default_call_policies is a model of CallPolicies with no precall or postcall behavior and a result_converter which handles by-value returns. Wrapped C++ functions and member functions use default_call_policies unless otherwise specified. You may find it convenient to derive new models of CallPolicies from default_call_policies.

namespace boost { namespace python
{
    struct default_call_policies
    {
        static bool precall(PyObject*);
        static PyObject* postcall(PyObject*, PyObject* result);
        typedef default_result_converter result_converter;
        template <class Sig> struct extract_return_type : mpl::front<Sig>{};
    };
}}

bool precall(PyObject*);

PyObject* postcall(PyObject*, PyObject* result);

default_result_converter is a model of ResultConverterGenerator which can be used to wrap C++ functions returning non-pointer types, char const*, and PyObject*, by-value.

namespace boost { namespace python
{
    struct default_result_converter
    {
        template <class T> struct apply;
    };
}}

template <class T> struct apply

This example comes from the Boost.Python implementation itself. Because the return_value_policy class template does not implement precall or postcall behavior, its default base class is default_call_policies:

template <class Handler, class Base = default_call_policies>
struct return_value_policy : Base
{
   typedef Handler result_converter;
};

return_arg and return_self instantiations are models of CallPolicies which return the specified argument parameter (usually *this) of a wrapped (member) function.

namespace boost { namespace python
{
   template <size_t arg_pos=1, class Base = default_call_policies>
   struct return_arg : Base
   {
      static PyObject* postcall(PyObject*, PyObject* result);
      struct result_converter{ template <class T> struct apply; };
      template <class Sig> struct extract_return_type : mpl::at_c<Sig, arg_pos>{};

   };
}}

PyObject* postcall(PyObject* args, PyObject* result);

namespace boost { namespace python
{
   template <class Base = default_call_policies>
   struct return_self
     : return_arg<1,Base>
   {};
}}

C++ module definition:

#include <boost/python/module.hpp>
#include <boost/python/class.hpp>
#include <boost/python/return_arg.hpp>

struct Widget
{
   Widget() :sensitive_(true){}
   bool get_sensitive() const { return sensitive_; }
   void set_sensitive(bool s) { this->sensitive_ = s; }
 private:
   bool sensitive_;
};

struct Label : Widget
{
   Label() {}

   std::string  get_label() const { return label_; }
   void set_label(const std::string &l){ label_ = l; }

 private:
   std::string label_;
};

using namespace boost::python;
BOOST_PYTHON_MODULE(return_self_ext)
{
   class_<widget>("Widget")
      .def("sensitive", &Widget::get_sensitive)
      .def("sensitive", &Widget::set_sensitive, return_self<>())
      ;

   class_<Label, bases<Widget> >("Label")
      .def("label", &Label::get_label)
      .def("label", &Label::set_label, return_self<>())
      ;
}

Python code:

>>> from return_self_ext import *
>>> l1 = Label().label("foo").sensitive(false)
>>> l2 = Label().sensitive(false).label("foo")

return_internal_reference instantiations are models of CallPolicies which allow pointers and references to objects held internally by a free or member function argument or from the target of a member function to be returned safely without making a copy of the referent. The default for its first template argument handles the common case where the containing object is the target (*this) of a wrapped member function.

namespace boost { namespace python
{
   template <std::size_t owner_arg = 1, class Base = default_call_policies>
   struct return_internal_reference : Base
   {
      static PyObject* postcall(PyObject*, PyObject* result);
      typedef reference_existing_object result_converter;
   };
}}

PyObject* postcall(PyObject* args, PyObject* result);

C++ module definition:

