`SWIG/Examples/ruby/class/`

## Wrapping a simple C++ class

`/cvsroot/SWIG/Examples/ruby/class/index.html,v 1.3 2002/11/30 22:10:05 beazley Exp`

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 shadow 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.