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<title>Issue 2583: There is no way to supply an allocator for basic_string(str, pos)</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="2583"><a href="lwg-defects.html#2583">2583</a>. There is no way to supply an allocator for <code>basic_string(str, pos)</code></h3>
<p><b>Section:</b> 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Pablo Halpern <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.cons">issues</a> in [string.cons].</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>
Container and <code>string</code> constructors in the standard follow two general rules:
</p>
<ol>
<li><p>Every constructor needs a version with and without an allocator argument (possibly through the use of 
default arguments).</p></li>
<li><p>Every constructor <em>except the copy constructor</em> for which an allocator is not provided uses a 
default-constructed allocator.</p></li>
</ol>
<p>
The first rule ensures emplacing a <code>string</code> into a container that uses a scoped allocator will correctly propagate 
the container's allocator to the new element.
<p/>
The current standard allows constructing a <code>string</code> as <code>basic_string(str, pos)</code> but not 
<code>basic_string(str, pos, alloc)</code>. This omission breaks the first rule and causes something like the following to fail:
</p>
<blockquote>
<pre>
typedef basic_string&lt;char, char_traits&lt;char&gt;, A&lt;char&gt;&gt; stringA;
vector&lt;stringA, scoped_allocator_adaptor&lt;A&lt;stringA&gt;&gt;&gt; vs;
stringA s;

vs.emplace_back(s, 2); // Ill-formed
</pre>
</blockquote>
<p>
</p>

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

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


<p id="res-2583"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4567.
</p>

<ol>
<li><p>Change 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a>, class template <code>basic_string</code> synopsis, as indicated</p>

<blockquote>
<pre>
basic_string(const basic_string&amp; str, size_type pos, <del>size_type n = npos,</del>
             const Allocator&amp; a = Allocator());
<ins>basic_string(const basic_string&amp; str, size_type pos, size_type n,
             const Allocator&amp; a = Allocator());</ins>             
</pre>
</blockquote>
</li>

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

<blockquote>
<pre>
basic_string(const basic_string&amp; str,
             size_type pos, <del>size_type n = npos,</del>
             const Allocator&amp; a = Allocator());
</pre>
<blockquote>
<p>
-3- <i>Throws</i>: <code>out_of_range</code> if <code>pos &gt; str.size()</code>.
<p/>
-4- <i>Effects</i>: Constructs an object of class <code>basic_string</code> and determines the effective length <code>rlen</code> 
of the initial string value as <del>the smaller of <code>n</code> and</del> <code>str.size() - pos</code>, as indicated in Table 65.
</p>
</blockquote>
<pre>
<ins>basic_string(const basic_string&amp; str, size_type pos, size_type n,
             const Allocator&amp; a = Allocator());</ins>             
</pre>
<blockquote>
<p>
<ins>-?- <i>Throws</i>: <code>out_of_range</code> if <code>pos &gt; str.size()</code>.</ins>
<p/>
<ins>-?- <i>Effects</i>: Constructs an object of class <code>basic_string</code> and determines the effective length <code>rlen</code> 
of the initial string value as the smaller of <code>n</code> and <code>str.size() - pos</code>, as indicated in Table 65.</ins>
</p>
<table border="1">
<caption>Table 65 &mdash; <code>basic_string(const basic_string&amp;, size_type, <del>size_type,</del> const Allocator&amp;)</code> 
<ins>and <code>basic_string(const basic_string&amp;, size_type, size_type, const Allocator&amp;)</code></ins> effects</caption>

<tr>
<th>Element</th>
<th>Value</th>
</tr>

<tr>
<td><code>data()</code></td>
<td>points at the first element of an allocated copy of
<code>rlen</code> consecutive elements of the string controlled
by <code>str</code> beginning at position <code>pos</code></td>
</tr>

<tr>
<td><code>size()</code></td>
<td><code>rlen</code></td>
</tr>

<tr>
<td><code>capacity()</code></td>
<td>a value at least as large as <code>size()</code></td>
</tr>

</table> 
</blockquote>
</blockquote>
</li>
</ol>





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