c++ - 错误 C2027 : use of undefined type 'boost::python::detail::reference_existing_object_requires_a_pointer_or_reference_return_type<R>'

Question

我收到错误reference_existing_object_requires_a_pointer_or_reference_return_type。

这是代码。

boost::shared_ptr<CDB::Basic> GetCdbWrapper(boost::shared_ptr<A> cmd)
    {
        return cmd->Cdb();
    }
}

virtual boost::shared_ptr<CDB::Basic> Cdb() { 
    boost::shared_ptr<CDB::Basic> CdbObj;
    return CdbObj;
}
boost::shared_ptr<CDB::Basic> GetCdb() { 
    return this->Cdb(); 
}


class_<A, bases<Core::CommandBase>, boost::shared_ptr<A>, boost::noncopyable, >("A",
    ":description:\n",
    boost::python::no_init
)

.def("Cdb", &A::GetCdb,
    ":description:\n",
    return_value_policy<reference_existing_object>()
);

我可以知道上面的代码有什么问题吗？我收到如下编译错误。

error C2027: use of undefined type 'boost::python::detail::reference_existing_object_requires_a_pointer_or_reference_return_type<R>'
1>        with
1>        [
1>            R=result_t
1>        ]
1>        c:\boost\boost_1_47_0\boost\python\detail\caller.hpp(200) : while compiling class template member function 'PyObject *boost::python::detail::caller_arity<1>::impl<F,Policies,Sig>::operator ()(PyObject *,PyObject *)'
1>        with
1>        [
1>            F=boost::shared_ptr<CDB::Basic> (__thiscall A::* )(void),
1>            Policies=boost::python::return_value_policy<boost::python::reference_existing_object>,
1>            Sig=boost::mpl::vector2<boost::shared_ptr<CDB::Basic>, A &>
1>        ]

Answer 1

如 return_internal_reference 中所述在文档中，返回的对象通过指针或引用引用现有的内部对象:

return_internal_reference [...] allow pointers and references to objects held internally [...] to be returned safely without making a copy of the referent.

Boost.Python 提供了一些概念检查，并且通常类型会强调失败的概念。在这种特殊情况下，编译器错误为:

reference_existing_object_requires_a_pointer_or_reference_return_type

This is because the GetCdb() function returns a boost::shared_ptr<CDB::Basic> by value, failing to meet the requirements for the return_internal_reference call policy. To resolve this, use the default call policy of returning a copy by value. This requires that the CDB::Basic be exposed through Boost.Python as being held by a boost::shared_ptr. Overall, this behavior is not too different than what is often used with a shared_ptr, where one creates copies of the shared_ptr to maintain shared ownership.

---

Here is an example demonstrating this behavior:

#include <boost/python.hpp>

// Mocks...
class spam {};

boost::shared_ptr<spam> get_spam()
{
  boost::shared_ptr<spam> spam_ptr(new spam());
  return spam_ptr;
}

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::class_<spam, boost::shared_ptr<spam>, boost::noncopyable>(
      "Spam", python::no_init);
  python::def("get_spam", &get_spam);
}

交互式使用:

>>> import example
>>> spam = example.get_spam()
>>> assert(type(spam) is example.Spam)