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<title>Issue 1138: Unusual return value for operator+</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++11">C++11</a> status.</em></p>
<h3 id="1138"><a href="lwg-defects.html#1138">1138</a>. Unusual return value for <code>operator+</code></h3>
<p><b>Section:</b> 27.4.4.1 <a href="https://wg21.link/string.op.plus">[string.op.plus]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-06-12 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Many of the <code>basic_string operator+</code> overloads return an rvalue-reference.  Is
that really intended?
</p>
<p>
I'm considering it might be a mild performance tweak to avoid making
un-necessary copies of a cheaply movable type, but it opens risk to dangling
references in code like:
</p>

<blockquote><pre>
auto &amp;&amp; s = string{"x"} + string{y};
</pre></blockquote>

<p>
and I'm not sure about:
</p>

<blockquote><pre>
auto s = string{"x"} + string{y};
</pre></blockquote>

<p><i>[
2009-10-11 Howard updated <i>Returns:</i> clause for each of these.
]</i></p>


<p><i>[
2009-11-05 Howard adds:
]</i></p>


<blockquote><p>
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
</p></blockquote>


<p id="res-1138"><b>Proposed resolution:</b></p>
<p>
Strike the <code>&amp;&amp;</code> from the return type in the following function
signatures:
</p>

<blockquote>
<p>
27.4 <a href="https://wg21.link/string.classes">[string.classes]</a> p2 Header Synopsis
</p>

<blockquote><pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs,
              const basic_string&lt;charT,traits,Allocator&gt;&amp; rhs);

template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(const basic_string&lt;charT,traits,Allocator&gt;&amp; lhs,
              basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);

template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs,
              basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);


template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(const charT* lhs,
              basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);

template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(charT lhs, basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);

template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs,
              const charT* rhs);

template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs, charT rhs);
</pre></blockquote>

<p>
 [string.op+]
</p>

<blockquote><pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs,
              const basic_string&lt;charT,traits,Allocator&gt;&amp; rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>lhs.append(rhs)<ins>)</ins></code>
</p></blockquote>
<pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(const basic_string&lt;charT,traits,Allocator&gt;&amp; lhs,
              basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>rhs.insert(0, lhs)<ins>)</ins></code>
</p></blockquote>
<pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs,
              basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>lhs.append(rhs)<ins>)</ins></code> [<i>Note:</i> Or equivalently
<code><ins>std::move(</ins>rhs.insert(0, lhs)<ins>)</ins></code> &mdash; <i>end note</i>]
</p></blockquote>
<pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(const charT* lhs,
              basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>rhs.insert(0, lhs)<ins>)</ins></code>.
</p></blockquote>
<pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(charT lhs, basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>rhs.insert(0, 1, lhs)<ins>)</ins></code>.
</p></blockquote>
<pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs,
              const charT* rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>lhs.append(rhs)<ins>)</ins></code>.
</p></blockquote>
<pre>
template&lt;class charT, class traits, class Allocator&gt;
  basic_string&lt;charT,traits,Allocator&gt;<del>&amp;&amp;</del>
    operator+(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; lhs, charT rhs);
</pre>
<blockquote><p>
<i>Returns:</i> <code><ins>std::move(</ins>lhs.append(1, rhs)<ins>)</ins></code>.
</p></blockquote>
</blockquote>

</blockquote>






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