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SWIG/Examples/tcl/class/

Wrapping a simple C++ class

$Header: /cvs/projects/SWIG/Examples/tcl/class/index.html,v 1.1.4.1 2001/08/30 04:18:46 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 {
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++ -tcl example.i

Some sample Tcl scripts

SWIG performs two forms of C++ wrapping-- a low level interface and a high level widget-like interface.
  • Click here to see a script that calls the C++ functions using the low-level interface.
  • Click here to see a the same script written with the high-level interface.

Key points

  • The low-level C++ interface works like this:

    • To create a new object, you call a constructor like this:
      set c [new_Circle 10.0]
      

    • To access member data, a pair of accessor functions are used. For example:
      Shape_x_set $c 15        ;# Set member data
      set x [Shape_x_get $c]   ;# Get member data
      
      Note: when accessing member data, the name of the base class must be used such as Shape_x_get

    • To invoke a member function, you simply do this
      puts "The area is [Shape_area $c]"
      

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

    • To invoke a destructor, simply do this
      delete_Shape $c     # Deletes a shape
      

    • Static member variables are wrapped as C global variables. For example:
      set n $Shape_nshapes    # Get a static data member
      set Shapes_nshapes 13   # Set a static data member
      

  • The high-level interface works like a Tk widget

    • To create a new object, you call a constructor like this:
      Circle c 10      # c becomes a name for the Circle object
      

    • To access member data, use cget and configure methods. For example:
      c configure -x 15        ;# Set member data
      set x [c cget -x]        ;# Get member data
      

    • To invoke a member function, you simply do this
      puts "The area is [c area]"
      

    • To invoke a destructor, simply destroy the object name like this:
      rename c ""         # c goes away
      

    • Static member variables are wrapped as C global variables. For example:
      set n $Shape_nshapes    # Get a static data member
      set Shapes_nshapes 13   # Set a static data member
      

General Comments

  • The low-level function interface is much faster than the high-level interface. In fact, all the higher level interface does is call functions in the low-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).