SWIG:Examples:python:class

SWIG/Examples/python/class/

Wrapping a simple C++ class

This example illustrates wrapping a simple C++ class to give a Python class.

The C++ Code

Suppose you have some C++ classes described by the following (and admittedly lame) header file:

/* File : example.h */

class Shape {
public:
  Shape() {
    nshapes++;
  }
  virtual ~Shape() {
    nshapes--;
  }
  double  x, y;
  void    move(double dx, double dy);
  virtual double area() = 0;
  virtual double perimeter() = 0;
  static  int nshapes;
};

class Circle : public Shape {
private:
  double radius;
public:
  Circle(double r) : radius(r) { }
  virtual double area();
  virtual double perimeter();
};

class Square : public Shape {
private:
  double width;
public:
  Square(double w) : width(w) { }
  virtual double area();
  virtual double perimeter();
};

The SWIG interface

A simple SWIG interface for this can be built by simply grabbing the header file like this:

/* File : example.i */
%module example

%{
#include "example.h"
%}

/* Let's just grab the original header file here */
%include "example.h"

Note: when creating a C++ extension, you must run SWIG with the -c++ option like this:

% swig -c++ -python example.i

A sample Python script

Click here to see a script that calls the C++ functions from Python.

Key points

To create a new object, you call a constructor like this:
```
c = example.new_Circle(10.0)
```
Member variables of the C++ class are wrapped as attributes of the Python class. For example:
```
c.x = 15   # Set member data
x = c.x    # Get member data
```
Member function are invoked as you would expect:
```
print "The area is ", c.area()
```
To invoke a destructor, simply call del on the object:
```
del c    # Deletes a shape
```

Static member variables are wrapped as C global variables. For example:

n = example.cvar.Shape_nshapes     # Get a static data member
example.cvar.Shapes_nshapes = 13   # Set a static data member

General Comments

SWIG does know how to properly perform upcasting of objects in an inheritance hierarchy (including multiple inheritance). Therefore it is perfectly safe to pass an object of a derived class to any function involving a base class.
C++ Namespaces - %nspace isn't yet supported for Python.