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POPython C-API Object Allocation‏
 

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    <p>I want to use the new and delete operators for creating and destroying my objects.</p> <p>The problem is python seems to break it into several stages. tp_new, tp_init and tp_alloc for creation and tp_del, tp_free and tp_dealloc for destruction. However c++ just has new which allocates and fully constructs the object and delete which destructs and deallocates the object.</p> <p>Which of the python tp_* methods do I need to provide and what must they do?</p> <p>Also I want to be able to create the object directly in c++ eg "PyObject *obj = new MyExtensionObject(args);" Will I also need to overload the new operator in some way to support this?</p> <p>I also would like to be able to subclass my extension types in python, is there anything special I need to do to support this?</p> <p>I'm using python 3.0.1.</p> <p>EDIT: ok, tp_init seems to make objects a bit too mutable for what I'm doing (eg take a Texture object, changing the contents after creation is fine, but change fundamental aspects of it such as, size, bitdept, etc will break lots of existing c++ stuff that assumes those sort of things are fixed). If I dont implement it will it simply stop people calling __init__ AFTER its constructed (or at least ignore the call, like tuple does). Or should I have some flag that throws an exception or somthing if tp_init is called more than once on the same object?</p> <p>Apart from that I think ive got most of the rest sorted.</p> <pre><code>extern "C" { //creation + destruction PyObject* global_alloc(PyTypeObject *type, Py_ssize_t items) { return (PyObject*)new char[type-&gt;tp_basicsize + items*type-&gt;tp_itemsize]; } void global_free(void *mem) { delete[] (char*)mem; } } template&lt;class T&gt; class ExtensionType { PyTypeObject *t; ExtensionType() { t = new PyTypeObject();//not sure on this one, what is the "correct" way to create an empty type object memset((void*)t, 0, sizeof(PyTypeObject)); static PyVarObject init = {PyObject_HEAD_INIT, 0}; *((PyObject*)t) = init; t-&gt;tp_basicsize = sizeof(T); t-&gt;tp_itemsize = 0; t-&gt;tp_name = "unknown"; t-&gt;tp_alloc = (allocfunc) global_alloc; t-&gt;tp_free = (freefunc) global_free; t-&gt;tp_new = (newfunc) T::obj_new; t-&gt;tp_dealloc = (destructor)T::obj_dealloc; ... } ...bunch of methods for changing stuff... PyObject *Finalise() { ... } }; template &lt;class T&gt; PyObjectExtension : public PyObject { ... extern "C" static PyObject* obj_new(PyTypeObject *subtype, PyObject *args, PyObject *kwds) { void *mem = (void*)subtype-&gt;tp_alloc(subtype, 0); return (PyObject*)new(mem) T(args, kwds) } extern "C" static void obj_dealloc(PyObject *obj) { ~T(); obj-&gt;ob_type-&gt;tp_free(obj);//most of the time this is global_free(obj) } ... }; class MyObject : PyObjectExtension&lt;MyObject&gt; { public: static PyObject* InitType() { ExtensionType&lt;MyObject&gt; extType(); ...sets other stuff... return extType.Finalise(); } ... }; </code></pre>
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