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<title>Issue 578: purpose of hint to allocator::allocate()</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="578"><a href="lwg-defects.html#578">578</a>. purpose of hint to allocator::allocate()</h3>
<p><b>Section:</b> 20.2.10.2 <a href="https://wg21.link/allocator.members">[allocator.members]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2006-05-17 <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#allocator.members">issues</a> in [allocator.members].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
        <p>

The     description    of     the     allocator    member     function
<code>allocate()</code>  requires  that  the <i>hint</i>  argument  be
either 0 or a  value previously returned from <code>allocate()</code>.
Footnote 227 further suggests that  containers may pass the address of
an adjacent element as this argument.

        </p>
        <p>

I  believe  that  either  the  footnote  is  wrong  or  the  normative
requirement that  the argument be  a value previously returned  from a
call to  <code>allocate()</code> is wrong. The latter  is supported by
the resolution  to issue 20-004 proposed in  c++std-lib-3736 by Nathan
Myers. In addition,  the <i>hint</i> is an ordinary  void* and not the
<code>pointer</code>  type returned  by  <code>allocate()</code>, with
the  two  types potentially  being  incompatible  and the  requirement
impossible to satisfy.

        </p>
        <p>

See also c++std-lib-14323 for some  more context on where this came up
(again).

        </p>
    

    <p id="res-578"><b>Proposed resolution:</b></p>
        <p>

Remove  the requirement  in  20.6.1.1, p4  that  the hint  be a  value
previously returned from <code>allocate()</code>. Specifically, change
the paragraph as follows:

        </p>
<p>
<del><i>Requires</i>: <i>hint</i> either 0 or previously obtained  from  member
<code>allocate</code>  and  not  yet passed  to member  <code>deallocate</code>.
The value hint may be used by an implementation to help improve performance
<sup>223)</sup>.</del> <ins>[<i>Note:</i> The value <i>hint</i> may be used by an
implementation to help improve performance. -- <i>end note</i>]</ins>
</p>
<blockquote><p>
<del>[Footnote: <sup>223)</sup>In a container member function, the address of an
adjacent element is often a good choice to pass for this argument.</del>
</p></blockquote>
    



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