Boost.Python: Polymorphism downcasting

13 Oct 2014

      Hi,

I have a list of base classes in C++, I want to access them in Python as a
list of their derived most classes.

Is there a build in means to cater for this in Boost.Python?

I've made an example the problem I ma facing:

// ------------------------------- Code ----------------------------------//
#include<memory>
#include<iostream>
#include<vector>
namespace boost { template<class T> T* get_pointer(std::shared_ptr<T>& p){
return p.get(); }}

struct Vehicle{ virtual ~Vehicle(){} friend bool operator==(const Vehicle&
lhs, const Vehicle& rhs) { return true; }};
struct Boat: public Vehicle{
    virtual ~Boat(){}
    friend bool operator==(const Boat& lhs, const Boat& rhs) { return true;
}
    char const* testBoatSpecificMethod() { return "Floating."; }
};
struct Truck: public Vehicle{
    virtual ~Truck(){}
    friend bool operator==(const Truck& lhs, const Truck& rhs) { return
true; }
    char const* testTruckSpecificMethod() { return "Trucking."; }
};

class Garage
{
    public:
Garage() {};
~Garage() {};
char const* test() {  std::string temp = "Vehicle List Size: " +
std::to_string(m_VehicleList.size()); return temp.c_str(); }
friend bool operator==(const Garage& lhs, const Garage& rhs) { return true;
}
std::vector<Vehicle>& vehicleList() { return m_VehicleList; }
    private:
        std::vector<Vehicle> m_VehicleList;
};

#include <boost/python.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>
BOOST_PYTHON_MODULE(garage_ext)
{
    using namespace boost::python;
    class_<Garage>("Garage")
        .def("test", &Garage::test)
        .def("vehicleList", &Garage::vehicleList,
return_internal_reference<1>());
   class_<Vehicle,std::shared_ptr<Vehicle>>("Vehicle");
   class_<Boat,std::shared_ptr<Boat>,bases<Vehicle>>("Boat")
        .def("testBoatSpecificMethod", &Boat::testBoatSpecificMethod);
   class_<Truck,std::shared_ptr<Truck>,bases<Vehicle>>("Truck")
        .def("testTruckSpecificMethod", &Truck::testTruckSpecificMethod);

   implicitly_convertible<std::shared_ptr<Boat>,std::shared_ptr<Vehicle>>();

 implicitly_convertible<std::shared_ptr<Truck>,std::shared_ptr<Vehicle>>();
   class_<std::vector<Vehicle> >("stl_vector_Vehicle")
       .def(vector_indexing_suite<std::vector<Vehicle> >());
}

// --------------------------- Test Script -------------------------------//
import garage_ext
g = garage_ext.Garage()
l = g.vehicleList()
l.append(garage_ext.Boat())
print "Expecting a Boat object:"
print str(l[0])
print g.vehicleList()[0].testBoatSpecificMethod()
garage_ext.f2("Done.")

// ------------------------------ Output ---------------------------------//
Expecting a Boat object
<garage_ext.Vehicle object at 0x7fb17a3bfb50>
Traceback (most recent call last):
  File "test_garage.py", line 7, in <module>
    print g.vehicleList()[0].testBoatSpecificMethod()
AttributeError: *'Vehicle' object has no attribute 'testBoatSpecificMethod'*

*'Vehicle' object has no attribute 'testBoatSpecificMethod'*
Here I want Vehicle to be a Boat object.

If there is not a build in or recommended/known Boost.Python means to
handle this problem,
I'm thinking of wrapping the list (Lots of accessors to be wrapped in my
library.) with a get accessor returning a boost::python::list, storing the
derived most types. Getting the derived most type possibly by calling
overriden 'getAsDerivedClass' method. I would like to avoid this. I dislike
having to add python usage specific methods to the library, for our design
and vision values / reasons.

Kind Regards,
Christoff

Christoff Kok

Christoff Kok

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