c++ - Create a propety with a call policy - boost::python -

i have following c++ classes expose python.

class plainolddata {      ... };  class fancyclass { public:     const plainolddata& getmypod() {return mypod;} private:     plainolddata mypod; }; 

because want python classes pythonic, expose mypod property. however, when try following:

// expose boost::python boost_python_module(mymod) {     class_<plainolddata>("plainolddata", init<>());      // fails     class_<fancyclass>("fancyclass", init<>())         .add_property("mypod", &fancyclass::getmypod);  } 

i following error: error c2027: use of undefined type 'boost::python::detail::specify_a_return_value_policy_to_wrap_functions_returning<t>'

but, if try specify call policy, such as:

    class_<fancyclass>("fancyclass", init<>())         .add_property("mypod", &fancyclass::getmypod, return_value_policy<copy_const_reference>()); 

i incredibly long error message.

is possible expose function property; doing wrong?

similar how python's property() passed python callable objects, boost::python::class_::add_property() function can accept python callable objects can created callpolicies, such returned boost::python::make_function().

for example, property in original code exposed like:

class_<fancyclass>("fancyclass", init<>())   .add_property("mypod", make_function(&fancyclass::getmypod,     return_value_policy<copy_const_reference>())); 

---

here complete minimal example:

#include <boost/python.hpp>  class egg {};  class spam { public:   const egg& get_egg() { return egg_; } private:   egg egg_; };  boost_python_module(example) {   namespace python = boost::python;   python::class_<egg>("egg");   python::class_<spam>("spam")     .add_property("egg", python::make_function(&spam::get_egg,       python::return_value_policy<python::copy_const_reference>()))     ; } 

interactive usage:

>>> import example >>> spam = example.spam() >>> assert(spam.egg not spam.egg) # unique identities spam.egg                                      # returns copy >>> egg1 = spam.egg >>> assert(egg1 not spam.egg) >>> egg2 = spam.egg >>> assert(egg1 not egg2)