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<title>Issue 3663: basic_string(const T&amp;, const Alloc&amp;) turns moves into copies</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#New">New</a> status.</em></p>
<h3 id="3663"><a href="lwg-active.html#3663">3663</a>. <code>basic_string(const T&amp;, const Alloc&amp;)</code> turns moves into copies</h3>
<p><b>Section:</b> 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a>, 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-01-21 <b>Last modified:</b> 2022-01-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.cons">issues</a> in [string.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This will do a copy not a move:
</p>
<blockquote>
<pre>
struct Str : std::string {
  Str() = default;
  Str(Str&amp;&amp; s) : std::string(std::move(s)) { }
};
Str s;
Str s2(std::move(s));
</pre>
</blockquote>
<p>
The problem is that the new C++17 constructor:
</p>
<blockquote><pre>
basic_string(const T&amp;, const Alloc&amp; = Alloc());
</pre></blockquote>
<p>
is an exact match, but the <code>basic_string</code> move constructor requires a derived-to-base conversion.
<p/>
This is a regression since C++14, because the move constructor was called in C++14.
<p/>
This problem also exists for <code>assign(const T&amp;)</code> and <code>operator=(const T&amp;)</code>.
<p/>
We can fix this by constraining those functions with <code>!derived_from&lt;T, basic_string&gt;</code>, so 
that the move constructor is the only viable function. Libstdc++ has done something very similar since 
2017, but using <code>!is_convertible&lt;const T*, const basic_string*&gt;</code> instead of <code>derived_from</code>.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Jonathan to revise P/R to use <code>is_base_of</code> or
<code>is_convertible</code> instead of <code>derived_from</code>.
</p>



<p id="res-3663"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>. 
</p>

<ol>
<li><p>Modify 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  constexpr explicit basic_string(const T&amp; t, const Allocator&amp; a = Allocator());
</pre>
<blockquote>
<p>
-7- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p>(7.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</p></li>
<li><p><ins>(7.?) &mdash; <code>derived_from&lt;T, basic_string&gt;</code> is <code>false</code> and</ins></p></li>
<li><p>(7.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</p></li>
</ol>
<p>
-8- <i>Effects:</i> Creates a variable, <code>sv</code>, as if by <code>basic_string_view&lt;charT, traits&gt; sv = t;</code> 
and then behaves the same as <code>basic_string(sv.data(), sv.size(), a)</code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
template&lt;class T&gt;
  constexpr basic_string&amp; operator=(const T&amp; t);
</pre>
<blockquote>
<p>
-27- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p>(27.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</p></li>
<li><p><ins>(27.?) &mdash; <code>derived_from&lt;T, basic_string&gt;</code> is <code>false</code> and</ins></p></li>
<li><p>(27.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</p></li>
</ol>
<p>
-28- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
basic_string_view&lt;charT, traits&gt; sv = t;
return assign(sv);
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  constexpr basic_string&amp; assign(const T&amp; t);
</pre>
<blockquote>
<p>
-4- <i>Constraints:</i>
</p>
<ol style="list-style-type:none">
<li><p>(4.1) &mdash; <code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> 
is <code>true</code> and</p></li>
<li><p><ins>(4.?) &mdash; <code>derived_from&lt;T, basic_string&gt;</code> is <code>false</code> and</ins></p></li>
<li><p>(4.2) &mdash; <code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code>.</p></li>
</ol>
<p>
-5- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
basic_string_view&lt;charT, traits&gt; sv = t;
return assign(sv.data(), sv.size());
</pre></blockquote>
</blockquote>
</blockquote>
</li>
</ol>






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