Mailman 3 June 2005 - Cplusplus-sig

wrapping a Singleton class
by Faheem Mitha June 12, 2005

June 12, 2005

Hi, Consider the following Singleton class, which seems to have problems being wrapped by Boost.Python. ******************************************************** class lustate { public: static lustate& instance(const vector<double>& lower, const vector<double>& upper) { static lustate lu(lower, upper); return lu; }; vector<double> l; vector<double> u; private: lustate(const vector<double>& _l, const vector<double>& _u): l(_l), u(_u) {}; }; **************************************************************** I tried BOOST_PYTHON_MODULE(mg) { boost::python::class_<lustate>("lustate", boost::python::no_init) // boost::python::class_<lustate>("lustate") .staticmethod("instance") ; } This compiles without error, but on importing I get ******************************************************************************* In [1]:import mg --------------------------------------------------------------------------- exceptions.KeyError Traceback (most recent call last) /home/faheem/co/paper/mg/c++/<console> KeyError: 'instance' ******************************************************************************** I think that static member functions still require the corresponding class to be instantiated in Python, since it is invoked like a regular bound method, though the values obtained are of course the same for every instantiation. This understanding is based on Googling + the example files staticmethod.cpp & staticmethod.py. In other words, it doesn't have the equivalent of lustate::instance at the Python level. Since the constructor is private, it cannot instantiate the class. Hence, it is stuck. Is this an accurate description of the situation? I'm not sure what the error means, though. Faheem.

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Re: [C++-sig] Custodian & ward
by "Jarno Seppänen" June 11, 2005

June 11, 2005

Hi, Niall asked: > ... and each FXWindow instance maintains a list of its children, and > deletes its children when it gets deleted, then how do I apply > custodian & ward to ensure that when the parent instance is deleted, > python knows not to delete the children? I've experienced the same problems when writing wrappers for LLVM. My conclusion is that BPL doesn't obey with_custodian_and_ward and/or return_value_policy in case of constructors. This is what I think should work, but eventually this crashes in a double delete: struct P; struct C { explicit C(P& parent); // transfers ownership of this to parent }; BOOST_PYTHON_MODULE(init_policy) { class_<C, boost::noncopyable> ("C", init<P&>() [with_custodian_and_ward<2, 1, return_value_policy<reference_existing_object> >()]); } I've attached a compiling example of the crash, and how to work around by using a factory function. BR, Jarno -- Geschenkt: 3 Monate GMX ProMail gratis + 3 Ausgaben stern gratis ++ Jetzt anmelden & testen ++ http://www.gmx.net/de/go/promail ++

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Function needs ownership of python object that is derived from a C++ class
by Thomas Kolb June 10, 2005

June 10, 2005

Hi all, I have a C++ class Base that is declared as follows: struct Base { virtual int f() = 0; }; and a free function call_f that needs ownership of the parameter: void Moses::call_f(Base* b) { std::cout << " b->f() = " << b->f() << std::endl; delete (b); } On pyhton side I derive from Base like that: class Derived(Base): def f(self): return 42 of course the following leads to a segmentation fault: d = Derived() call_f(d) First thing I tried was to do this auto_ptr stuff as described in FAQ ("How can I wrap a function which needs to take ownership of a raw pointer"), but this doesn't work because of: Boost.Python.ArgumentError: Python argument types in call_f(Derived) did not match C++ signature: call_f(std::auto_ptr<Moses::Base>) then I tried to wrap call_f as follows below: call_f_Wrapper(PyObject* p) { handle<> hP(p); Moses::Base* bP = extract<Moses::Base*>(object(hP)); call_f(bP); } but this leads to segmentation faults too. Last thing I did was the stuff described in PythonInfo Wiki in section "ownership of C++ object extended in Python", but that also doesn't work and so, I guess I need your help. Best Regards, Franz

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std::wstring argument troubles ?
by Nicolas Lelong June 8, 2005

June 8, 2005

Hi, I updated my boost libraries last week (via CVS on the 06/02) and I'm now experiencing problems when calling functions with wstring arguments, which worked seamlessly with my previous boost libs, using python 2.4. I may be missing something obvious, did anything change concerning wstring support (I was previously using a CVS snapshot around v1.31 of boost) for instance, I have a class BrushTextMenu::Entry defined roughly as follows :: class BrushTextMenu : public ... { public: struct Entry { Entry() {}; explicit Entry(std::wstring const& _text) : text(_text) {}; std::wstring text; }; which is wrapped as follows :: python::class_<BrushTextMenu, bases<Brush>, Brush_Holder<BrushTextMenu>::type, boost::noncopyable> class_BrushTextMenu("BrushTextMenu", no_init); { python::scope class_scope = class_BrushTextMenu; python::class_<BrushTextMenu::Entry>("Entry") .def( init<std::wstring const&>() ) ; } now, whenever I call the Entry constructor, I get error messages concerning signature mismatch :: b = BrushTextMenu.Entry(u"r") ==> Traceback (most recent call last): File "<string>", line 1, in ? Boost.Python.ArgumentError: Python argument types in Entry.__init__(Entry, unicode) did not match C++ signature: __init__(struct _object *, class std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> >) __init__(struct _object *) or b = BrushTextMenu.Entry("r") ==> Traceback (most recent call last): File "<string>", line 1, in ? Boost.Python.ArgumentError: Python argument types in Entry.__init__(Entry, str) did not match C++ signature: __init__(struct _object *, class std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> >) __init__(struct _object *) Thanks in advance, Nicolas.

