<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 931: type trait extent&lt;T, I&gt;</title>
<meta property="og:title" content="Issue 931: type trait extent&lt;T, I&gt;">
<meta property="og:description" content="C++ library issue. Status: C++11">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue931.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#C++11">C++11</a> status.</em></p>
<h3 id="931"><a href="lwg-defects.html#931">931</a>. type trait <code>extent&lt;T, I&gt;</code></h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Yechezkel Mett <b>Opened:</b> 2008-11-04 <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#meta.unary.prop">active issues</a> in [meta.unary.prop].</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.unary.prop">issues</a> in [meta.unary.prop].</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>
The draft (N2798) says in 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> Table 44: 
</p>
<blockquote>
<table border="1">
<caption>Table 44 -- Type property queries</caption>
<tr><th>Template</th><th>Value</th></tr>
<tr>
<td>
<code>template &lt;class T, unsigned I = 0&gt; struct extent;</code>
</td>
<td>
If <code>T</code> is not an array type (8.3.4), or if it has rank less than
<code>I</code>, or if <code>I</code> is 0
and <code>T</code> has type "array of unknown bound of <code>U</code>", then 0; otherwise, the
size of the <code>I</code>'th dimension of <code>T</code>
</td>
</tr>
</table>
</blockquote>

<p>
Firstly it isn't clear from the wording if <code>I</code> is 0-based or 1-based 
("the <code>I</code>'th dimension" sort of implies 1-based). From the following 
example it is clear that the intent is 0-based, in which case it 
should say "or if it has rank less than or equal to <code>I</code>".
</p>
<p>
Sanity check:
</p>
<p>
The example says <code>assert((extent&lt;int[2], 1&gt;::value) == 0);</code>
</p>
<p>
Here the rank is 1 and <code>I</code> is 1, but the desired result is 0.
</p>

<p><i>[
Post Summit:
]</i></p>


<blockquote>
<p>
Do not use "size" or "value", use "bound". Also, move the
cross-reference to 8.3.4 to just after "bound".
</p>
<p>
Recommend Tentatively Ready.
</p>
</blockquote>



<p id="res-931"><b>Proposed resolution:</b></p>
<p>
In Table 44 of 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a>, third row, column "Value",
change the cell content:
</p>

<blockquote>
<table border="1">
<caption>Table 44 -- Type property queries</caption>
<tr><th>Template</th><th>Value</th></tr>
<tr>
<td>
<code>template &lt;class T, unsigned I = 0&gt; struct extent;</code>
</td>
<td>
If <code>T</code> is not an array type <del>(8.3.4)</del>, or if it has rank less than
<ins> or equal to</ins> <code>I</code>, or if <code>I</code> is 0
and <code>T</code> has type "array of unknown bound of <code>U</code>", then 0; otherwise, the
<del>size</del> <ins>bound (8.3.4)</ins> of the <code>I</code>'th dimension of <code>T</code><ins>,
where indexing of <code>I</code> is zero-based.</ins>
</td>
</tr>
</table>
</blockquote>

<p><i>[
Wording supplied by Daniel.
]</i></p>







</body>
</html>
