<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 635: domain of allocator::address</title>
<meta property="og:title" content="Issue 635: domain of allocator::address">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue635.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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#Resolved">Resolved</a> status.</em></p>
<h3 id="635"><a href="lwg-defects.html#635">635</a>. domain of <code>allocator::address</code></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#Resolved">Resolved</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2007-02-08 <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#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The table of allocator requirements in 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> describes
<code>allocator::address</code> as:
</p>
<blockquote><pre>
a.address(r)
a.address(s)
</pre></blockquote>
<p>
where <code>r</code> and <code>s</code> are described as:
</p>
<blockquote><p>
a value of type <code>X::reference</code> obtained by the expression <code>*p</code>. 
</p></blockquote>

<p>
and <code>p</code> is 
</p>

<blockquote><p>
a value of type <code>X::pointer</code>, obtained by calling <code>a1.allocate</code>, 
where <code>a1 == a</code>
</p></blockquote>

<p>
This all implies that to get the address of some value of type <code>T</code> that
value must have been allocated by this allocator or a copy of it.
</p>

<p>
However sometimes container code needs to compare the address of an external value of
type <code>T</code> with an internal value.  For example <code>list::remove(const T&amp; t)</code>
may want to compare the address of the external value <code>t</code> with that of a value
stored within the list.  Similarly <code>vector</code> or <code>deque insert</code> may
want to make similar comparisons (to check for self-referencing calls).
</p>

<p>
Mandating that <code>allocator::address</code> can only be called for values which the
allocator allocated seems overly restrictive.
</p>

<p><i>[
post San Francisco:
]</i></p>


<blockquote><p>
Pablo recommends NAD Editorial, solved by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2768.pdf">N2768</a>.
</p></blockquote>

<p><i>[
2009-04-28 Pablo adds:
]</i></p>


<blockquote><p>
Tentatively-ready NAD Editorial as fixed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2768.pdf">N2768</a>.
</p></blockquote>

<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote><p>
Fixed by N2768.
</p></blockquote>

<p><i>[
2009-07-28 Reopened by Alisdair.  No longer solved by concepts.
]</i></p>


<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
<del>NAD Editorial</del><ins>Resolved</ins>.  Addressed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2982.pdf">N2982</a>.
</p></blockquote>



<p id="res-635"><b>Proposed resolution:</b></p>
<p>
Change 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a>:
</p>

<blockquote>
<p>
<code>r</code> : a value of type <code>X::reference</code> <del>obtained by the expression *p</del>.
</p>
<p>
<code>s</code> : a value of type <code>X::const_reference</code> <del>obtained by the 
expression <code>*q</code> or by conversion from a value <code>r</code></del>.
</p>
</blockquote>

<p><i>[
post Oxford:  This would be rendered NAD Editorial by acceptance of
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2257.html">N2257</a>.
]</i></p>


<p><i>[
Kona (2007):  This issue is section 8 of N2387.  There was some discussion of it but
no resolution to this issue was recorded.  Moved to Open.
]</i></p>







</body>
</html>
