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POExtending a virtual C++ class exposed via Boost.Python
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  1. POExtending a virtual C++ class exposed via Boost.Python
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    copied!<p>I am trying to expose this C++ class using Boost.Python:</p> <pre><code>class VAlgorithm { public: VAlgorithm(const char *name); virtual ~VAlgorithm(); virtual bool Initialize() = 0; virtual bool Process() = 0; virtual bool Finalize() = 0; virtual const char *GetName() const; // Default implementation in cpp file } </code></pre> <p>My final goal is to define children of VAlgorithm in the python shell as python classes. Following <a href="http://wiki.python.org/moin/boost.python/OverridableVirtualFunctions" rel="nofollow">this example</a>, I defined a callback class:</p> <pre><code>class VAlgorithm_callback: public VAlgorithm { public: VAlgorithm_callback(PyObject *p, const char *name) : self(p), VAlgorithm(name) { } const char * GetName() const { return call_method&lt;const char *&gt;(self, "GetName"); } static const char * GetName_default(const VAlgorithm&amp; self_) { return self_.VAlgorithm::GetName(); } private: PyObject *self; }; </code></pre> <p>Right now I am exposing only the class itself and the GetName() method. Since it is a virtual class, I placed this code inside BOOST_PYTHON_MODULE:</p> <pre><code>class_&lt;VAlgorithm, VAlgorithm_callback, boost::noncopyable&gt;("VAlgorithm", no_init) // .def("GetName", &amp;VAlgorithm_callback::GetName_default); // </code></pre> <p>I can compile this and load the module in the python shell. Then I try to define a child class and call the GetName() default implementation defined in the C++ code:</p> <pre><code>&gt;&gt;&gt; class ConcAlg1(VAlgorithm): ... pass ... &gt;&gt;&gt; c1 = ConcAlg1("c1") Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; RuntimeError: This class cannot be instantiated from Python &gt;&gt;&gt; class ConcAlg2(VAlgorithm): ... def __init__(self, name): ... pass ... &gt;&gt;&gt; c2 = ConcAlg2("c2") &gt;&gt;&gt; c2.GetName() Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; Boost.Python.ArgumentError: Python argument types in VAlgorithm.GetName(ConcAlg2) did not match C++ signature: GetName(VAlgorithm) &gt;&gt;&gt; class ConcAlg3(VAlgorithm): ... def __init__(self, name): ... super(ConcAlg3, self).__init__(self, name) ... &gt;&gt;&gt; c3 = ConcAlg3("c3") Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; File "&lt;stdin&gt;", line 3, in __init__ RuntimeError: This class cannot be instantiated from Python </code></pre> <p>I am not an expert (just facing these issues for the first time), but it seems to me that ConcAlg1 and ConcAlg3 try to instantiate a VAlgorithm object and fail because of the no_init parameter used when exposing VAlgorithm (I Can't omit it or the code won't compile), and ConcAlg2 can't call GetName() because somehow it is not recognized as a child of VAlgorithm. I must be doing something trivially wrong but I can't figure out what (I am a novice of Boost.Python and extension). Can anyone please help me? Thanks</p>
 

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