Mailman 3 March 2015 - Cplusplus-sig

Passing std::wstring to Python callback
by Bogdan Cristea April 3, 2015

April 3, 2015

Hi I am using boost python for providing python bindings for a C++ library. I am using Python 2.7. My library needs to call callbacks that are defined in Python. There callbacks can take a string parameter. C++ side I am always using std::wstring. Everything works on as long as std::wstring contains only ASCII chars, but once I get something like “é” my application crashes. This issue seems to be related to the way I pass std::wstring to Python callbacks: as long as I make sure that the passed string has only ASCII chars it is ok. My problem is how do I pass unicode strings to Python. thanks Bogdan

2 1

Memory management in Boost.Python
by Ernie Lee April 2, 2015

April 2, 2015

Hello, I am working on adding Python binding to C++ project and run into the following problem: For some reason Python tries to delete objects that are still held by smart pointer instances in our C++ code. We are using std::shared_ptr for memory management and classes exposed through Boost::Python have HeldType set to std::shared_ptr<…>. This problem manifests itself in the following scenarios: - in Python i subclass C++ class, which acts as ‘callback’, and create an instance of it and pass it to C++ code, where it is held in std::shared_ptr instance. Python script itself does not use this callback object after that point. When Python script terminates i receive memory error, which seems to be triggered by attempting to double-free this callback object. - in Python i subclass C++ class, create an instance of it (call it ‘A') and store it in container-like object (call it ‘B') (B is defined in C++ and exposed to Python). B stores shared_ptr to A. As soon as Python execution point leaves current scope my object A gets deleted while B still holds shared_ptr to A. It looks to me that ether i have a bug in my wrapping code (any idea what this could be?) or in some cases Python disregards HeldType and tries to determine object lifespan by its own means (in both cases from Python point of view created object is no longer in scope so i am guessing thats why it tries to delete it). If the latter is correct, any idea how to force Python to use smart pointer class specified in HeldType? Thank you, Ernie.

2 6

Boost.Python "contains" compile fix
by Huebl, Axel March 30, 2015

March 30, 2015

Hi, adding more Python 3 features, the "contains" method was added (in 1.55.0?) http://www.boost.org/doc/libs/1_55_0/libs/python/doc/news.html Unfortunately, I found a nasty compile bug when compiling with nvcc that I tried to fix in https://github.com/boostorg/python/pull/14 Does anyone know how to test my patch to see if this function is still working as expected? Thank you for your help, Axel

3 5

Boost python exposing abstract class , function returning boost::shared_ptr
by warin March 30, 2015

March 30, 2015

> I'm not top posting. I have an abstract class A2, with derived class B2. I want to map a function returning a boost::shared_ptr of A. This abstract class is wrapped : struct A2{ virtual ~A2(){} virtual int ret() =0; }; struct B2: public A2{ virtual ~B2(){} int ret() { return 1; } }; // wrapper struct A2Wrap : A2, wrapper<A2> { inline int ret() { return this->get_override("ret")(); } }; // function to map boost::shared_ptr<A2> f1() { return boost::shared_ptr<A2>(new B2()); } // expose to python BOOST_PYTHON_MODULE(grids) { class_<A2Wrap, boost::shared_ptr<A2Wrap> , boost::noncopyable >("A2") .def("ret",pure_virtual(&A2::ret)); class_<B2,boost::shared_ptr<B2>,bases<A2>>("B2"); def("f1",f1); } I get the following result on execution : >import grids >>> a = grids.f1() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: No to_python (by-value) converter found for C++ type: boost::shared_ptr<A2> Any idea for this problem ? I imagine to express the conversion from boost::shared_ptr<A2> to boost::shared_ptr<A2Wrap>.

2 4

Reuse PyObject for non python instanciated C++ objects
by Goode, Carl R (PG GT LGT EN MT 4 2) March 25, 2015

