Re: [Boost-users] Copy of a C++ object extended with Boost.Python

Thanks for you response ! When you say "that one wrapper helper", are you speaking about the object class of Boost.python, or about a home made class ? On 24 août, 22:01, OvermindDL1 wrote:

On Mon, Aug 24, 2009 at 1:06 PM, 4ian wrote:

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I have a simple C++ object, exposed to python with Boost.python, like this :

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------------------------------------------------- BOOST_PYTHON_MODULE(MyModule) {   class_<MyObject>("MyObject")... } -------------------------------------------------

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I call then python, making an instance of MyObject accessible :

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------------------------------------------------- MyObject anInstance;

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object main_module((handle<>( borrowed( PyImport_AddModule ( "__main__" ) ) ) ) ); object main_namespace = main_module.attr( "__dict__" ); object myModule( (handle<>(PyImport_ImportModule("MyModule"))) ); main_namespace["anInstance"] = ptr(&anInstance);

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try {   handle<> ignored(( PyRun_String( "anInstance.aNewMember = 1;print anInstance.aNewMember;",                                    Py_file_input,                                    main_namespace.ptr(),                                    main_namespace.ptr() ) ) ); } catch ( error_already_set ) {   PyErr_Print(); } -------------------------------------------------

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You can observe that I've added a member ( aNewMember ) to MyObject instance. But if I make a copy of my object :

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------------------------------------------------- MyObject anotherInstance = anInstance;

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main_namespace["anotherInstance"] = ptr(&anotherInstance);

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try {   handle<> ignored(( PyRun_String( "print anotherInstance.aNewMember;",                                    Py_file_input,                                    main_namespace.ptr(),                                    main_namespace.ptr() ) ) ); } catch ( error_already_set ) {   PyErr_Print(); } -------------------------------------------------

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I have a error saying that MyObject hasn't any "aNewMember" member. What I have added with python has not been copied/transfered. Is there a workaround to add permanently some new members to wrapped objects ?

You are adding an entry to the __dict__ of the wrapper class that contains the MyObject, but on the line MyObject anotherInstance = anInstance; you are creating a copy of only the C++ object, not the wrapper object that has the __dict__.  You need to make a copy of it through the Python bindings, not directly on the C++ copy-constructor because then you are just copying the MyObject, not the wrapper object.

I have worked around it by putting a dict in MyObject and exposing it to python as __dict__ though, just make sure you overload your copy constructor so you copy the dict too and not just make a copy of the dict handle, that is important.  I did this work-around in an odd way, not sure it would work directly in Boost.Python, but I would think so.  I think the proper way in Boost.Python would be to (multi-)subclass your MyObject from that one wrapper helper so you can get the python self from your class's this, then you can copy the self (not the handle, again this is important) to the new one as well. _______________________________________________ Boost-users mailing list Boost-us...@lists.boost.orghttp://lists.boost.org/mailman/listinfo.cgi/boost-users