Mailman 3 May 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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Exposing c++ classes with static arrays using Boost.Python and py++
by Abhi Oct. 3, 2006

Oct. 3, 2006

While running py++ on a class with a static array inside it, py++ generates Boost.Python bindings using a derived class for the original C++ class. Let me illustrate with the help of the following example: class AttributesTest { public: int staticIntArray[10]; }; Notice this class has an array inside it Running py++ generates the following code, with a wrapper class that derives from the AttributesTest class. struct AttributesTest_wrapper : AttributesTest, bp::wrapper< AttributesTest > { AttributesTest_wrapper(AttributesTest const & arg ) : AttributesTest( arg ) , bp::wrapper< AttributesTest >() {} AttributesTest_wrapper( ) : AttributesTest( ) , bp::wrapper< AttributesTest >() {} pyplusplus::containers::static_sized::array_1_t< int, 10 > pyplusplus_staticIntArray_wrapper(){ return pyplusplus::containers::static_sized::array_1_t< int, 10 >( staticIntArray ); } }; and BOOST_PYTHON_MODULE(PYPP_YodaPacketCommon){ if( true ){ typedef bp::class_< AttributesTest_wrapper > AttributesTest_exposer_t; AttributesTest_exposer_t AttributesTest_exposer = AttributesTest_exposer_t( "AttributesTest" ); bp::scope AttributesTest_scope( AttributesTest_exposer ); AttributesTest_exposer.def( bp::init< >()[bp::default_call_policies()] ); pyplusplus::containers::static_sized::register_array_1< int, 10, bp::default_call_policies >( "__array_1_int_10" ); AttributesTest_exposer.add_property( "staticIntArray" , bp::make_function( (pyplusplus::containers::static_sized::array_1_t< int, 10 > ( AttributesTest_wrapper::* )( ) )(&AttributesTest_wrapper::pyplusplus_staticIntArray_wrapper) , bp::with_custodian_and_ward_postcall< 0, 1, bp::default_call_policies >() ) );; Note that it maps the python name "AttributesTest" to the derived class AttributesTest_wrapper. Why not just keep it simple and rather than creating a derived class just map the variable "staticIntArray" via a global method, such as: .add_property( "staticIntArray" , bp::make_function( (&::someglobal_staticIntArray_wrapper) , bp::with_custodian_and_ward_postcall< 0, 1, bp::default_call_policies >() ) );; and define the someglobal_staticIntArray_wrapper method as: pyplusplus::containers::static_sized::array_1_t< int, 10 > someglobal_staticIntArray_wrapper(){ return pyplusplus::containers::static_sized::array_1_t< int, 10 >( staticIntArray ); } Is there any drawback with just mapping the array variable to a global method rather than mapping the whole class. thanks - Abhi

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Working with python unicode strings without wstring
by Rod Cloutier Aug. 6, 2006

Aug. 6, 2006

Hi, I am trying to wrap an external library that uses "const wchar_t *" a arguments for several methods. I know that if they would have used std::wstring the conversion would have been straight forward. Is there anyway to convert python unicode string to wchar_t pointers ? Can I make the following code work without writting a wrapper function that uses a wstring as an argument ? in C++: void printText( const wchar_t* str ) { std::wcout << str; } in python: MyModule.printText( u"The message" ) Thanks Rodrigue Cloutier

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Redirecting PyErr_Print
by Mrinmay May 31, 2006

May 31, 2006

I want to build a application that scan the mail message and file attachment ,can u send me a demo Application in vc++. I will be grateful to u. Mailed:- mrinmaybiswas(a)rediffmail.com

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Passing C++ objects to Python by reference
by Garrick Chin May 29, 2006

May 29, 2006

The below class is defined in C++ and exposed in Python: class A { public: A() :value("unchanged") { } void setValue(std::string newValue) { value = newValue; } std::string value; }; The following method is defined in Python: def changeA(a): a.setValue("changed") When changeA() is passed an object created in Python: a1 = A() changeA(a1) print a1.value #prints "changed" the passed in object gets changed, but when the equivalent is done in C++: A a2; object changeA = dictionary["changeA"]; changeA(a2) std::cout << a2 << "\n"; //prints "unchanged" it appears that the C++ object is passed to Python by value instead of reference. Is there a way to specify passing by reference instead of value to boost::python?

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held_ptr and polymorphism
by Garrick Chin May 28, 2006

May 28, 2006

I compiled the \python\test\polymorphism2.cpp with and without HELD_BY_AUTO_PTR defined to toggle holding the C++ defined Python objects with and without std::auto_ptr into two separate modules: polymorphism2_ext polymorphism2_auto_ptr_ext I ran the following test: class ADerived(A): def f(self): return "ADerived::f()" call_f(ADerived()) with two different imports: "from polymoprhism2_ext import" and "from polymoprhism2_auto_ptr_ext import" With the non-auto_ptr module, call_f() correctly prints "ADerived::f()". _With_ the auto_ptr module, call_f() incorrectly prints "A::f()". Is this a bug? How does one store particular C++ defined Python objects in a smart pointer without losing polymorphic behavior?

