SWIG:Examples:tcl:class

SWIG/Examples/tcl/class/

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

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 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.
C++ Namespaces - %nspace isn't yet supported for Tcl.