<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 119: Should virtual functions be allowed to strengthen the exception specification?</title>
<meta property="og:title" content="Issue 119: Should virtual functions be allowed to strengthen the exception specification?">
<meta property="og:description" content="C++ library issue. Status: TC1">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue119.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#TC1">TC1</a> status.</em></p>
<h3 id="119"><a href="lwg-defects.html#119">119</a>. Should virtual functions be allowed to strengthen the exception specification?</h3>
<p><b>Section:</b> 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Judy Ward <b>Opened:</b> 1998-12-15 <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#res.on.exception.handling">active issues</a> in [res.on.exception.handling].</p>
<p><b>View all other</b> <a href="lwg-index.html#res.on.exception.handling">issues</a> in [res.on.exception.handling].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>Section 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a> states: </p>

<p>&quot;An implementation may strengthen the exception-specification
for a function by removing listed exceptions.&quot; </p>

<p>The problem is that if an implementation is allowed to do this for
virtual functions, then a library user cannot write a class that
portably derives from that class. </p>

<p>For example, this would not compile if ios_base::failure::~failure
had an empty exception specification: </p>

<pre>#include &lt;ios&gt;
#include &lt;string&gt;

class D : public std::ios_base::failure {
public:
        D(const std::string&amp;);
        ~D(); // error - exception specification must be compatible with 
              // overridden virtual function ios_base::failure::~failure()
};</pre>


<p id="res-119"><b>Proposed resolution:</b></p>
<p>Change Section 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a> from:</p>

<p>&nbsp;&nbsp;&nbsp;&nbsp; &quot;may strengthen the
exception-specification for a function&quot;</p>

<p>to:</p>

<p>&nbsp;&nbsp;&nbsp;&nbsp; &quot;may strengthen the
exception-specification for a non-virtual function&quot;. </p>





</body>
</html>
