<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3048: transform_reduce(exec, first1, last1, first2, init) discards execution policy</title>
<meta property="og:title" content="Issue 3048: transform_reduce(exec, first1, last1, first2, init) discards execution policy">
<meta property="og:description" content="C++ library issue. Status: C++20">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3048.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++20">C++20</a> status.</em></p>
<h3 id="3048"><a href="lwg-defects.html#3048">3048</a>. <code>transform_reduce(exec, first1, last1, first2, init)</code> discards execution policy</h3>
<p><b>Section:</b> 26.10.6 <a href="https://wg21.link/transform.reduce">[transform.reduce]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Billy Robert O'Neal III <b>Opened:</b> 2017-12-15 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Since there exists only one common <i>Effects</i> element for both the parallel and the non-parallel form of
<code>transform_reduce</code> without explicit operation parameters, the current specification of a function call 
<code>std::transform_reduce(exec, first1, last1, first2, init)</code> has the same effect as if the 
<code>ExecutionPolicy</code> would have been ignored. Presumably this effect is unintended.
</p>

<p><i>[
2018-01-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>

<p><i>[2018-3-17 Adopted in Jacksonville]</i></p>



<p id="res-3048"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4713">N4713</a>.</p>

<ol>
<li><p>Modify 26.10.6 <a href="https://wg21.link/transform.reduce">[transform.reduce]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class InputIterator1, class InputIterator2, class T&gt;
  T transform_reduce(InputIterator1 first1, InputIterator1 last1,
                     InputIterator2 first2,
                     T init);
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects:</i> Equivalent to:</ins> 
</p>
<blockquote><pre>
<ins>return transform_reduce(first1, last1, first2, init, plus&lt;&gt;(), multiplies&lt;&gt;());</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class ExecutionPolicy,
         class ForwardIterator1, class ForwardIterator2, class T&gt;
  T transform_reduce(ExecutionPolicy&amp;&amp; exec,
                     ForwardIterator1 first1, ForwardIterator1 last1,
                     ForwardIterator2 first2,
                     T init);
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> Equivalent to: 
</p>
<blockquote><pre>
return transform_reduce(<ins>std::forward&lt;ExecutionPolicy&gt;(exec),</ins> first1, last1, first2, init, plus&lt;&gt;(), 
multiplies&lt;&gt;());
</pre></blockquote>
</blockquote>
</blockquote>

</li>
</ol>




</body>
</html>