March 25, 2015

I have a use case where we have a C++ framework that is exposed through boost python. If I use a C++ factory to create objects, I want to always return the same python object once it is exposed. I've done this through a postcall policy, but I was wondering if there is a better way. Below is a simple example of my problem and my proposed solution. This works, with the only drawback that I create a PyObject* that I throw out every time. I do not want my framework (in this case, struct X) to know anything about python. Also, I'm familiar with "has_backed_reference" and using a heldType wrapper, but that would only apply to objects instantiated through python. struct X; std::map<std::string, boost::shared_ptr<X> > xmap; std::map<X*, PyObject*> xpymap; struct X // a container element { std::string id; std::string check; X(std::string s) : id(s) { } void printId() { std::cout << "ID is " << id << std::endl; } void change_string(std::string nstring) { check = nstring; } void print_string() { std::cout << "String is now: " << check << std::endl; } boost::shared_ptr<X> static XFactory(std::string val) { std::map<std::string,boost::shared_ptr<X> >::iterator it = xmap.find(val); if (it == xmap.end()) { boost::shared_ptr<X> nptr = boost::make_shared<X>(val); xmap[val] = nptr; return nptr; } return xmap[val]; } }; // Custom call policy to overwrite the postcall from C++ back to python // template <typename BasePolicy = boost::python::default_call_policies> struct carls_policy : BasePolicy { template <typename ArgumentPackage> PyObject* postcall(const ArgumentPackage& args, PyObject* result) { // Chain to base policy. result = BasePolicy::postcall(args, result); // Extract shared_ptr from result // boost::shared_ptr<X> xptr = boost::python::extract< boost::shared_ptr<X> >(result); if (!xptr) { return result; } std::map< X*, PyObject* >::iterator it = xpymap.find(xptr.get()); if (it == xpymap.end()) { xpymap[xptr.get()] = result; Py_INCREF(xpymap[xptr.get()]); // Increment to store in map } else { Py_DECREF(result); // Not needed anymore } Py_INCREF(xpymap[xptr.get()]); // Inremented to return to python return xpymap[xptr.get()]; } }; BOOST_PYTHON_MODULE(rvp) { boost::python::class_<X, boost::shared_ptr<X> > ("X", boost::python::no_init) .def("XFactory", &X::XFactory, carls_policy<>()).staticmethod("XFactory") .def("printId", &X::printId) .def("change_string", &X::change_string) .def("print_string", &X::print_string) ; } Implementation: >>> import rvp >>> x1 = rvp.X.XFactory('id1') >>> x2 = rvp.X.XFactory('id2') >>> x3 = rvp.X.XFactory('id1') >>> x1 is x3 True This message and any attachments are solely for the use of intended recipients. The information contained herein may include trade secrets, protected health or personal information, privileged or otherwise confidential information. Unauthorized review, forwarding, printing, copying, distributing, or using such information is strictly prohibited and may be unlawful. If you are not an intended recipient, you are hereby notified that you received this email in error, and that any review, dissemination, distribution or copying of this email and any attachment is strictly prohibited. If you have received this email in error, please contact the sender and delete the message and any attachment from your system. Thank you for your cooperation

1 0

Linking with Python3 Libs
by Dong Ta March 25, 2015

March 25, 2015

Hi, I'm trying to figure out how to build Python modules using Boost Python against Python3; and I'm currently stuck at getting cmake to find Python 3. I have already installed Python3 via brew install python3; but cmake still can't find PythonLibs 3. What am I missing here? Here's the header of my *CMakeLists.txt * cmake_minimum_required(VERSION 3.0) find_package(PythonLibs 3 REQUIRED) and here's the error message Could NOT find PythonLibs: Found unsuitable version "2.7.6", but required is at least "3" (found /usr/lib/libpython2.7.dylib)

1 0

Subclassing C++ classes in Python
by Ernie Lee March 16, 2015

March 16, 2015

Hi there, I am creating binding for some C++ classes and I need to be able to subclass exposed C++ classes in Python (and pass subclassed instances back in to C++ code). I was able to make this work by following this example: http://www.boost.org/doc/libs/1_57_0/libs/python/doc/tutorial/doc/html/pyth… and creating binding for both: original C++ classes and ‘wrapper’ structs. This however create inconvenience and path for errors because now users need to remember to use different base class when subclassing C++ classes. Is there is a way to achieve subclassing functionality by exposing one class instead of two? When I tries to use class exposed as ’wrapper’ as substitute for C++ original class i am getting a segfault error (see example below). Is there is something wrong with a way i create a wrapper-class or it is not intended to work as substitute for a base class? Thanks, Ernie. Python code: ———————— from subclass import * class B(A_Wrapper): def __init__(self): A_Wrapper.__init__(self) def info(self): print 'B info!' a = A(); a.info(); print '1' b = B(); b.info(); print '2' aw = A_Wrapper(); print '3'; aw.info(); print '4' # <-- segfault before '4' got printed C++ code: ———————— #include <iostream> #include <boost/python.hpp> class A { public: virtual ~A() { std::cout << "A destructor for object: " << this << std::endl; } virtual void info() { std::cout << "C++ A info for object" << this << std::endl; } }; struct A_Wrapper : A, boost::python::wrapper<A> { A_Wrapper() : A() {} void info() {this->get_override("info")(); } }; BOOST_PYTHON_MODULE(subclass) { boost::python::class_<A, boost::noncopyable>("A") .def("info", &A::info) ; boost::python::class_<A_Wrapper, boost::noncopyable>("A_Wrapper", boost::python::init <>() ) .def("info", &A_Wrapper::info) ; }

2 2

[pybindgen] How to wrap char** argv[]?
by stefano franchi March 12, 2015

March 12, 2015

[I posted the same question on Ubuntu's Launchpad's answers site, where earlier discussions of pybindgen took place. However, there seems to be no activity on that site, so i am reposting here] Problem with pybindgen: I stumbled upon a library that includes a function using pointers to the standard main function's arguments, and therefore uses a signature of char** argv[] (that is, a pointer to the standard char* argv[]): in main I have the standard signature with reference to its arguments: int main( int argc, char* argv[] ) { // initialize libstage - call this first Stg::Init( &argc, &argv ); ----------etc.----------------- and then the signature of the called function becomes: void Init( int* argc, char** argv[] ); --------------------------------- Given this situation, which parameter am I supposed to pass to add_function('Init')? I saw the answer how to pass char** argv, but I am not sure it is applicable here, as I don't understand the code in that answer too well, I am afraid. Thanks, Stefano -- __________________________________________________ Stefano Franchi stefano.franchi(a)gmail.com <stefano(a)tamu.edu> http://stefano.cleinias.org

1 0