Programming Python, 3rd Edition

Wrapping C++ Classes with SWIG

One of the more clever tricks SWIG can perform is class wrapper generation. Given a C++ class declaration and special command-line settings, SWIG generates the following:

• A C++-coded Python extension module with accessor functions that interface with the C++ class’s methods and members

• A Python-coded module with a wrapper class (called a “shadow” or “proxy” class in SWIG-speak) that interfaces with the C++ class accessor functions module

As before, to use SWIG in this domain, write and debug your class as though it would be used only from C++. Then, simply run SWIG in your makefile to scan the C++ class declaration and compile and link its output. The end result is that by importing the shadow class in your Python scripts, you can utilize C++ classes as though they were really coded in Python. Not only can Python programs make and use instances of the C++ class, they can also customize it by subclassing the generated shadow class.

A Simple C++ Extension Class

To see how this works, we need a C++ class. To illustrate, let’s code a simple one to be used in Python scripts.^[*] The following C++ files define a Number class with three methods (add, sub, display), a data member (data), and a constructor and destructor. Example 22-18 shows the header file.

Example 22-18. PP3E\Integrate\Extend\Swig\Shadow\number.h

class Number { public: Number(int start); // constructor ~Number( ); // destructor void add(int value); // update data member void sub(int value); int square( ...