EQ2EMu/EQ2/source/depends/boost_1_72_0/libs/python/doc/reference/return_arg.qbk

[section boost/python/return_arg.hpp]
[section Introduction]
`return_arg` and `return_self` instantiations are models of [link concepts.callpolicies `CallPolicies`] which return the specified argument parameter (usually `*this`) of a wrapped (member) function. 
[endsect]
[section Class `return_arg`]
[table
[[Parameter][Requirements][Description][Default]]
 [[arg_pos][A positive compile-time constant of type `std::size_t`.][the position of the argument to be returned.][1]]
[[Base][A model of [link concepts.callpolicies `CallPolicies`]][Used for policy composition. Any `result_converter` it supplies will be overridden by `return_arg`, but its `precall` and `postcall` policies are composed as described here [link concepts.callpolicies `CallPolicies`].][default_call_policies]]
 ]
``
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>{};

   };
}}
``
[endsect]
[section Class `return_arg` static functions]
``PyObject* postcall(PyObject* args, PyObject* result);``
[variablelist
[[Requires][`PyTuple_Check(args) != 0` and `PyTuple_Size(args) != 0`]]
[[Returns][PyTuple_GetItem(args,arg_pos-1)]]
]
[endsect]
[section Class template `return_self`]
``
namespace boost { namespace python
{
   template <class Base = default_call_policies>
   struct return_self 
     : return_arg<1,Base>
   {};
}}
``
[endsect]
[section Example]
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") 
``
[endsect]
[endsect]