Mailman 3 April 2006 - Cplusplus-sig

C++ singleton
by Emmanuel Taurel Oct. 31, 2007

Oct. 31, 2007

Hello everybody, I am actually writing an interface to python for an existing C++ library using Boost. This library has a C++ singleton class. This class has only one constructor (with arguments) which is "protected" because it's a singleton. I am not able to build a Boost interface to this class because the compiler complains that the constructor I have given to Boost in the class_xxx constructor argument is not public (which is true) ! Therefore, my question is simple : How is it possible to interface such a class with only one constructor which is protected and a static method to create/access the object ? Thank's very much for your answer. Sincerely Emmanuel Taurel (etaurel(a)cells.es) PS : By the way, is there a searchable archive of this mailing list ?

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optimizing away calls to the python runtime -- was [detecting if a function is written in python or C++]
by Mathieu Lacage May 15, 2006

May 15, 2006

hi, I apologize for this second email but the more I think about what I am trying to do, the less I see how to do it. So, here is a hopefully detailed explanation of what I am trying to achieve. Here are my c++ classes: class Event { virtual void notify (void) = 0; }; class Simulator { void schedule (Event *event); double now_s (void); // return current time. void run (void); }; sample c++ use code: class MyEvent : public Event { virtual void notify (void) { std::cout << "now=" Simulator::now_s () << std::endl; delete this; } }; Simulator->schedule (new MyEvent ()); Simulator->run (); // invokes MyEvent::notify sometime later. This sort of code requires a lot of "forwarding code". That is, you often have to write a bunch of Event-deriving classes whose sole purpose in life is to forward the call to their notify method to a call to another class' method with event-specific parameters. So, I have a small make_event c++ template which can be used to do this sort of thing: class MyClass { void random_stuff (double id) { std::cout << "id="<<id<<"now=" Simulator::now_s () << std::endl; } }; MyClass *obj = new MyClass (); Simulator->schedule (make_event (&MyClass::random_stuff, obj, 10.0)); Simulator->run (); Okay, so, to wrap this, I wrapped my Simulator class and my EventWrap class: struct EventWrap : yans::Event { EventWrap( PyObject* self_); virtual ~EventWrap(); virtual void notify (void); private: PyObject *m_self; }; EventWrap::EventWrap (PyObject* self_) : m_self(self_) { Py_INCREF(m_self); } EventWrap::~EventWrap() { Py_DECREF(m_self); } void EventWrap::notify (void) { call_method<void>(m_self, "notify"); delete this; } namespace boost { namespace python { template <> struct has_back_reference<EventWrap> : mpl::true_ {}; }} void export_event (void) { class_<EventWrap, std::auto_ptr<EventWrap>, boost::noncopyable> event ("Event"); event.def ("notify", pure_virtual(&Event::notify)); } void simu_insert_in_s (double delta, std::auto_ptr<EventWrap> ev) { Simulator::insert_in_s (delta, ev.get ()); ev.release (); } void export_simulator (void) { def ("run", Simulator::run); def ("insert_in_s", simu_insert_in_s); } which allows me to write the following python code: class __MyEvent__(Event): def set_callback (self, function, *args): self.__function_ = function; self.__args_ = args; def notify (self): self.__function_ (*self.__args_); def make_event(function, *args): ev = __MyEvent__ (); ev.set_callback (function, *args); return ev def my_nothing0 (): print "my model now=%f" % simulator.now_s (); simulator.insert_in_s (4.0, make_event (my_nothing1, 99)); simulator.run () okay, so all of this stuff works really nicely but there is a catch. I would like to be able to write code such as: simulator.insert_in_s (4.0, make_event (simulator.now_s)); simulator.run () and ensure that once simulator.run has reached the Simulator::run c++ method, no python code will be executed ever. Namely, I want call to make_event to generate a nice C++ Event class which will not call back into python's __MyEvent__.notify before calling simulator.now_s and Simulator::now_s. i.e., it should call back Simulator::now_s directly. The more I think about this, the less I see a way to do this with boost. Have I missed something ? Mathieu --

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Best way to use python as an API inside a Qt/C++ app?
by craig harris May 12, 2006

May 12, 2006

I am using Qt as the GUI for a C++ app. I would also like to use python as an API for the application. For instance when a menu item is pressed, a stored python command is executed. Is this wise? And what is the best approach (tool) to implementing this? PyQt, boost.python, other?