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Polymorphism and Dangling Reference exception
by Garrick Chin May 28, 2006

May 28, 2006

Hello all, I am trying to run a basic polymorphism test for boost::python using the following simple code: polymorphism_test.cpp: struct Shape { virtual ~Shape() {} virtual std::string draw(const std::string& param) = 0; }; struct ShapeWrap : Shape, wrapper<Shape> { std::string draw(const std::string& param) { return this->get_override("draw")(param); } }; std::string printShape(Shape* shape) { return shape->draw("blah"); } BOOST_PYTHON_MODULE(test) { class_<ShapeWrap, boost::noncopyable>("Shape") .def("draw", pure_virtual(&Shape::draw)) ; def("printShape", &printShape); } polymorphism_test.py: import test class Circle(test.Shape): def draw(self, param): return "circle: " + param print test.printShape(Circle()) When the Python interpeter hits the "print test.printShape(Circle())" line, it throws the following exception: ReferenceError: Attempt to return dangling reference to object of type: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > I do not understand why this would be thrown. I am not returning any references or pointers, and I basically copied this code from the documentation. What would be causing this exception? Thank you for your time.

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Re: [C++-sig] help passing image data
by J.D. Yamokoski May 25, 2006

May 25, 2006

Understood. I guess using Py_Finalize was a lazy way for me to sort of reset the Python interpreter. I just wanted to make sure that variables created inside a script don't build in the global dictionary and maybe cause strange runtime errors over the course of running the program. I am sure I could just make sure to clean out anything new created in the dictionary or something like that. So then does Boost support multiple Python threads created in C? On Thu May 25 12:10:56 EDT 2006, "Ralf W. Grosse-Kunstleve" <rwgk(a)yahoo.com> wrote: > --- "J.D. Yamokoski" <yamokosk(a)ufl.edu> wrote: >> Are there any other C/API calls that are not compatible with >> Boost? > > I don't think so. > >> And what is the proper way to shut down the interpreter or reset >> it using Boost? > > It is not supported. This is because the groups funding > Boost.Python V2 > development had no interest in embedding. > Actually, how serious is this limitation in practice? If you > simply start the > interpreter when your main is called and then reuse it all the > time, what is > lost compared to shutting down and restarting the interpreter > multiple times? > > > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around > http://mail.yahoo.com > _______________________________________________ > C++-sig mailing list > C++-sig(a)python.org > http://mail.python.org/mailman/listinfo/c++-sig > >

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call policies help needed
by Roman Yakovenko May 25, 2006

May 25, 2006

Hi. Please consider next case: class A{ ... }; struct B{ B( A& a ) : a( a ) {} A& a; }; In order to export class B, I have to create access function for member variable a. A& get_a( B& b ){ return b.a }; BOOST_PYTHON_MODULE(module){ class_< A > ...; class_< B >( "B", bp::init< A& >( )[with_custodian_and_ward<1,2>()] ) .add_property( "a", make_funtion( &get_a, ???????? ) ); } I don't know what call policies pyplusplus, code generator, should give for make_function by default. Why? Because there are different use case: 1. Python code: a = module.A() b = module.B( a ) In this case life time of "a" is already managed by boost.python, so the return value policy for make_function could be "return_existing_object". I think, that in this case it is safe to use it.( Am I wrong ? ) 2. Python code: a = module.get_the_a_object() # returns reference to existing object a, that was created # from C++, life time of this object is not managed # by boost.python b = module.B( a ) In this case it is unsafe to use "return_existing_object". The safer alternatives are: 1. to use "copy_[non]_const_reference", will not work for all classes, also this is not an expected behaviour: b.a.s = 2 will not take effect, because b.a will return copy of a 2. to use "return_internal_reference". Here I make small assumption, that developer has some guaranties: object "a" will not be destroyed, if there is at least one "b" object. Obviously this guarantee exists in C++ code, but C++ code does not have garbage collector. In Python, object "b", could leave longer then object "a". So I am not sure about this option too. 3. "return_existing_object" 4. I am sure, I missed something. Thoughts? Thanks for help. -- Roman Yakovenko C++ Python language binding http://www.language-binding.net/

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Re: [C++-sig] help passing image data
by J.D. Yamokoski May 25, 2006

May 25, 2006

Are there any other C/API calls that are not compatible with Boost? And what is the proper way to shut down the interpreter or reset it using Boost? I will comment this out and see if it has an effect.. I am not guessing it will since finalize was not being called when I ran into the problems I was having. On Wed May 24 18:22:07 EDT 2006, "Ralf W. Grosse-Kunstleve" <rwgk(a)yahoo.com> wrote: > void CPythonDLL::finalize() > { > Py_Finalize(); > } > > CPythonDLL::~CPythonDLL(void) >> { >> int nIsInit = Py_IsInitialized(); >> >> if (nIsInit) Py_Finalize(); >> } > > Boost.Python doesn't support Py_Finalize(). You have to comment > this out. > (I am not familiar with embedding and didn't look closely at your > message, but > this stood out to me as a real problem.) > > > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around > http://mail.yahoo.com > _______________________________________________ > C++-sig mailing list > C++-sig(a)python.org > http://mail.python.org/mailman/listinfo/c++-sig > >

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