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<title>Issue 2707: path construction and assignment should have "string_type&amp;&amp;" overloads</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++17">C++17</a> status.</em></p>
<h3 id="2707"><a href="lwg-defects.html#2707">2707</a>. <code>path</code> construction and assignment should have "<code>string_type&amp;&amp;</code>" overloads</h3>
<p><b>Section:</b> 31.12.6 <a href="https://wg21.link/fs.class.path">[fs.class.path]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Eric Fiselier <b>Opened:</b> 2016-05-09 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#fs.class.path">issues</a> in [fs.class.path].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently construction of a <code>path</code> from the native <code>string_type</code> always performs a copy, even when 
that string is passed as a rvalue. This is a large pessimization as paths are commonly constructed from temporary strings.
<p/>
One pattern I frequently see is:
</p>
<blockquote><pre>
path foo(path const&amp; p) {
  auto s = p.native();
  mutateString(s);
  return s;
}
</pre></blockquote>
<p>
Implementations should be allowed to move from <code>s</code> and avoid an unnecessary allocation.
I believe <code>string_type&amp;&amp;</code> constructor and assignment operator overloads should be added to support this.
</p>


<p id="res-2707"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4582.
</p>

<ol>
<li><p>Change 31.12.6 <a href="https://wg21.link/fs.class.path">[fs.class.path]</a>, class <code>path</code> synopsis, as indicated:</p>
<blockquote class="note">
<p>
[<i>Drafting note</i>: Making the <code>string_type&amp;&amp;</code> constructors and assignment operators <code>noexcept</code> 
would over-constrain implementations which may need to perform construct additional state]
</p>
</blockquote>

<blockquote>
<pre>
namespace std::filesystem {
  class path {
  public:
    [&hellip;]
    <i>// 27.10.8.4.1, constructors and destructor</i>
    path() noexcept;
    path(const path&amp; p);
    path(path&amp;&amp; p) noexcept;
    <ins>path(string_type&amp;&amp; source);</ins>
    template &lt;class Source&gt;
    path(const Source&amp; source);
    [&hellip;]
    
    <i>// 27.10.8.4.2, assignments</i>
    path&amp; operator=(const path&amp; p);
    path&amp; operator=(path&amp;&amp; p) noexcept;
    <ins>path&amp; operator=(string_type&amp;&amp; source);
    path&amp; assign(string_type&amp;&amp; source);</ins>
    template &lt;class Source&gt;
    path&amp; operator=(const Source&amp; source);
    template &lt;class Source&gt;
    path&amp; assign(const Source&amp; source)
    template &lt;class InputIterator&gt;
    path&amp; assign(InputIterator first, InputIterator last);    
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Add a new paragraph following 31.12.6.5.1 <a href="https://wg21.link/fs.path.construct">[fs.path.construct]</a>/3:</p>

<blockquote>
<pre>
<ins>path(string_type&amp;&amp; source);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Constructs an object of class <code>path</code> with <code>pathname</code> having the original value of 
<code>source</code>. <code>source</code> is left in a valid but unspecified state.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Add a new paragraph following 31.12.6.5.2 <a href="https://wg21.link/fs.path.assign">[fs.path.assign]</a>/4:</p>

<blockquote>
<pre>
<ins>path&amp; operator=(string_type&amp;&amp; source);
path&amp; assign(string_type&amp;&amp; source);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Modifies <code>pathname</code> to have the original value of <code>source</code>. <code>source</code> 
is left in a valid but unspecified state.</ins>
<p/>
<ins>-?- <i>Returns</i>:<code>*this</code></ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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