SWIG:Examples:ruby:class

SWIG/Examples/ruby/class/

Wrapping a simple C++ class

This example illustrates C++ class wrapping performed by SWIG. C++ classes are simply transformed into Ruby classes that provide methods to access 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++ -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 Ruby interpreter, the corresponding C++ destructor is automatically invoked. (Note: destructors are currently not inherited. This might change later. Until then, use -make_default).

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 provides 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.
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.