<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 842: ConstructibleAsElement and bit containers</title>
<meta property="og:title" content="Issue 842: ConstructibleAsElement and bit containers">
<meta property="og:description" content="C++ library issue. Status: CD1">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue842.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#CD1">CD1</a> status.</em></p>
<h3 id="842"><a href="lwg-defects.html#842">842</a>. <code>ConstructibleAsElement</code> and bit containers</h3>
<p><b>Section:</b> 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>, 23.3.14 <a href="https://wg21.link/vector.bool">[vector.bool]</a>, 22.9.2 <a href="https://wg21.link/template.bitset">[template.bitset]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2008-06-03 <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#container.requirements">issues</a> in [container.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>
23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> p. 3 says:
</p>

<blockquote><p>
Objects stored in these components shall be constructed using
<code>construct_element</code> (20.6.9). For each operation that inserts an
element of type <code>T</code> into a container (<code>insert</code>,
<code>push_back</code>, <code>push_front</code>, <code>emplace</code>, etc.) with
arguments <code>args... T</code> shall be <code>ConstructibleAsElement</code>,
as described in table 88. [<i>Note:</i> If the component is instantiated
with a scoped allocator of type <code>A</code> (i.e., an allocator for which
<code>is_scoped_allocator&lt;A&gt;::value</code> is <code>true</code>), then
<code>construct_element</code> may pass an inner allocator argument to
<code>T</code>'s constructor. <i>-- end note</i>]
</p></blockquote>

<p>
However <code>vector&lt;bool, A&gt;</code> (23.3.14 <a href="https://wg21.link/vector.bool">[vector.bool]</a>) and <code>bitset&lt;N&gt;</code> 
(22.9.2 <a href="https://wg21.link/template.bitset">[template.bitset]</a>) store bits, not <code>bool</code>s, and <code>bitset&lt;N&gt;</code>
does not even have an allocator.  But these containers are governed by this clause.  Clearly this
is not implementable.
</p>


<p id="res-842"><b>Proposed resolution:</b></p>
<p>
Change 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> p. 3:
</p>

<blockquote><p>
Objects stored in these components shall be constructed using
<code>construct_element</code> (20.6.9)<ins>, unless otherwise specified</ins>.
For each operation that inserts an
element of type <code>T</code> into a container (<code>insert</code>,
<code>push_back</code>, <code>push_front</code>, <code>emplace</code>, etc.) with
arguments <code>args... T</code> shall be <code>ConstructibleAsElement</code>,
as described in table 88. [<i>Note:</i> If the component is instantiated
with a scoped allocator of type <code>A</code> (i.e., an allocator for which
<code>is_scoped_allocator&lt;A&gt;::value</code> is <code>true</code>), then
<code>construct_element</code> may pass an inner allocator argument to
<code>T</code>'s constructor. <i>-- end note</i>]
</p></blockquote>

<p>
Change 23.3.14 <a href="https://wg21.link/vector.bool">[vector.bool]</a>/p2:
</p>

<blockquote><p>
Unless described below, all operations have the same requirements and semantics as the primary <code>vector</code> template, 
except that operations dealing with the <code>bool</code> value type map to bit values in the container storage<ins>,
and <code>construct_element</code> (23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>) is not used to construct these values</ins>.
</p></blockquote>

<p>
Move 22.9.2 <a href="https://wg21.link/template.bitset">[template.bitset]</a> to clause 20.
</p>






</body>
</html>
