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What am I doing wrong?</title> +<link rel="stylesheet" href="../boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> +<link rel="home" href="../index.html" title="Boost.Python"> +<link rel="up" href="../faq.html" title="Chapter 4. Frequently Asked Questions (FAQs)"> +<link rel="prev" href="../faq.html" title="Chapter 4. Frequently Asked Questions (FAQs)"> +<link rel="next" href="is_return_internal_reference_eff.html" title="Is return_internal_reference efficient?"> +</head> +<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> +<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../images/boost.png"></td></tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="../faq.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../faq.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="is_return_internal_reference_eff.html"><img src="../images/next.png" alt="Next"></a> +</div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="faq.i_m_getting_the_attempt_to_retur"></a><a class="link" href="i_m_getting_the_attempt_to_retur.html" title="I'm getting the "attempt to return dangling reference" error. What am I doing wrong?">I'm getting the + "attempt to return dangling reference" error. What am I doing wrong?</a> +</h3></div></div></div> +<p> + That exception is protecting you from causing a nasty crash. It usually happens + in response to some code like this: + </p> +<pre class="programlisting"><span class="identifier">period</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">get_floating_frequency</span><span class="special">()</span> <span class="keyword">const</span> +<span class="special">{</span> + <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">call_method</span><span class="special"><</span><span class="identifier">period</span> <span class="keyword">const</span> <span class="special">&>(</span> + <span class="identifier">m_self</span><span class="special">,</span><span class="string">"get_floating_frequency"</span><span class="special">);</span> +<span class="special">}</span> +</pre> +<p> + And you get: + </p> +<pre class="programlisting"><span class="identifier">ReferenceError</span><span class="special">:</span> <span class="identifier">Attempt</span> <span class="identifier">to</span> <span class="keyword">return</span> <span class="identifier">dangling</span> <span class="identifier">reference</span> <span class="identifier">to</span> <span class="identifier">object</span> <span class="identifier">of</span> <span class="identifier">type</span><span class="special">:</span> +<span class="keyword">class</span> <span class="identifier">period</span> +</pre> +<p> + In this case, the Python method invoked by <code class="computeroutput"><span class="identifier">call_method</span></code> + constructs a new Python object. You're trying to return a reference to a + C++ object (an instance of <code class="computeroutput"><span class="keyword">class</span> <span class="identifier">period</span></code>) contained within and owned by that + Python object. Because the called method handed back a brand new object, + the only reference to it is held for the duration of <code class="computeroutput"><span class="identifier">get_floating_frequency</span><span class="special">()</span></code> above. When the function returns, the Python + object will be destroyed, destroying the instance of <code class="computeroutput"><span class="keyword">class</span> + <span class="identifier">period</span></code>, and leaving the returned + reference dangling. That's already undefined behavior, and if you try to + do anything with that reference you're likely to cause a crash. Boost.Python + detects this situation at runtime and helpfully throws an exception instead + of letting you do that. + </p> +</div> +<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> +<td align="left"></td> +<td align="right"><div class="copyright-footer">Copyright © 2002-2015 David + Abrahams, Stefan Seefeld<p> + Distributed under the Boost Software License, Version 1.0. (See accompanying + file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) + </p> +</div></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="../faq.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../faq.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="is_return_internal_reference_eff.html"><img src="../images/next.png" alt="Next"></a> +</div> +</body> +</html> |