/copyable/movable?</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#WP">WP</a> status.</em></p>
<h3 id="4061"><a href="lwg-defects.html#4061">4061</a>. Should <code>std::basic_format_context</code> be default-constructible/copyable/movable?</h3>
<p><b>Section:</b> 28.5.6.7 <a href="https://wg21.link/format.context">[format.context]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-03-24 <b>Last modified:</b> 2024-07-08</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.context">issues</a> in [format.context].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Per 28.5.6.7 <a href="https://wg21.link/format.context">[format.context]</a>, it seems that <code>std::basic_format_context</code> has a default 
constructor that is effectively defaulted, which means that it is default constructible if and only 
if <code>OutIt</code> is default constructible. Currently only libstdc++ makes it conditionally default 
constructible, while libc++ and MSVC STL (together with fmtlib) make it never default constructible.
<p/>
It seems that <code>basic_format_context</code> objects are supposed to be created by the implementation 
in some internal way, and user codes are only supposed to modify existing <code>basic_format_context</code> 
objects during formatting.
</p>

<p><i>[2024-05-08; Reflector poll]</i></p>

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

<p><i>[St. Louis 2024-06-29; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4061"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4971" title=" Working Draft, Programming Languages — C++">N4971</a>.
</p>

<ol>
<li><p>Modify 28.5.6.7 <a href="https://wg21.link/format.context">[format.context]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Out, class charT&gt;
  class basic_format_context {
    basic_format_args&lt;basic_format_context&gt; args_; // <i>exposition only</i>
    Out out_;                                      // <i>exposition only</i>

    <ins>basic_format_context(const basic_format_context&amp;) = delete;
    basic_format_context&amp; operator=(const basic_format_context&amp;) = delete;</ins>
  public:
    using iterator = Out;
    using char_type = charT;
    template&lt;class T&gt; using formatter_type = formatter&lt;T, charT&gt;;
    
    basic_format_arg&lt;basic_format_context&gt; arg(size_t id) const noexcept;
    std::locale locale();
    
    iterator out();
    void advance_to(iterator it);
  };
}
</pre>
</blockquote>
</li>
</ol>





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