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POHow to pass a C++ object to another C++ object with Boost.Python
 

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  1. POHow to pass a C++ object to another C++ object with Boost.Python
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    <p>I have some C++ code that defines two classes, A and B. B takes an instance of A during construction. I have wrapped A with Boost.Python so that Python can create instances of A, as well as subclasses. I want to do the same with B.</p> <pre class="lang-cpp prettyprint-override"><code>class A { public: A(long n, long x, long y) : _n(n), _x(x), _y(y) {}; long get_n() { return _n; } long get_x() { return _x; } long get_y() { return _y; } private: long _n, _x, _y; }; class B { public: B(A a) : _a(a) {}; doSomething() { ... }; private: A _a; }; </code></pre> <p>While wrapping B, I needed to work out how to pass an instance of A to B's constructor. I did some digging and the <a href="http://misspent.wordpress.com/2009/09/27/how-to-write-boost-python-converters/">solution</a> I found was to write a "converter" class:</p> <pre class="lang-cpp prettyprint-override"><code>struct A_from_python_A { static void * convertible(PyObject* obj_ptr) { // assume it is, for now... return obj_ptr; } // Convert obj_ptr into an A instance static void construct(PyObject* obj_ptr, boost::python::converter::rvalue_from_python_stage1_data* data) { // extract 'n': PyObject * n_ptr = PyObject_CallMethod(obj_ptr, (char*)"get_n", (char*)"()"); long n_val = 0; if (n_ptr == NULL) { cout &lt;&lt; "... an exception occurred (get_n) ..." &lt;&lt; endl; } else { n_val = PyInt_AsLong(n_ptr); Py_DECREF(n_ptr); } // [snip] - also do the same for x, y // Grab pointer to memory into which to construct the new A void* storage = ( (boost::python::converter::rvalue_from_python_storage&lt;A&gt;*) data)-&gt;storage.bytes; // in-place construct the new A using the data // extracted from the python object new (storage) A(n_val, x_val, y_val); // Stash the memory chunk pointer for later use by boost.python data-&gt;convertible = storage; } // register converter functions A_from_python_A() { boost::python::converter::registry::push_back( &amp;convertible, &amp;construct, boost::python::type_id&lt;A&gt;()); } }; </code></pre> <p>Then I register this with:</p> <pre class="lang-cpp prettyprint-override"><code>BOOST_PYTHON_MODULE(interpolation_ext) { // register the from-python converter for A A_from_python_A(); class_&lt;A&gt;("A", init&lt;long, long, long&gt;()) ; class_&lt;B&gt;("B", init&lt;object&gt;()) ; } </code></pre> <p>Convertible and construct are methods that answer the "is this convertible?" and "how to convert?" questions respectively. I have observed that the construct() method is non-trivial - it has to reach into A's PyObject*, extract all relevant fields, then rebuild a C++ instance that it then passes to B's constructor. Because A contains some private fields, it has to do this via public access mechanisms (whereas with a pure Python object it wouldn't have to, right?). This seems to work.</p> <p>However, <em>is the field extraction in the 'construct' function really necessary?</em> It seems laborious. If A is a compound object, it could get very complicated, and possibly require one converter to invoke another. I perhaps understand the requirement if A is a Python class, but if the A instance originated from the C++ side, is there a way to determine that this is the case, and then simply get a handle (e.g. pointer) to this 'native' object, as a shortcut?</p> <p>Here's the associated python code:</p> <pre class="lang-python prettyprint-override"><code>from my_ext import A, B a = A(1,2,3) b = B(a) b.doSomething() </code></pre>
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