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Custodian & ward
by Niall Douglas June 7, 2005

June 7, 2005

Hi, If there is: struct FXWindow { FXWindow(FXWindow *parent=0); }; ... and each FXWindow instance maintains a list of its children, and deletes its children when it gets deleted, then how do I apply custodian & ward to ensure that when the parent instance is deleted, python knows not to delete the children? Something like: class_< FXWindow >("FXWindow", init< FXWindow*>()[with_custodian_and_ward_postcall<1,2>()]) BTW the docs on the reference page for with_custodian_and_ward are confusing - the text suggests that the first arg determines the life of the second, whereas the code for return_internal_reference() is pretty clear that the second arg determines the life of the first. I am assuming that the latter is the correct orientation? Cheers, Niall

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Custodian & ward
by Niall Douglas June 7, 2005

June 7, 2005

Hi, If there is: struct FXWindow { FXWindow(FXWindow *parent=0); }; ... and each FXWindow instance maintains a list of its children, and deletes its children when it gets deleted, then how do I apply custodian & ward to ensure that when the parent instance is deleted, python knows not to delete the children? Something like: class_< FXWindow >("FXWindow", init< FXWindow*>()[with_custodian_and_ward_postcall<1,2>()]) BTW the docs on the reference page for with_custodian_and_ward are confusing - the text suggests that the first arg determines the life of the second, whereas the code for return_internal_reference() is pretty clear that the second arg determines the life of the first. I am assuming that the latter is the correct orientation? Cheers, Niall

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pyste #include format
by Scott McKay June 7, 2005

June 7, 2005

Could this line in pyste.py context['Include'] = lambda header: infos.CodeInfo('#include <%s>\n' % header, 'include') possibly be changed to something like this instead to honour the existing syntax but also allow you to explicitly specify the include format? context['Include'] = infos.Include with the following addition to infos.py def Include( header ): header = header.strip() if ( header[0] == '"' or header[0] == '<' ) : CodeInfo('#include %s\n' % header, 'include') else: CodeInfo('#include <%s>\n' % header, 'include') Cheers Scott

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And now for a raw constructor
by Studious Apprentice June 7, 2005

June 7, 2005

Thanks again Dave for the help provided ! I would like to know if there is a way to do the same for a constructor. I planned to use a .def("__init__", rawfunction(constructorfwd)) but this wont work for I do not have a constructed object to start with ;-) and on the top of that the object needs to be constructed at a specific place. Browsing through the Boost Python code, it looks like I could use the detail::make_keyword_range_constructor<Signature,NArgs> function or even use the objects::make_holder<Arity> however I do know which parameters I should provide... Thanks in advance, Marc

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Boost Python - class_<T> As object
by Dan Eloff June 7, 2005

June 7, 2005

Hi all, I've not posted to this discussion list before, but I have been subscribed and following it loosely for a year now. I was looking through the boost python docs and found something interesting: class_<T> as objects Due to the dynamic nature of Boost.Python objects, any class_<T> may also be one of these types! The following code snippet wraps the class (type) object. We can use this to create wrapped instances. Example: object vec345 = ( class_<Vec2>("Vec2", init<double, double>()) .def_readonly("length", &Point::length) .def_readonly("angle", &Point::angle) )(3.0, 4.0); assert(vec345.attr("length") == 5.0); This is nice, but I'm wondering about the inline class_<> declaration. Specifically I don't want to copy and paste the entire class_<> stuff everytime I want to create a python object of a exported c++ class and use it in c++. This violates the rule of duplication as a bad thing. How could I avoid this? Looking at it as I write, I think it may be possible to save the entire thing as a C++ variable and then use it elsewhere. class_<Vec2> g_Vec2 = class_<Vec2>("Vec2", init<double, double>()) .def_readonly("length", &Point::length) .def_readonly("angle", &Point::angle); And then I use it like so ?: object vec345 = g_Vec2(3.0,4.0); object vec_other = g_Vec2(7.0,7.0); And use it inside the module declaration as well: BOOST_PYTHON_MODULE(mymodule) ( g_Vec2; ... ) I may be totally off here, but hopefully I've given a clear picture of what I'm trying to do. I just want to put everything in one place if I can. Thanks for reading! -Dan

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Re: [C++-sig] Raw member and constructor functions
by Studious Apprentice June 6, 2005

June 6, 2005

Thanks again! It now works ;-) Sorry for my mistake, I modified the code because I got an conversion error at compilation from tuple to object, but the error was in the member function not in the forwarding function, so I added the extract which was indeed stupid ;-) Thanks again ! Marc "Studious Apprentice" <apprentice_99(a)hotmail.com> writes: >Thanks a lot for this fast answer ! This looks like the way to go. The >following code does compile but the result under python is unexpected : > >#include <boost/python.hpp> >#include <boost/python/raw_function.hpp> using namespace boost::python; > >namespace { > class X { > public: > object memberraw(const tuple& args, const dict& kw) > { > return make_tuple(args, kw); > } > }; > > object memberrawfwd(tuple raw, dict kw) > { > tuple arg = extract<tuple&>(raw.slice(1, _))(); Why didn't you just do it the way I told you to? Did that fail somehow? This should be tuple arg( raw.slice(1,_) ); > return extract<X&>(raw[0])(). > memberraw(arg, kw); > } > >}; > >BOOST_PYTHON_MODULE(PythonTest) >{ > class_<X>("X") > .def("memberraw", raw_function(memberrawfwd)) > ; >} -- Dave Abrahams Boost Consulting www.boost-consulting.com _______________________________________________ C++-sig mailing list C++-sig(a)python.org http://mail.python.org/mailman/listinfo/c++-sig

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