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<title>Issue 1404: pointer_traits should have a size_type member</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++11">C++11</a> status.</em></p>
<h3 id="1404"><a href="lwg-defects.html#1404">1404</a>. <code>pointer_traits</code> should have a <code>size_type</code> member</h3>
<p><b>Section:</b> 20.2.3 <a href="https://wg21.link/pointer.traits">[pointer.traits]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> INCITS <b>Opened:</b> 2010-08-25 <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#pointer.traits">issues</a> in [pointer.traits].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US-106</b></p>
<p>
<code>pointer_traits</code> should have a <code>size_type</code> member for completeness.
</p>
<p>
Add <code>typedef <i>see below</i> size_type;</code> to the generic
<code>pointer_traits</code> template and <code>typedef size_t
size_type;</code> to <code>pointer_traits&lt;T*></code>. Use
<code>pointer_traits::size_type</code> and
<code>pointer_traits::difference_type</code> as the defaults for
<code>allocator_traits::size_type</code> and
<code>allocator_traits::difference_type</code>.
</p>
<p>
See Appendix 1 - Additional Details
</p>

<p><i>[
Post-Rapperswil, Pablo provided wording:
]</i></p>


<p>
The original ballot comment reads simply: "<code>pointer_traits</code> should have a 
<code>size_type</code> for completeness."  The additional details reveal, however,
that the desire for a <code>size_type</code> is actually driven by the needs
of <code>allocator_traits</code>.  The <code>allocator_traits</code> template should get its
default <code>difference_type</code> from <code>pointer_traits</code> but if it did,
it should get its <code>size_type</code> from the same source.  Unfortunately,
there is no obvious meaning for <code>size_type</code> in <code>pointer_traits</code>.
</p>
<p>
Alisdair suggested, however, that the natural relationship between
<code>difference_type</code> and <code>size_type</code> can be expressed simply by the
<code>std::make_unsigned&lt;T></code> metafunction.  Using this metafunction,
we can easily define <code>size_type</code> for <code>allocator_traits</code> without
artificially adding <code>size_type</code> to <code>pointer_traits</code>.
</p>

<blockquote><p>
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
</p></blockquote>

<p><i>[
Adopted at 2010-11 Batavia
]</i></p>





<p id="res-1404"><b>Proposed resolution:</b></p>
<p>
In [allocator.requirements], Table 42, change two rows as follows:
</p>
<blockquote>
<table border="1">
  <tr>
    <td><code>X::size_type</code></td>
    <td>unsigned integral type</td>
    <td>a type that can represent the size of the largest object in the
    allocation model</td>
    <td><code>
      <del>size_t</del>
      <ins>make_unsigned&lt;X::difference_type&gt;::type</ins>
    </code></td>
  </tr>
  <tr>
    <td><code>X::difference_type</code></td>
    <td>signed integral type</td>
    <td>a type that can represent the difference between any two pointers in
    the allocation model</td>
    <td><code>
      <del>ptrdiff_t</del>
      <ins>pointer_traits&lt;X::pointer&gt;::difference_type</ins>
    </code></td>
  </tr>
</table>
</blockquote>
<p>
In [allocator.traits.types], Change the definition of <code>difference_type</code> and
<code>size_type</code> as follows:
</p>
<blockquote><p>
  <code>typedef</code> <i>see below</i> <code>difference_type;</code>
  </p><blockquote><p>
    <i>Type:</i> <code>Alloc::difference_type</code> if such a type exists,
    else <code><del>ptrdiff_t</del>
    <ins>pointer_traits&lt;pointer&gt;::difference_type</ins></code>.
  </p></blockquote>
  <p>
  <code>typedef</code> <i>see below</i> <code>size_type;</code>
  </p><blockquote><p>
    <i>Type:</i> <code>Alloc::size_type</code> if such a type exists,
    else <code><del>size_t</del>
    <ins>make_unsigned&lt;difference_type&gt;::type</ins></code>.
  </p></blockquote>
</blockquote>





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