Programming Python, Second Edition

Wrapping C++ Classes with SWIG

One of the neater tricks SWIG can perform is class wrapper generation -- given a C++ class declaration and special command-line settings, SWIG generates:

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

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

As before, simply run SWIG in your makefile to scan the C++ class declaration and compile its outputs. 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 Little C++ Class (But Not Too Much)

To see how this all works, we need a C++ class. To illustrate, let’s code a simple one to be used in Python scripts.^[149] The following C++ files define a Number class with three methods (add, sub, display), a data member (data), and a constructor and destructor. Example 19-19 shows the header file.

Example 19-19. PP2E\Integrate\Extend\Swig\Shadow\number.h

class Number
{
public:
    Number(int start);
    ~Number(  );
    void add(int value);
    void sub(int value);
    void display(  );
    int data;
};

And Example 19-20 is the C++ class’s implementation file; each method prints a message when called to trace class operations.

Example 19-20. PP2E\Integrate\Extend\Swig\Shadow\number.cxx ...