Wrapping a simple C++ class

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

/* 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

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

%{
#include "example.h"
%}

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

% swig -c++ -python example.i

A sample Python script

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.

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