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<title>I'm getting the "attempt to return dangling reference" error. What am I doing wrong?</title>
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<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.
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Abrahams, Stefan Seefeld<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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