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<title>Issue 3127: basic_osyncstream::rdbuf needs a const_cast</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="3127"><a href="lwg-defects.html#3127">3127</a>. <code>basic_osyncstream::rdbuf</code> needs a <code>const_cast</code></h3>
<p><b>Section:</b> 31.11.3.1 <a href="https://wg21.link/syncstream.osyncstream.overview">[syncstream.osyncstream.overview]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2018-06-29 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#syncstream.osyncstream.overview">issues</a> in [syncstream.osyncstream.overview].</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>
The current specification of <code>basic_osyncstream::rdbuf()</code> is
</p>
<blockquote>
<pre>
    syncbuf_type* rdbuf() const noexcept { return &amp;sb; }
</pre>
</blockquote>
<p>
This is ill-formed because the exposition-only member <code>sb</code> is <code>const</code> inside this <code>const</code> member function,
but the return type is a pointer to non-<code>const</code> <code>syncbuf_type</code>. It needs to cast away the constness, consistent
with the other streams with embedded stream buffers (such as string and file streams).
</p>

<p><i>[2018-07-20 Status set to Tentatively Ready after five positive votes on the reflector.]</i></p>

<p><i>[2018-11, Adopted in San Diego]</i></p>



<p id="res-3127"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4750">N4750</a>.
</p>

<ol>
<li>
<p>Change 31.11.3.1 <a href="https://wg21.link/syncstream.osyncstream.overview">[syncstream.osyncstream.overview]</a>, class template <code>basic_osyncstream</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class charT, class traits, class Allocator&gt;
  class basic_osyncstream : public basic_ostream&lt;charT, traits&gt; {
  public:
    [&hellip;]

    // 31.11.3.3 <a href="https://wg21.link/syncstream.osyncstream.members">[syncstream.osyncstream.members]</a>, member functions
    void emit();
    streambuf_type* get_wrapped() const noexcept;
    syncbuf_type* rdbuf() const noexcept { return <ins>const_cast&lt;syncbuf_type*&gt;(</ins>&amp;sb<ins>)</ins>; }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>
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