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++ -tcl example.i

Some sample Tcl scripts

• 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).

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