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<title>Issue 42: String ctors specify wrong default allocator</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#TC1">TC1</a> status.</em></p>
<h3 id="42"><a href="lwg-defects.html#42">42</a>. String ctors specify wrong default allocator</h3>
<p><b>Section:</b> 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Nathan Myers <b>Opened:</b> 1998-08-06 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#basic.string">active issues</a> in [basic.string].</p>
<p><b>View all other</b> <a href="lwg-index.html#basic.string">issues</a> in [basic.string].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>The basic_string&lt;&gt; copy constructor: </p>

<pre>basic_string(const basic_string&amp; str, size_type pos = 0,
             size_type n = npos, const Allocator&amp; a = Allocator()); </pre>

<p>specifies an Allocator argument default value that is
counter-intuitive. The natural choice for a the allocator to copy from
is str.get_allocator(). Though this cannot be expressed in
default-argument notation, overloading suffices. </p>

<p>Alternatively, the other containers in Clause 23 (deque, list,
vector) do not have this form of constructor, so it is inconsistent,
and an evident source of confusion, for basic_string&lt;&gt; to have
it, so it might better be removed. </p>


<p id="res-42"><b>Proposed resolution:</b></p>
<p> In 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a>, replace the declaration of the copy
constructor as follows: </p>

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

<p>In 27.4.3.2 <a href="https://wg21.link/string.require">[string.require]</a>, replace the copy constructor declaration
as above. Add to paragraph 5, Effects:</p>

<blockquote>
  <p>In the first form, the Allocator value used is copied from
  <code>str.get_allocator()</code>.</p>
</blockquote>


<p><b>Rationale:</b></p>
<p>The LWG believes the constructor is actually broken, rather than
just an unfortunate design choice.</p>

<p>The LWG considered two other possible resolutions:</p>

<p>A. In 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a>, replace the declaration of the copy
constructor as follows:</p>

<blockquote>
  <pre>basic_string(const basic_string&amp; str, size_type pos = 0,
             size_type n = npos);
basic_string(const basic_string&amp; str, size_type pos,
             size_type n, const Allocator&amp; a); </pre>
</blockquote>

<p>In 27.4.3.2 <a href="https://wg21.link/string.require">[string.require]</a>, replace the copy constructor declaration
as above. Add to paragraph 5, Effects: </p>

<blockquote>
  <p>When no <code>Allocator</code> argument is provided, the string is constructed using the
  value <code>str.get_allocator()</code>. </p>
</blockquote>

<p>B. In 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a>, and also in 27.4.3.2 <a href="https://wg21.link/string.require">[string.require]</a>, replace
the declaration of the copy constructor as follows: </p>

<blockquote>
  <pre>basic_string(const basic_string&amp; str, size_type pos = 0,
             size_type n = npos); </pre>
</blockquote>

<p>The proposed resolution reflects the original intent of the LWG. It
was also noted by Pete Becker that this fix &quot;will cause
a small amount of existing code to now work correctly.&quot;</p>

<p><i>[
Kona: issue editing snafu fixed - the proposed resolution now correctly
reflects the LWG consensus.
]</i></p>






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