<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 928: Wrong concepts used for tuple's comparison operators</title>
<meta property="og:title" content="Issue 928: Wrong concepts used for tuple's comparison operators">
<meta property="og:description" content="C++ library issue. Status: NAD Concepts">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue928.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#NAD_Concepts">NAD Concepts</a> status.</em></p>
<h3 id="928"><a href="lwg-closed.html#928">928</a>. Wrong concepts used for <code>tuple</code>'s comparison operators</h3>
<p><b>Section:</b> 22.4.9 <a href="https://wg21.link/tuple.rel">[tuple.rel]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Concepts">NAD Concepts</a>
 <b>Submitter:</b> Joe Gottman <b>Opened:</b> 2008-10-28 <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#tuple.rel">active issues</a> in [tuple.rel].</p>
<p><b>View all other</b> <a href="lwg-index.html#tuple.rel">issues</a> in [tuple.rel].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Concepts">NAD Concepts</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In the latest working draft for C++0x, <code>tuple</code>'s <code>operator==</code> and <code>operator&lt;</code>
are declared as 
</p>

<blockquote><pre>
template&lt;class... TTypes, class... UTypes&gt; 
  requires EqualityComparable&lt;TTypes, UTypes&gt;... 
  bool operator==(const tuple&lt;TTypes...&gt;&amp; t, const tuple&lt;UTypes...&gt;&amp; u);
</pre></blockquote>

<p>
and
</p>

<blockquote><pre>
template&lt;class... TTypes, class... UTypes&gt; 
  requires LessThanComparable&lt;TTypes, UTypes&gt;... 
  bool operator&lt;(const tuple&lt;TTypes...&gt;&amp; t, const tuple&lt;UTypes...&gt;&amp; u);
</pre></blockquote>

<p>
But the concepts <code>EqualityComparable</code> and <code>LessThanComparable</code> only take one 
parameter, not two.  Also, even if <code>LessThanComparable</code> could take two 
parameters, the definition of <code>tuple::operator&lt;()</code> should also require 
</p>

<blockquote><pre>
LessThanComparable&lt;UTypes, TTypes&gt;... // (note the order) 
</pre></blockquote>

<p>
since the algorithm for <code>tuple::operator&lt;</code> is the following (pseudo-code)
</p>

<blockquote><pre>
for (size_t N = 0; N &lt; sizeof...(TTypes); ++N) { 
    if (get&lt;N&gt;(t) &lt; get&lt;N&gt;(u) return true; 
    else if ((get&lt;N&gt;(u) &lt; get&lt;N&gt;(t)) return false; 
} 

return false; 
</pre></blockquote>

<p>
Similar problems hold for <code>tuples</code>'s other comparison operators.
</p>

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


<blockquote><p>
Recommend Tentatively Ready.
</p></blockquote>



<p id="res-928"><b>Proposed resolution:</b></p>
<p>
In 22.4.1 <a href="https://wg21.link/tuple.general">[tuple.general]</a> and 22.4.9 <a href="https://wg21.link/tuple.rel">[tuple.rel]</a> change:
</p>

<blockquote><pre>
template&lt;class... TTypes, class... UTypes&gt;
  requires <del>EqualityComparable</del><ins>HasEqualTo</ins>&lt;TTypes, UTypes&gt;...
  bool operator==(const tuple&lt;TTypes...&gt;&amp;, const tuple&lt;UTypes...&gt;&amp;);

template&lt;class... TTypes, class... UTypes&gt;
  requires <del>LessThanComparable</del><ins>HasLess</ins>&lt;TTypes, UTypes&gt;... <ins>&amp;&amp; HasLess&lt;UTypes, TTypes&gt;...</ins>
  bool operator&lt;(const tuple&lt;TTypes...&gt;&amp;, const tuple&lt;UTypes...&gt;&amp;);

template&lt;class... TTypes, class... UTypes&gt;
  requires <del>EqualityComparable</del><ins>HasEqualTo</ins>&lt;TTypes, UTypes&gt;...
  bool operator!=(const tuple&lt;TTypes...&gt;&amp;, const tuple&lt;UTypes...&gt;&amp;);

template&lt;class... TTypes, class... UTypes&gt;
  requires <del>LessThanComparable</del><ins>HasLess</ins>&lt;<del>U</del><ins>T</ins>Types, <del>T</del><ins>U</ins>Types&gt;... <ins>&amp;&amp; HasLess&lt;UTypes, TTypes&gt;...</ins>
  bool operator&gt;(const tuple&lt;TTypes...&gt;&amp;, const tuple&lt;UTypes...&gt;&amp;);

template&lt;class... TTypes, class... UTypes&gt;
  requires <del>LessThanComparable</del><ins>HasLess</ins>&lt;<del>U</del><ins>T</ins>Types, <del>T</del><ins>U</ins>Types&gt;... <ins>&amp;&amp; HasLess&lt;UTypes, TTypes&gt;...</ins>
  bool operator&lt;=(const tuple&lt;TTypes...&gt;&amp;, const tuple&lt;UTypes...&gt;&amp;);

template&lt;class... TTypes, class... UTypes&gt;
  requires <del>LessThanComparable</del><ins>HasLess</ins>&lt;TTypes, UTypes&gt;... <ins>&amp;&amp; HasLess&lt;UTypes, TTypes&gt;...</ins>
  bool operator&gt;=(const tuple&lt;TTypes...&gt;&amp;, const tuple&lt;UTypes...&gt;&amp;);
</pre></blockquote>





</body>
</html>
