Wrapping a simple C++ class

$Header: /cvs/projects/SWIG/Examples/python/class/index.html,v 2001/08/30 04:12:40 beazley Exp $

This example illustrates the most primitive form of C++ class wrapping performed by SWIG. In this case, C++ classes are simply transformed into a collection of C-style functions that provide access to class members.

The C++ Code

Suppose you have some C++ classes described by the following (and admittedly lame) header file:
/* File : example.h */

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

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

class Square : public Shape {
  double width;
  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)

  • To access member data, a pair of accessor functions are used. For example:
    example.Shape_x_set(c,15)    # Set member data
    x = example.Shape_x_get(c)    # Get member data
    Note: when accessing member data, the name of the class in which the member data was must be used. In this case, Shape_x_get() and Shape_x_set() are used since 'x' was defined in Shape.

  • To invoke a member function, you simply do this
    print "The area is ", example.Shape_area(c)

  • Type checking knows about the inheritance structure of C++. For example:
    example.Shape_area(c)       # Works (c is a Shape)
    example.Circle_area(c)      # Works (c is a Circle)
    example.Square_area(c)      # Fails (c is definitely not a Square)

  • To invoke a destructor, simply do this
    example.delete_Shape(c)     # Deletes a shape
    (Note: destructors are currently not inherited. This might change later).

  • 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

  • This low-level interface is not the only way to handle C++ code. Shadow classes provide a much higher-level interface.

  • 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.

  • A wide variety of C++ features are not currently supported by SWIG. Here is the short and incomplete list:

    • Overloaded methods and functions. SWIG wrappers don't know how to resolve name conflicts so you must give an alternative name to any overloaded method name using the %name directive like this:
      void foo(int a);  
      %name(foo2) void foo(double a, double b);

    • Overloaded operators. Not supported at all. The only workaround for this is to write a helper function. For example:
      %inline %{
          Vector *vector_add(Vector *a, Vector *b) {
                ... whatever ...

    • Namespaces. Not supported at all. Won't be supported until SWIG2.0 (if at all).

    • Dave's snide remark: Like a large bottle of strong Tequilla, it's better to use C++ in moderation.