[C++-sig] Convert OpenCV class reference return value

[Chris Schnaufer]

I have a C++ library where const references to OpenCV types are returned. Now I'm porting the interface to Python and have hit upon what looks to be an old issue; there doesn't appear to be a way to have boost.python intrinsically convert a const reference OpenCV type to a Python object (tuple or numpy). My guess is that I'm missing something, but maybe not. So I'm providing more details

Right now I'm writing wrapper classes/functions to handle the interface but would prefer to define a conversion of the types in a way that boost.python can just do the conversion without individual class/function wrappers explicitly declared for everything.

For example, if a method returns a member variable of type cv::Vec3d as 'const cv::Vec3d&', I would like to declare one "const cv:Vec3d& to Python" converter and have that be used/specified where needed; and another for Python to const cv:Vec3d&. Sans individual wrapper classes/functions.

Going over the library I've come closest with one of: 1) declare things in such a way as to get a [paraphrasing] "unable to convert pointer to const cv::Vec3d&() type" error [in as_to_python_function.hpp - so close!], or 2) use return_value_policy<copy_const_ref> which Python subsequently can't convert.

I think declaring something along the lines of "class_<cv::Vec3d>..." might work but then I'm redefining the OpenCV class which I don't like to do since my library shouldn't make OpenCV types visible in Python.

I am new to boost.python so I'm probably wrong, but it appears the best solution would be to have as_to_python_function.cpp split into reference and copy-constructor classes to handle the appropriate differences. Right now the same class handles both cases.

Any help is appreciated.

I am including a simple example:

#include <boost/python.hpp>
namespace p = boost::python;

class Vec3d
{
                public:
                  Vec3d() {v[0] = v[1] = v[2] = 0.0;}
                  virtual ~Vec3d() {}
                private:
                  double v[3];
};

class UseVec3d
{
                public:
                                UseVec3d() {}
                                UseVec3d(const Vec3d& vec) : v(vec) {}
                                virtual ~UseVec3d() {};
                                const Vec3d& get_vec() const {return v;}
                                void set_vec(const Vec3d& vec) {v = vec;}
                private:
                  Vec3d                 v;
};

BOOST_PYTHON_MODULE(test)
{
                p::class_<UseVec3d>("UseVec3d")
                                .def("get_vec", &UseVec3d::get_vec)   // Error's here
                ;
}

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/cplusplus-sig/attachments/20170403/64e52cee/attachment.html>