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Howto expose exception classes using boost.python?
by Juho Mäkinen May 9, 2006

May 9, 2006

I have stumbled across this problem when I started looking the right way how to expose a bunch of C++ Exception classes to python. I have a "master" exception class, which is delivered from std::exception. All other exception classes in our project are base on this. As far as I know, Python requires that the exceptions must be delivered from PyExc_Exception, or a class which is delivered from PyExc_Exception, but I haven't found any way how to expose a C++ class which delivers from a python class. There's also PyErr_NewException() available within Python C API. This would be the ideal way how I'd like to use our exceptions. PyErr_SetObject(PyExc_MyException, InstanceOfMyException) -or in python- raise mymodule.MyException(arg1, arg2) I know that I can expose C++ exceptions with boost.python (the PodBayDoorException example) using boost::python::register_exception_translator, but I want to be able to distinguish my different C++ exceptions at the python side. (eg. It's not enough that all my exceptions are translated into PyExc_RuntimeError, or some other standard python exception) All ideas how to expose exception hierarchy from C++ to Python are appreciated. Thanks in advance, - Juho Mäkinen

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Simple boost example available
by Nicolas P. Rougier April 29, 2006

April 29, 2006

Hi all, I put on http://www.loria.fr/~rougier/software/boost/ a simple example showing some boost python features in action. Hope this can help someone. Nicolas Rougier.

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VC 2005 support in boost.python
by Henry Padilla April 28, 2006

April 28, 2006

I play Civilization 4, that uses Boost. We can't compile the SDK because there's some incompatibility issues with VC 2005. I was wondering if anyone still had incompatibility issues with VC 2005 or if the macro BOOST_WORKAROUND was still needed. Thanks for the help. Tom P.

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Fwd: Virtual Functions with Default Implementations
by Roman Yakovenko April 27, 2006

April 27, 2006

Hi. It seems to me that I miss something very important ( or very obvious ) Here is a relevant piece of code from tutorials( http://tinyurl.com/hw5hb ) struct Base { virtual ~Base() {} virtual int f() { return 0; } }; struct BaseWrap : Base, wrapper<Base> { int f() { if (override f = this->get_override("f")) return f(); // *note* return Base::f(); } int default_f() { return this->Base::f(); } }; class_<BaseWrap, boost::noncopyable>("Base") .def("f", &Base::f, &BaseWrap::default_f) ; Why boost.python requests to register BaseWrap::default_f? It seems to me that BaseWrap::f implementation deals with this. So what did I miss? -- Roman Yakovenko C++ Python language binding http://www.language-binding.net/ -- Roman Yakovenko C++ Python language binding http://www.language-binding.net/

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status of signatures?
by Ingo Lütkebohle April 27, 2006

April 27, 2006

Hi, what is the status of the boost-signatures patch (originally by Ralf IIRC)? Is it being taken forwarded? Is it planned for inclusion into boost::python sometime soon? I think more complete documentation generation like implemented by that patch is one of the most important things still missing! Having read through the list I recognize there are awkward cases with unwieldy signatures but are they a showstopper? I figure that most easy cases might be covered already. Best Regards Ingo

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singleton wrapping
by Libor Capak April 24, 2006

April 24, 2006

hello, i'm quite new to boost::python. i'd like to ask how you solve singleton wrapping. i have simple template: template<typename T> class Singleton { static T* singleton; public: //.. blabla .. static T& instance() { mustBe(singleton); return *singleton; } // .. blabla .. }; and simple class, for example: class A : public Singleton<A> { public: void foo(); }; and now to export into python module i'm using wrapper class: class A_wrap { static void foo() { A.instance().foo(); } }; BOOST_PYTHON_MODULE(test) { class_<A_wrap>("A", no_init) .def("foo", &A_wrap::foo) .staticmethod("foo") ; } is there any trick how to skip writing class A_wrap and still use method A::foo() as staticmethod from python? Libor Capak

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Wrapping pointers
by Dominic Sacré April 22, 2006

April 22, 2006

Hi, I'm having trouble exporting C++ pointers to Python. My code looks like this: class A { A *p; ... }; BOOST_PYTHON_MODULE(test) { class_<A>("A", init<>()) .def_readonly("p", &A::p) ... ; } Now every time I try to access A::p from Python I get this error: TypeError: No to_python (by-value) converter found for C++ type: A* I searched the list archives and found a message about the same issue, where the only suggestion was to use .def_readonly("p", make_getter(&A::p, return_internal_reference<1>())) instead, but this doesn't help either. On the contrary, this causes a segfault every time I use A::p (although p points to a valid object, at least in C++). So what should I do to access this pointer from Python? Thanks, Dominic

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