1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>How can I wrap a function which needs to take ownership of a raw pointer?</title> 5<link rel="stylesheet" href="../boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Boost.Python"> 8<link rel="up" href="../faq.html" title="Chapter 5. Frequently Asked Questions (FAQs)"> 9<link rel="prev" href="how_can_i_find_the_existing_pyob.html" title="How can I find the existing PyObject that holds a C++ object?"> 10<link rel="next" href="compilation_takes_too_much_time_.html" title="Compilation takes too much time and eats too much memory! What can I do to make it faster?"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../images/boost.png"></td></tr></table> 14<hr> 15<div class="spirit-nav"> 16<a accesskey="p" href="how_can_i_find_the_existing_pyob.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="compilation_takes_too_much_time_.html"><img src="../images/next.png" alt="Next"></a> 17</div> 18<div class="section"> 19<div class="titlepage"><div><div><h3 class="title"> 20<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 21 a function which needs to take ownership of a raw pointer?</a> 22</h3></div></div></div> 23<p> 24 <span class="bold"><strong>Q:</strong></span> Part of an API that I'm wrapping goes 25 something like this: 26 </p> 27<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> 28<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> 29</pre> 30<p> 31 However: 32 </p> 33<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> 34<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> 35<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> 36<span class="identifier">del</span> <span class="identifier">a</span> 37<span class="identifier">del</span> <span class="identifier">b</span> 38<span class="preprocessor"># python</span> <span class="identifier">interpreter</span> <span class="identifier">crashes</span> 39<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> 40</pre> 41<p> 42 Even binding the lifetime of a to b via <code class="computeroutput"><span class="identifier">with_custodian_and_ward</span></code> 43 doesn't prevent the python object a from ultimately trying to delete the 44 object it's pointing to. Is there a way to accomplish a 'transfer-of-ownership' 45 of a wrapped C++ object? 46 </p> 47<p> 48 --Bruce Lowery 49 </p> 50<p> 51 Yes: Make sure the C++ object is held by auto_ptr: 52 </p> 53<pre class="programlisting"><span class="identifier">class_</span><span class="special"><</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"><</span><span class="identifier">A</span><span class="special">></span> <span class="special">>(</span><span class="string">"A"</span><span class="special">)</span> 54 <span class="special">...</span> 55 <span class="special">;</span> 56</pre> 57<p> 58 Then make a thin wrapper function which takes an auto_ptr parameter: 59 </p> 60<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">&</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"><</span><span class="identifier">A</span><span class="special">></span> <span class="identifier">a</span><span class="special">)</span> 61<span class="special">{</span> 62 <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> 63 <span class="identifier">a</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span> 64<span class="special">}</span> 65</pre> 66<p> 67 Wrap that as B.add. Note that pointers returned via <code class="computeroutput"><span class="identifier">manage_new_object</span></code> 68 will also be held by <code class="computeroutput"><span class="identifier">auto_ptr</span></code>, 69 so this transfer-of-ownership will also work correctly. 70 </p> 71</div> 72<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 73<td align="left"></td> 74<td align="right"><div class="copyright-footer">Copyright © 2002-2015 David 75 Abrahams, Stefan Seefeld<p> 76 Distributed under the Boost Software License, Version 1.0. (See accompanying 77 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>) 78 </p> 79</div></td> 80</tr></table> 81<hr> 82<div class="spirit-nav"> 83<a accesskey="p" href="how_can_i_find_the_existing_pyob.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="compilation_takes_too_much_time_.html"><img src="../images/next.png" alt="Next"></a> 84</div> 85</body> 86</html> 87
