SWIG:Examples:ruby:class

SWIG/Examples/ruby/class/

Wrapping a simple C++ class

This example illustrates wrapping a simple C++ class to give a Ruby class.

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++ -ruby example.i

A sample Ruby script

Click here to see a script that calls the C++ functions from Ruby.

Key points

To create a new object, you call a constructor like this:
```
c = Example::Circle.new(10)
```
To access member data, a pair of accessor methods are used. For example:
```
c.x = 15    # Set member data
x = c.x     # Get member data
```
To invoke a member function, you simply do this
```
print "The area is ", c.area, "\n"
```
When a instance of Ruby level wrapper class is garbage collected by the Ruby interpreter, the corresponding C++ destructor is automatically invoked.

Static member variables are wrapped as Ruby class accessor methods. For example:

n = Shape.nshapes     # Get a static data member
Shapes.nshapes = 13   # Set a static data member

General Comments

Ruby module of SWIG differs from other language modules in wrapping C++ interfaces. They provide lower-level interfaces and optional higher-level interfaces know as proxy classes. Ruby module needs no such redundancy due to Ruby's sophisticated extension API.
SWIG does know how to properly perform upcasting of objects in an inheritance hierarchy except for multiple inheritance.
C++ Namespaces - %nspace isn't yet supported for Ruby.