bump version to 12.2.2-pve1

[ceph.git] / ceph / src / boost / libs / python / doc / reference / to_python_converter.qbk

[section boost/python/to_python_converter.hpp]
[section Introduction]
`to_python_converter` registers a conversion from objects of a given C++ type into a Python object. 
[endsect]
[section Class template `to_python_converter`]
`to_python_converter` adds a wrapper around a static member function of its second template parameter, handling low-level details such as insertion into the converter registry.

In the table below, x denotes an object of type T
[table
[[Parameter][Requirements][Description]]
[[T][][The C++ type of the source object in the conversion]]
[[Conversion][`PyObject* p = Conversion::convert(x)`,
`if p == 0`, `PyErr_Occurred() != 0`.][A class type whose static member function convert does the real work of the conversion.]]
[[bool has_get_pytype=false][`PyTypeObject const * p = Conversion::get_pytype()`]
[Optional member - if Conversion has `get_pytype` member supply `true` for this parameters. If present `get_pytype` is used to document the return type of functions using this conversion. The `get_pytype` may be implemented using the classes and functions from pytype_function.hpp NOTE : For backward compatibility this parameter may be passed after checking if BOOST_PYTHON_SUPPORTS_PY_SIGNATURES is defined (see [link function_invocation_and_creation.function_documentation.boost_python_pytype_function_hpp.example here]).]
]]

``
namespace boost { namespace python
{
  template <class T, class Conversion, bool convertion_has_get_pytype_member=false>
  struct to_python_converter
  {
      to_python_converter();
  };
}}
``
[section Class template `to_python_converter` constructor]
``to_python_converter();``
[variablelist
[[Effects][Registers a `to_python` converter which uses `Conversion::convert()` to do its work.]]
]
[endsect]
[endsect]
[section Example]
This example presumes that someone has implemented the standard noddy example module from the Python documentation, and placed the corresponding declarations in "noddy.h". Because noddy_NoddyObject is the ultimate trivial extension type, the example is a bit contrived: it wraps a function for which all information is contained in the type of its return value.

In C++:
``
#include <boost/python/reference.hpp>
#include <boost/python/module.hpp>
#include "noddy.h"

struct tag {};
tag make_tag() { return tag(); }

using namespace boost::python;

struct tag_to_noddy
{
    static PyObject* convert(tag const& x)
    {
        return PyObject_New(noddy_NoddyObject, &noddy_NoddyType);
    }
    static PyTypeObject const* get_pytype()
    {
        return &noddy_NoddyType;
    }
};

BOOST_PYTHON_MODULE(to_python_converter)
{
    def("make_tag", make_tag);
    to_python_converter<tag, tag_to_noddy, true>(); //"true" because tag_to_noddy has member get_pytype
}
``
In Python:
``
>>> import to_python_converter
>>> def always_none():
...     return None
...
>>> def choose_function(x):
...     if (x % 2 != 0):
...         return to_python_converter.make_tag
...     else:
...         return always_none
...
>>> a = [ choose_function(x) for x in range(5) ]
>>> b = [ f() for f in a ]
>>> type(b[0])
<type 'NoneType'>
>>> type(b[1])
<type 'Noddy'>
>>> type(b[2])
<type 'NoneType'>
>>> type(b[3])
<type 'Noddy'>
``
[endsect]
[endsect]