Easy extending guide

Introduction

”... Boost.Python library is designed to wrap C++ interfaces non-intrusively, so that you should not have to change the C++ code at all in order to wrap it.”

The previous statement is almost true. There are few use cases that the library doesn’t support. This guide will list some of them and will offer few possible solutions.

Pointer to function

Boost.Python doesn’t handle “pointer to function” functionality. You cannot pass it as function argument or keep it, as a member variable.

The simple work-around is to use command design pattern

Problematic function arguments types

C arrays

Boost.Python doesn’t handle C arrays, the only exception are char* and wchar_t*.

Consider the following function:

int write( int* data, size_t size );

The technical reasons are not the only one that prevent Boost.Python to expose such functions, there is a mental one: such interface is not intuitive for Python developers. They expect to pass single argument. For example, built-in file.write method takes a single argument - sequence of characters.

Work-around:

  1. With small help from the developer, Py++ generates code which feets well into Python developer mental model. Pure virtual member functions are a special case, which Py++ doesn’t handle right now.
  2. Use STL containers, std::vector<...> and others.

Immutable by reference

Python defines few fundamental types as “immutable”. The value of an instance of the immutable type could not be changed after construction. Try to avoid passing the immutable types by reference.

Immutable types:

  • char
  • signed char
  • unsigned char
  • wchar_t
  • short int
  • short unsigned int
  • bool
  • int
  • unsigned int
  • long int
  • long unsigned int
  • long long int
  • long long unsigned int
  • float
  • double
  • long double
  • complex double
  • complex long double
  • complex float
  • std::string
  • std::wstring
  • C++ enum is mapped to Python int type
  • smart pointers

Work around:

  • Just don’t pass them by reference :-)
  • With small help from the developer, Py++ generates code which work-arounds this issue, but the resulting interface is ugly.

void*

In most cases, void* is used when a developer has to deal with a memory block. Python provides support for this functionality, but I still didn’t find an easy and intuitive way to expose it. There is no work-around for this issue.

If you use void* to pass a reference to some object, than Boost.Python and Py++ support such use case.

Memory managment

  • Use std::auto_ptr to transfer ownership and responsibility for an object destruction.
  • The only well supported smart pointer class is boost::shared_ptr. I suggest you to use it all the time, especially in cases where you want to create object from Python and pass ownership to C++ code. You don’t want the headache associated with this task.