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<title>Issue 2579: Inconsistency wrt Allocators in basic_string assignment vs. basic_string::assign</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="2579"><a href="lwg-defects.html#2579">2579</a>. Inconsistency wrt Allocators in <code>basic_string</code> assignment vs. <code>basic_string::assign</code></h3>
<p><b>Section:</b> 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Marshall Clow <b>Opened:</b> 2016-01-05 <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#string.assign">issues</a> in [string.assign].</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>
In issue <a href="lwg-defects.html#2063" title="Contradictory requirements for string move assignment (Status: C++17)">2063</a><sup><a href="https://cplusplus.github.io/LWG/issue2063" title="Latest snapshot">(i)</a></sup>, we changed the <i>Effects</i> of <code>basic_string::assign(basic_string&amp;&amp;)</code> to match the 
behavior of <code>basic_string::operator=(basic_string&amp;&amp;)</code>, making them consistent.
<p/>
We did not consider <code>basic_string::assign(const basic_string&amp;)</code>, and its <i>Effects</i> differ from 
<code>operator=(const basic_string&amp;)</code>.
<p/>
Given the following definition:
</p>
<blockquote><pre>
typedef std::basic_string&lt;char, std::char_traits&lt;char&gt;, MyAllocator&lt;char&gt;&gt; MyString;

MyAllocator&lt;char&gt; alloc1, alloc2;
MyString string1("Alloc1", alloc1);
MyString string2(alloc2);
</pre></blockquote>
<p>
the following bits of code are not equivalent:
</p>
<blockquote><pre>
string2 = string1;       // (a) calls operator=(const MyString&amp;)
string2.assign(string1); // (b) calls MyString::assign(const MyString&amp;)
</pre></blockquote>
<p>
What is the allocator for <code>string2</code> after each of these calls?
</p>
<ol style="list-style-type:lower-alpha">
<li><p>If <code>MyAllocator&lt;char&gt;::propagate_on_container_copy_assignment</code> is true, then it should be <code>alloc2</code>, 
otherwise it should be <code>alloc1</code>.</p></li>
<li><p><code>alloc2</code></p></li>
</ol>
<p>	
27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a>/1 says that (b) is equivalent to <code>assign(string1, 0, npos)</code>, which eventually calls 
<code>assign(str.data() + pos, rlen)</code>. No allocator transfer there.
</p>

<p><i>[2016-02, Issues Telecon]</i></p>

<p>
P0; move to Tentatively Ready.
</p>


<p id="res-2579"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4567.
</p>

<ol>
<li><p>Modify 27.4.3.7.3 <a href="https://wg21.link/string.assign">[string.assign]</a> p.1 as indicated:</p>

<blockquote>
<pre>
basic_string&amp; assign(const basic_string&amp; str);
</pre>
<blockquote>
<p>
-1- <i>Effects</i>: Equivalent to <code><ins>*this = str</ins><del>assign(str, 0, npos)</del></code>.
<p/>
-2- <i>Returns</i>: <code>*this</code>.
</p>
</blockquote>
</blockquote>

</li>
</ol>





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