#include <boost/python/module.hpp>
#include <boost/python/class.hpp>
#include <boost/python/return_internal_reference.hpp>

class Bar
{
 public:
   Bar(int x) : x(x) {}
   int get_x() const { return x; }
   void set_x(int x) { this->x = x; }
 private:
   int x;
};

class Foo
{
 public:
   Foo(int x) : b(x) {}

   // Returns an internal reference
   Bar const& get_bar() const { return b; }

 private:
   Bar b;
};

using namespace boost::python;
BOOST_PYTHON_MODULE(internal_refs)
{
   class_<Bar>("Bar", init<int>())
      .def("get_x", &Bar::get_x)
      .def("set_x", &Bar::set_x)
      ;

   class_<Foo>("Foo", init<int>())
      .def("get_bar", &Foo::get_bar
          , return_internal_reference<>())
      ;
}

Python code:

>>> from internal_refs import *
>>> f = Foo(3)
>>> b1 = f.get_bar()
>>> b2 = f.get_bar()
>>> b1.get_x()
3
>>> b2.get_x()
3
>>> b1.set_x(42)
>>> b2.get_x()
42

return_value_policy instantiations are simply models of CallPolicies which are composed of a ResultConverterGenerator and optional Base CallPolicies.

namespace boost { namespace python
{
  template <class ResultConverterGenerator, class Base = default_call_policies>
  struct return_value_policy : Base
  {
      typedef ResultConverterGenerator result_converter;
  };
}}

C++ module definition:

#include <boost/python/module.hpp>
#include <boost/python/class.hpp>
#include <boost/python/copy_const_reference.hpp>
#include <boost/python/return_value_policy.hpp>

// classes to wrap
struct Bar { int x; }

struct Foo {
   Foo(int x) : { b.x = x; }
   Bar const& get_bar() const { return b; }
 private:
   Bar b;
};

// Wrapper code
using namespace boost::python;
BOOST_PYTHON_MODULE(my_module)
{
   class_<Bar>("Bar");

   class_<Foo>("Foo", init<int>())
      .def("get_bar", &Foo::get_bar
          , return_value_policy<copy_const_reference>())
      ;
}

Python code:

>>> from my_module import *
>>> f = Foo(3)         # create a Foo object
>>> b = f.get_bar()    # make a copy of the internal Bar object

This header provides facilities for establishing a lifetime dependency between two of a function's Python argument or result objects. The ward object will not be destroyed until after the custodian as long as the custodian object supports weak references (Boost.Python extension classes all support weak references). If the custodian object does not support weak references and is not None, an appropriate exception will be thrown. The two class templates with_custodian_and_ward and with_custodian_and_ward_postcall differ in the point at which they take effect.

In order to reduce the chance of inadvertently creating dangling pointers, the default is to do lifetime binding before the underlying C++ object is invoked. However, before invocation the result object is not available, so with_custodian_and_ward_postcall is provided to bind lifetimes after invocation. Also, if a C++ exception is thrown after with_custodian_and_ward<>::precall but before the underlying C++ object actually stores a pointer, the lifetime of the custodian and ward objects will be artificially bound together, so one might choose with_custodian_and_ward_postcall instead, depending on the semantics of the function being wrapped.

Please note that this is not the appropriate tool to use when wrapping functions which transfer ownership of a raw pointer across the function-call boundary. Please see the FAQ if you want to do that.

namespace boost { namespace python
{
   template <std::size_t custodian, std::size_t ward, class Base = default_call_policies>
   struct with_custodian_and_ward : Base
   {
      static bool precall(PyObject* args);
   };
}}

bool precall(PyObject* args);

namespace boost { namespace python
{
   template <std::size_t custodian, std::size_t ward, class Base = default_call_policies>
   struct with_custodian_and_ward_postcall : Base
   {
      static PyObject* postcall(PyObject* args, PyObject* result);
   };
}}

PyObject *postcall(PyObject* args, PyObject* result);

The following example shows how with_custodian_and_ward_postcall is used by the library to implement return_internal_reference

template <std::size_t owner_arg = 1, class Base = default_call_policies>
struct return_internal_reference
    : with_custodian_and_ward_postcall<0, owner_arg, Base>
{
   typedef reference_existing_object result_converter;
};