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<title>Issue 2609: [filesys.ts] [PDTS] Unclear why range-based-for functions return different types</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#TS">TS</a> status.</em></p>
<h3 id="2609"><a href="lwg-defects.html#2609">2609</a>. [filesys.ts] [PDTS] Unclear why range-based-for functions return different types</h3>
<p><b>Section:</b> 6 [filesys.ts::fs.filesystem.synopsis] <b>Status:</b> <a href="lwg-active.html#TS">TS</a>
 <b>Submitter:</b> FI-2 <b>Opened:</b> 2014-01-20 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#filesys.ts::fs.filesystem.synopsis">issues</a> in [filesys.ts::fs.filesystem.synopsis].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TS">TS</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: filesys.ts</b></p>
<p>It is unclear why the range-for support functions (begin()/end()) for
<code>directory_iterator</code> and <code>recursive_directory_iterator</code> return different types for <code>begin()</code>
and <code>end()</code>, namely that <code>begin()</code> returns a reference to <code>const</code> and <code>end()</code> returns a value.</p>

  <p>
    <i>[2014-02-07: Beman Dawes provides comments from the Boost implementation:]</i>
  </p>


  <pre>
  //  begin() and end() are only used by a range-based for statement in the context of
  //  auto - thus the top-level const is stripped - so returning const is harmless and
  //  emphasizes begin() is just a pass through.</pre>

  <p><i>[2014-02-08: Daniel responds to Beman]</i></p>

  
  <p>
  The difference in return types becomes relevant, when testing whether e.g. <code>directory_iterator</code>
  would satisfy the <code>Traversable</code> requirements as currently specified in 
  <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3763.html">N3763</a>. Expressed in
  code form these requirements impose that the following assertion holds:
  </p>
  <blockquote><pre>
  static_assert(std::is_same&lt;
      decltype(std::range_begin(std::declval&lt;directory_iterator&gt;())),
      decltype(std::range_end(std::declval&lt;directory_iterator&gt;()))
    &gt;::value, "No Traversable type");
  </pre></blockquote>
  <p>
  Both <code>directory_iterator</code> and <code>recursive_directory_iterator</code> won't satisfy this requirement
  currently.
  </p>
  <p><i>[
    2014-02-11 Issaquah: Change begin() argument and return to pass-by-value. See wiki notes for rationale.
    Beman to provide wording for review next meeting.
  ]</i></p>


  <p><i>[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]</i></p>




<p id="res-2609"><b>Proposed resolution:</b></p>
<ol>
<li>
  <p><i>Change 6 [fs.filesystem.synopsis]:</i></p>
  
  <pre>
  class directory_iterator;

  // enable directory_iterator range-based for statements
  <del>const</del> directory_iterator<del>&amp;</del> begin(<del>const</del> directory_iterator<del>&amp;</del> iter) noexcept;
  directory_iterator end(const directory_iterator&amp;) noexcept;

  class recursive_directory_iterator;

  // enable recursive_directory_iterator range-based for statements
  <del>const</del> recursive_directory_iterator<del>&amp;</del> begin(<del>const</del> recursive_directory_iterator<del>&amp;</del> iter) noexcept;
  recursive_directory_iterator end(const recursive_directory_iterator&amp;) noexcept;
  </pre>
</li>
<li>
  <p><i>
    Change 13.2 [directory_iterator.nonmembers]:
  </i></p>
  <p>
    These functions enable use of <code>directory_iterator</code> with C++11
    range-based for statements.
  </p>
  <pre><del>const</del> directory_iterator<del>&amp;</del> begin(<del>const</del>directory_iterator<del>&amp;</del> iter) noexcept;</pre>
  <blockquote>
    <p>
      <i>Returns: </i><code>iter</code>.
    </p>
  </blockquote>
  <pre>directory_iterator end(const directory_iterator&amp;) noexcept;</pre>
  <blockquote>
    <p>
      <i>Returns: </i><code>directory_iterator()</code>.
    </p>
  </blockquote>
</li>
<li>
  <p><i>
    Change 14.2 [rec.dir.itr.nonmembers]:
  </i></p>
  <p>
    These functions enable use of <code>recursive_directory_iterator</code>
    with C++11 range-based for statements.
  </p>
  <pre><del>const</del> recursive_directory_iterator<del>&amp;</del> begin(<del>const</del>recursive_directory_iterator<del>&amp;</del> iter) noexcept;</pre>
  <blockquote>
    <p>
      <i>Returns: </i><code>iter</code>.
    </p>
  </blockquote>
  <pre>recursive_directory_iterator end(const recursive_directory_iterator&amp;) noexcept;</pre>
  <blockquote>
    <p>
      <i>Returns: </i><code>recursive_directory_iterator()</code>.
    </p>
  </blockquote>
</li>
</ol>







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