boost python explicit typecast needed

boost-python, c++

Solution

When it comes to `boost::shared_ptr`, Boost.Python generally provides the desired functionality. In this particular case, there is no need to explicitly provide custom `to_python` converters as long as the module declaration defines that `Base` is held by `boost::shared_ptr<Base>`, and Boost.Python is told that `A` inherits from `Base`.

BOOST_PYTHON_MODULE(example) {
  using namespace boost::python;
  class_<Base, boost::shared_ptr<Base>, 
         boost::noncopyable>("Base", no_init);
  class_<A, bases<Base>,
         boost::noncopyable>("A", no_init);
  def("factory",  &factory);
  def("consumer", &consumer);
}

Boost.Python does not currently support custom lvalue converters, as it requires changes to the core library. Therefore, the `consumer` function either needs accept the `boost:shared_ptr<A>` by value or by const-reference. Either of the following signatures should work:

void consumer(boost::shared_ptr<A> a)
void consumer(const boost::shared_ptr<A>& a)

Here is a complete example:

#include <boost/python.hpp>
#include <boost/make_shared.hpp>

class Base
{
public:
  virtual ~Base() {}
};

class A
  : public Base
{
public:
  A(int value) : value_(value) {}
  int value() { return value_; };
private:
  int value_;
};

boost::shared_ptr<Base> factory()
{
  return boost::make_shared<A>(42);
}

void consumer(const boost::shared_ptr<A>& a)
{
  std::cout << "The value of object is " << a->value() << std::endl;
}

BOOST_PYTHON_MODULE(example) {
  using namespace boost::python;
  class_<Base, boost::shared_ptr<Base>, 
         boost::noncopyable>("Base", no_init);
  class_<A, bases<Base>,
         boost::noncopyable>("A", no_init);
  def("factory",  &factory);
  def("consumer", &consumer);
}

And the usage:

>>> from example import *
>>> x = factory()
>>> type(x)
<class 'example.A'>
>>> consumer(x)
The value of object is 42
>>> 

Since the module declaration specified that `Base` was a base-class for `A`, Boost.Python was able to resolves the type returned from `factory()` to `example.A`.

Problem

Original source