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<title>Issue 274: a missing/impossible allocator requirement</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#CD1">CD1</a> status.</em></p>
<h3 id="274"><a href="lwg-defects.html#274">274</a>. a missing/impossible allocator requirement</h3>
<p><b>Section:</b> 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2000-11-02 <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#allocator.requirements">active issues</a> in [allocator.requirements].</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.requirements">issues</a> in [allocator.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
I see that table 31 in 20.1.5, p3 allows T in std::allocator&lt;T&gt; to be of
any type. But the synopsis in 20.4.1 calls for allocator&lt;&gt;::address() to
be overloaded on reference and const_reference, which is ill-formed for
all T = const U. In other words, this won't work:
</p>

<p>
template class std::allocator&lt;const int&gt;;
</p>

<p>
The obvious solution is to disallow specializations of allocators on
const types. However, while containers' elements are required to be
assignable (which rules out specializations on const T's), I think that
allocators might perhaps be potentially useful for const values in other
contexts. So if allocators are to allow const types a partial
specialization of std::allocator&lt;const T&gt; would probably have to be
provided.
</p>


<p id="res-274"><b>Proposed resolution:</b></p>
<p>Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from</p>

    <blockquote><p>
    any type
    </p></blockquote>

<p>to</p>
    <blockquote><p>
    any non-const, non-reference type
    </p></blockquote>

<p><i>[Redmond: previous proposed resolution was "any non-const,
non-volatile, non-reference type".  Got rid of the "non-volatile".]</i></p>




<p><b>Rationale:</b></p>
<p>
Two resolutions were originally proposed: one that partially
specialized std::allocator for const types, and one that said an
allocator's value type may not be const.  The LWG chose the second.
The first wouldn't be appropriate, because allocators are intended for
use by containers, and const value types don't work in containers.
Encouraging the use of allocators with const value types would only
lead to unsafe code.
</p>
<p>
The original text for proposed resolution 2 was modified so that it
also forbids volatile types and reference types.
</p>

<p><i>[Cura&ccedil;ao: LWG double checked and believes volatile is correctly
excluded from the PR.]</i></p>







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