Code generator for binding C++ APIs to Ruby with no sweat.
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Code generator for binding C++ APIs to Ruby with no sweat. Rarity consists of a few header files you will need to include in your project and a script that will generate your bindings by checking out a YAML file containing your bindings description.

Warning: this software hasn't been nearly tested enough to be declared stable. It is not recommended for production use. You are very much encouraged to participate the testing effort by sending feedback and bug reports.


By default, you need to go into your project's directory and execute the following (replacing $RARITY_PATH with the path to the rarity.rb script)

  ruby $RARITY_PATH/rarity.rb

This will create a rarity-bindings.cpp file in the current directory that you will need to link to your project. You will also have to add the include directory $RARITY_PATH/include and link with a Ruby library.

You can also specify your own input directory and output file like this:

  ruby $RARITY_PATH/rarity.rb --input directory --output file.cpp

There's also a third option that allows you to create a Ruby module that will wrap all the bindings:

  ruby $RARITY_PATH/rarity.rb --input directory --output file.cpp --module MyRubyBindings

How to generate bindings for a class ?

The first thing to do is to add a component called 'RarityClass' to the class using inheritence. Any class that needs to be used from both C++ and Ruby must include this component. After including the component, our code will look like this:


class MyClass : public RarityClass
  MyClass(const std::string& name) : RarityClass("MyClass"), name(name)

  const std::string& GetName() const
    return (name);
 void SetName(const std::string& str)
   name = str;
 static void ClassMethod(void)
   std::cout << "Executing Class Method" << std::endl;

  std::string name;

Notice that your constructor needs to call RarityClass's constructor, which takes as parameter the name of the Ruby class that will be bound with this class.

Now to the actual binding part. For Rarity to generate the bindings, you will need to create a binding YAML file that will look like this:


  include: 'myclass.hpp' # The path to the file including your class
    initialize: # The 'initialize' method will call the C++ constructor.
        - std::string
      return: void
      return: std::string
        - std::string
      return: void
      static: true # If the method uses the static qualifier, you must set this flag to true.
      return: void

The Rarity script will recursively look for YAML files whose names start with 'bindings-', so your YAML file must begin with those characters (e.g.: bindings-myclass.yml).

How to use the bindings from a C++ application?

Rarity also comes with a set of tools for easily using the bindings. A few objects allow you to use Ruby objects from C++ and handle exceptions properly. Let's write a Ruby script and C++ main function using our previous bindings:


class MyRubyClass
  def initialize
    puts "Initializing Ruby class"
    @my_class = "Name set from Ruby"
  def run my_class = nil
    my_class ||= @my_class
    puts "[Ruby] #{my_class.get_name}"
  def run_class_member


#include "rarity.hpp"
#include "myclass.hpp"

int main(void)
  RarityInitialize(); // Must be called before any construction of RarityClass instances
    MyClass my_class("C++-created MyClass");
    Ruby::Constant my_ruby_class("MyRubyClass");
    Ruby::Object   my_ruby_instance = my_ruby_class.Apply("new");

    my_ruby_instance.Apply("run", 1, &my_class); // Method name, argument count, argument list of pointers to Rarity objects
    my_ruby_instance.Apply("run"); // Call the same script method using the default parameter value

  catch (const std::exception* e) // Ruby exceptions are converted to std::exception-compatible objects
    std::cerr << "Caught exception " << e->what() << std::endl;
  Ruby::Constant("GC").Apply("start"); // Forces Ruby's garbage collector to start
  return (0);

And that's it. We've seen pretty much everything Rarity offers.

Notes on API generation

Naming convention

In Ruby, method names are supposed to be written in snake case. Note that regardless of your naming convention in C++, the Ruby bindings will use snake case (this means that in our previous example, the API generated for MyClass is actually:


It is possible to override this behavior by setting an attribute 'alias' in the method's description.

Natively supported types

The generated bindings will support the type you described (at least as pointers). Moreover, the native following types can be converted from one side to the other (or both).

<-  = Ruby to C++ conversion supported
->  = C++ to Ruby conversion supported
<-> = Ruby to C++ and C++ to Ruby conversions supported

std::string   <-> String
unsigned int  <-> Fixnum
int           <-> Fixnum
float         <-> Float
std::function <-  Proc
std::vector   <-> Array
Ruby::Object  <-> Any ruby object


It is possible not to expose any constructor to Ruby, in which case it will be impossible from Ruby scripts to instantiate the class.

Memory Management

Depending on the context in which objects are created, they might not answer to the same garbage collecting rules.

If you create an object from Ruby using the "new" method, your object will answer to Ruby's garbage collector. The linked C++ object will be destroyed when Ruby's garbage collector collects its Ruby counterpart. However, if your object is instantiated from C++, you will have to delete it yourself.

Consequently, it is possible for a Ruby object to outlive its C++ counterpart. In that case, the Ruby object will not crash the application; however, it will throw an exception.