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diff --git a/libs/python/doc/html/faq/how_can_i_find_the_existing_pyob.html b/libs/python/doc/html/faq/how_can_i_find_the_existing_pyob.html new file mode 100644 index 0000000000..bf4147c9d2 --- /dev/null +++ b/libs/python/doc/html/faq/how_can_i_find_the_existing_pyob.html @@ -0,0 +1,103 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>How can I find the existing PyObject that holds a C++ object?</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="does_boost_python_work_with_mac_.html" title="Does Boost.Python work with Mac OS X?"> +<link rel="next" 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?"> +</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="does_boost_python_work_with_mac_.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="how_can_i_wrap_a_function_which0.html"><img src="../images/next.png" alt="Next"></a> +</div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="faq.how_can_i_find_the_existing_pyob"></a><a class="link" href="how_can_i_find_the_existing_pyob.html" title="How can I find the existing PyObject that holds a C++ object?">How can I find + the existing PyObject that holds a C++ object?</a> +</h3></div></div></div> +<div class="blockquote"><blockquote class="blockquote"><p> + "I am wrapping a function that always returns a pointer to an already-held + C++ object." + </p></blockquote></div> +<p> + One way to do that is to hijack the mechanisms used for wrapping a class + with virtual functions. If you make a wrapper class with an initial PyObject* + constructor argument and store that PyObject* as "self", you can + get back to it by casting down to that wrapper type in a thin wrapper function. + For example: + </p> +<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{</span> <span class="identifier">X</span><span class="special">(</span><span class="keyword">int</span><span class="special">);</span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">X</span><span class="special">();</span> <span class="special">...</span> <span class="special">};</span> +<span class="identifier">X</span><span class="special">*</span> <span class="identifier">f</span><span class="special">();</span> <span class="comment">// known to return Xs that are managed by Python objects</span> + + +<span class="comment">// wrapping code</span> + +<span class="keyword">struct</span> <span class="identifier">X_wrap</span> <span class="special">:</span> <span class="identifier">X</span> +<span class="special">{</span> + <span class="identifier">X_wrap</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">self</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">v</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">self</span><span class="special">(</span><span class="identifier">self</span><span class="special">),</span> <span class="identifier">X</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="special">{}</span> + <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">self</span><span class="special">;</span> +<span class="special">};</span> + +<span class="identifier">handle</span><span class="special"><></span> <span class="identifier">f_wrap</span><span class="special">()</span> +<span class="special">{</span> + <span class="identifier">X_wrap</span><span class="special">*</span> <span class="identifier">xw</span> <span class="special">=</span> <span class="keyword">dynamic_cast</span><span class="special"><</span><span class="identifier">X_wrap</span><span class="special">*>(</span><span class="identifier">f</span><span class="special">());</span> + <span class="identifier">assert</span><span class="special">(</span><span class="identifier">xw</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span> + <span class="keyword">return</span> <span class="identifier">handle</span><span class="special"><>(</span><span class="identifier">borrowed</span><span class="special">(</span><span class="identifier">xw</span><span class="special">-></span><span class="identifier">self</span><span class="special">));</span> +<span class="special">}</span> + +<span class="special">...</span> + +<span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f_wrap</span><span class="special">());</span> +<span class="identifier">class_</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="identifier">X_wrap</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span><span class="special">>(</span><span class="string">"X"</span><span class="special">,</span> <span class="identifier">init</span><span class="special"><</span><span class="keyword">int</span><span class="special">>())</span> + <span class="special">...</span> + <span class="special">;</span> +</pre> +<p> + Of course, if X has no virtual functions you'll have to use <code class="computeroutput"><span class="keyword">static_cast</span></code> instead of <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> + with no runtime check that it's valid. This approach also only works if the + <code class="computeroutput"><span class="identifier">X</span></code> object was constructed + from Python, because <code class="computeroutput"><span class="identifier">X</span></code>s constructed + from C++ are of course never <code class="computeroutput"><span class="identifier">X_wrap</span></code> + objects. + </p> +<p> + Another approach to this requires you to change your C++ code a bit; if that's + an option for you it might be a better way to go. work we've been meaning + to get to anyway. When a <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code> + is converted from Python, the shared_ptr actually manages a reference to + the containing Python object. When a shared_ptr<X> is converted back + to Python, the library checks to see if it's one of those "Python object + managers" and if so just returns the original Python object. So you + could just write <code class="computeroutput"><span class="identifier">object</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span></code> to get + the Python object back. To exploit this you'd have to be able to change the + C++ code you're wrapping so that it deals with shared_ptr instead of raw + pointers. + </p> +<p> + There are other approaches too. The functions that receive the Python object + that you eventually want to return could be wrapped with a thin wrapper that + records the correspondence between the object address and its containing + Python object, and you could have your f_wrap function look in that mapping + to get the Python object out. + </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="does_boost_python_work_with_mac_.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="how_can_i_wrap_a_function_which0.html"><img src="../images/next.png" alt="Next"></a> +</div> +</body> +</html> |