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C++ Boost


Header <boost/python/to_python_converter.hpp>


Class Template to_python_converter
Class Template to_python_converter synopsis
Class Template to_python_converter constructor


to_python_converter registers a conversion from objects of a given C++ type into a Python object.


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.
to_python_converter template parameters
In the table below, x denotes an object of type T
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.

Class template to_python_converter synopsis

namespace boost { namespace python
  template <class T, class Conversion>
  struct to_python_converter

Class template to_python_converter constructor

Effects: Registers a to_python converter which uses Conversion::convert() to do its work.


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.

C++ module definition

#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);

    def("make_tag", make_tag);
    to_python_converter<tag, tag_to_noddy>();

Python code

>>> 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'>

Revised 13 November, 2002

© Copyright Dave Abrahams 2002. All Rights Reserved.