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<title>How can I wrap a function which needs to take ownership of a raw pointer?</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="faq.how_can_i_wrap_a_function_which0"></a><a class="link" href="how_can_i_wrap_a_function_which0.html" title="How can I wrap a function which needs to take ownership of a raw pointer?">How can I wrap
      a function which needs to take ownership of a raw pointer?</a>
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<p>
        <span class="bold"><strong>Q:</strong></span> Part of an API that I'm wrapping goes
        something like this:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span> <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">void</span> <span class="identifier">add</span><span class="special">(</span> <span class="identifier">A</span><span class="special">*</span> <span class="special">);</span> <span class="special">}</span>
<span class="identifier">where</span> <span class="identifier">B</span><span class="special">::</span><span class="identifier">add</span><span class="special">()</span> <span class="identifier">takes</span> <span class="identifier">ownership</span> <span class="identifier">of</span> <span class="identifier">the</span> <span class="identifier">pointer</span> <span class="identifier">passed</span> <span class="identifier">to</span> <span class="identifier">it</span><span class="special">.</span>
</pre>
<p>
        However:
      </p>
<pre class="programlisting"><span class="identifier">a</span> <span class="special">=</span> <span class="identifier">mod</span><span class="special">.</span><span class="identifier">A</span><span class="special">()</span>
<span class="identifier">b</span> <span class="special">=</span> <span class="identifier">mod</span><span class="special">.</span><span class="identifier">B</span><span class="special">()</span>
<span class="identifier">b</span><span class="special">.</span><span class="identifier">add</span><span class="special">(</span> <span class="identifier">a</span> <span class="special">)</span>
<span class="identifier">del</span> <span class="identifier">a</span>
<span class="identifier">del</span> <span class="identifier">b</span>
<span class="preprocessor"># python</span> <span class="identifier">interpreter</span> <span class="identifier">crashes</span>
<span class="preprocessor"># later</span> <span class="identifier">due</span> <span class="identifier">to</span> <span class="identifier">memory</span> <span class="identifier">corruption</span><span class="special">.</span>
</pre>
<p>
        Even binding the lifetime of a to b via <code class="computeroutput"><span class="identifier">with_custodian_and_ward</span></code>
        doesn't prevent the python object a from ultimately trying to delete the
        object it's pointing to. Is there a way to accomplish a 'transfer-of-ownership'
        of a wrapped C++ object?
      </p>
<p>
        --Bruce Lowery
      </p>
<p>
        Yes: Make sure the C++ object is held by auto_ptr:
      </p>
<pre class="programlisting"><span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">auto_ptr</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;(</span><span class="string">"A"</span><span class="special">)</span>
  <span class="special">...</span>
  <span class="special">;</span>
</pre>
<p>
        Then make a thin wrapper function which takes an auto_ptr parameter:
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">b_insert</span><span class="special">(</span><span class="identifier">B</span> <span class="special">&amp;</span><span class="identifier">b</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">auto_ptr</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">{</span>
  <span class="identifier">b</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">a</span><span class="special">.</span><span class="identifier">get</span><span class="special">());</span>
  <span class="identifier">a</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
        Wrap that as B.add. Note that pointers returned via <code class="computeroutput"><span class="identifier">manage_new_object</span></code>
        will also be held by <code class="computeroutput"><span class="identifier">auto_ptr</span></code>,
        so this transfer-of-ownership will also work correctly.
      </p>
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