<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3628: "Effects: Equivalent to:" and uninitialized memory algorithms</title>
<meta property="og:title" content="Issue 3628: &quot;Effects: Equivalent to:&quot; and uninitialized memory algorithms">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3628.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#New">New</a> status.</em></p>
<h3 id="3628"><a href="lwg-active.html#3628">3628</a>. "<i>Effects:</i> Equivalent to:" and uninitialized memory algorithms</h3>
<p><b>Section:</b> 26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2021-10-23 <b>Last modified:</b> 2022-01-29</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#specialized.algorithms">active issues</a> in [specialized.algorithms].</p>
<p><b>View all other</b> <a href="lwg-index.html#specialized.algorithms">issues</a> in [specialized.algorithms].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Most uninitialized memory algorithms (26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a>) are specified by 
plain "<i>Effects:</i> Equivalent to:". According to 16.3.2.4 <a href="https://wg21.link/structure.specifications">[structure.specifications]</a>/4, 
such wording requires propagation of "<i>Constraints</i>" of selected constructors. The first two 
overloads of <code>std::reduce</code> (26.10.4 <a href="https://wg21.link/reduce">[reduce]</a>) are specified similarly.
<p/>
I feel the wording for uninitialized memory algorithms is incorrect, because it means that the 
constraints, especially for algorithms in the <code>std</code>, depend on "<code>Constraints</code>" in 
the specifications of many standard library types (but not any user-defined type), which is 
implementable but brings serious inconsistency.
<p/>
Perhaps we should add "<i>Mandates:</i>" to these algorithms (except for algorithms in <code>std::ranges</code>).
</p>

<p><i>[2022-01-29; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
<br/>
Tim:
"P2. Not for this particular case (I'm pretty sure there'll be agreement
that this shouldn't induce any constraint), but for the more general
issue of "<em>Effects:</em> Equivalent to" propagating <em>Constraints:</em>;
I'm not sure that's the right approach in general (unlike the other elements,
<em>Constraints:</em> requires special handling beyond "use this code" and is
pretty hard to work through if we have a lengthy code block) - and it
certainly doesn't really make a lot of sense to propagate <em>Constraints:</em>
but not actual core-language constraints."
</p>



<p id="res-3628"><b>Proposed resolution:</b></p>





</body>
</html>
