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<title>Issue 2069: Inconsistent exception spec for basic_string move constructor</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++14">C++14</a> status.</em></p>
<h3 id="2069"><a href="lwg-defects.html#2069">2069</a>. Inconsistent exception spec for <code>basic_string</code> move constructor</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++14">C++14</a>
 <b>Submitter:</b> Bo Persson <b>Opened:</b> 2011-07-01 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</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++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Sub-clause 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> contains these constructors in paragraphs 2 and 3:
</p>
<blockquote><pre>
basic_string(const basic_string&lt;charT,traits,Allocator&gt;&amp; str);
basic_string(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; str) noexcept;
</pre><blockquote>
<p>
[&hellip;]
<p/>
-3- <i>Throws</i>: The second form throws nothing if the allocator's move constructor throws nothing.
</p></blockquote></blockquote>
<p>
How can it <i>ever</i> throw anything if it is marked <code>noexcept</code>?
</p>
<p><i>[2011-07-11: Daniel comments and suggests wording changes]</i></p>

<p>
Further, according to paragraph 18 of the same sub-clause:
</p>
<blockquote><pre>
basic_string(const basic_string&amp; str, const Allocator&amp; alloc);
basic_string(basic_string&amp;&amp; str, const Allocator&amp; alloc);
</pre><blockquote>
<p>
[&hellip;]
<p/>
-18- <i>Throws</i>: The second form throws nothing if <code>alloc == str.get_allocator()</code> 
unless the copy constructor for <code>Allocator</code> throws.
</p>
</blockquote></blockquote>
<p>
The constraint &quot;unless the copy constructor for <code>Allocator</code> throws&quot;
is redundant, because according to Table 28 &mdash; Allocator requirements, the expressions
</p>
<blockquote><pre>
X a1(a);
X a(b);
</pre></blockquote>
<p>
impose the requirement: &quot;Shall not exit via an exception&quot;.
</p>

<p><i>[
2011 Bloomington.
]</i></p>


<p>
Move to Ready.
</p>



<p id="res-2069"><b>Proposed resolution:</b></p>
<p>This wording is relative to the FDIS.</p>

<ol>
<li><p>Change 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> p3 as indicated (This move constructor has a wide
contract and is therefore safely marked as <code>noexcept</code>):</p>

<blockquote><pre>
basic_string(const basic_string&lt;charT,traits,Allocator&gt;&amp; str);
basic_string(basic_string&lt;charT,traits,Allocator&gt;&amp;&amp; str) noexcept;
</pre><blockquote>
<p>
-2- <i>Effects</i>: Constructs an object of class <code>basic_string</code> as indicated in Table 64. 
In the second form, <code>str</code> is left in a valid state with an unspecified value.
<p/>
<del>-3- <i>Throws</i>: The second form throws nothing if the allocator's move constructor throws nothing.</del>
</p>
</blockquote></blockquote>
</li>

<li><p>Change 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> p18 as indicated (This move-like constructor may throw,
if the allocators don't compare equal, but not because of a potentially throwing allocator
copy constructor, only because the allocation attempt may fail and throw an exception):</p>

<blockquote><pre>
basic_string(const basic_string&amp; str, const Allocator&amp; alloc);
basic_string(basic_string&amp;&amp; str, const Allocator&amp; alloc);
</pre><blockquote>
<p>
[&hellip;]
<p/>
-18- <i>Throws</i>: The second form throws nothing if <code>alloc == str.get_allocator()</code> 
<del>unless the copy constructor for <code>Allocator</code> throws</del>.
</p>
</blockquote></blockquote>
</li>

</ol>






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