Boost Python

Diane Trout

2006 March 23 - SoCal Piggies

Boost Python

• Yet another way of making C/C++ code available to python
• Related work
  • Pyrex
  • SWIG

Pyrex

• Has poor support for C++ but allows you to mostly write python-like code for integrating C libraries

Swig

• Popular and it generates wrappers for things other than python
• However, its handling of C++ is functional but hackish

C++, The Problem Language

• C++ is possibly the most complicated language in common usage.
• Binary compatibility with C++ is a giant PITA
• The name mangling used to implement function overloading is compiler specific and rather arcane.
  • E.g. void Mussa::Nway() becomes _ZN9NwayPaths4pendEv
  • or Sequence::load_annot(string, int, int) becomes _ZN8Sequence10load_annotESsii
  • (Functions returning STL iterators are even longer)

Swigs solution

• Swig handles wrapping C++ by converting C++ calls into C functions that make the this pointer explicit, and then escapes from the complexity of the C++ ABI by using the nice standard C ABI
• Unfortunately this means that to make something look like a class in your target language swig produces a shadow class in the implementation language, that uses the C representation.

Boost Python

• There is no shadow class
• Uses C++ Template Magic
• http://boost.org/libs/python/doc/internals.html
  • The template handles registering types, and marshaling between C++ and python

Hello Example

//hello.cpp
#include <boost/python.hpp>
using namespace boost::python;
char const* greet() {
  return "hi there";
}
BOOST_PYTHON_MODULE(hello) {
  def("greet", greet);
}
// g++ -o hello.so -shared -I/usr/include/python2.3 \\
// -lpython2.3 -lboost_python hello.cpp

Simple Class

#include <string>
struct World {
  void set(std::string msg) { this->msg = msg; }
  std::string greet() { return msg; }
  std::string msg; // not accessible
};
#include <boost/python.hpp>
using namespace boost::python;
BOOST_PYTHON_MODULE(simple) {
    class_<World>("World")
      .def("greet", &World::greet)
      .def("set", &World::set) ;
}
// g++ -shared -I/usr/include/python2.3 -lpython2.3 -lboost_python \\
// -o simple.so simple.cpp

Property

#include <string>
struct World {
  void set(std::string msg) { this->msg = msg; }
  std::string greet() { return msg; }
  std::string msg; // not accessible
};
#include <boost/python.hpp>
using namespace boost::python;
BOOST_PYTHON_MODULE(simple) {
    class_<World>("World")
      .def("greet", &World::greet)
      .add_property("msg", &World::greet, &World::set) ;
}
// g++ -shared -I/usr/include/python2.3 -lpython2.3 -lboost_python \\
// -o property.so property.cpp