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<title>Issue 3568: basic_istream_view needs to initialize value_</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="3568"><a href="lwg-defects.html#3568">3568</a>. <code>basic_istream_view</code> needs to initialize <code><i>value_</i></code></h3>
<p><b>Section:</b> 25.6.6.2 <a href="https://wg21.link/range.istream.view">[range.istream.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2021-06-15 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
<a href="https://wg21.link/P2325R3" title=" Views should not be required to be default constructible">P2325R3</a> removes the default member initializers for <code>basic_istream_view</code>'s exposition-only 
<code><i>stream_</i></code> and <code><i>value_</i></code> members. Consequently, copying a <code>basic_istream_view</code> 
before the first call to <code>begin</code> may produce undefined behavior since doing so necessarily copies the 
uninitialized <code><i>value_</i></code>. We should restore <code><i>value_</i></code>'s default member initializer and 
remove this particular footgun.
</p>

<p><i>[2021-06-23; Reflector poll]</i></p>

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

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3568"><b>Proposed resolution:</b></p>
<p>
Wording relative to the <a href="https://github.com/cplusplus/draft/releases/download/n4892/n4892.pdf">post-202106 
working draft</a> (as near as possible). This PR is currently being implemented in MSVC.
</p>

<ol>
<li><p>Modify 25.6.6.2 <a href="https://wg21.link/range.istream.view">[range.istream.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;movable Val, class CharT, class Traits>
    requires default_initializable&lt;Val&gt; &amp;&amp;
             <i>stream-extractable</i>&lt;Val, CharT, Traits&gt;
  class basic_istream_view : public view_interface&lt;basic_istream_view&lt;Val, CharT, Traits&gt;&gt; {
    [&hellip;]
    Val <i>value_</i> <ins>= Val()</ins>; <i>// exposition only</i>
  };
}
</pre>
</blockquote>
</li>

</ol>




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