Classes

Rice provides two ways to define Ruby classes: creating new Ruby-only classes and wrapping existing C++ classes.

Ruby-Only Classes

To create a Ruby class without wrapping a C++ type, use the non-template version of define_class:

#include <rice/rice.hpp>

using namespace Rice;

extern "C"
void Init_test()
{
  Class rb_cTest = define_class("Test");
}

This creates a Ruby class called Test that inherits from Object. You can then add methods using define_method with function pointers or lambdas.

Wrapping C++ Classes

To wrap an existing C++ class, use the template version of define_class<T>():

class MyClass
{
public:
  MyClass();
  void do_something();
};

extern "C"
void Init_test()
{
  Data_Type<MyClass> rb_cMyClass =
    define_class<MyClass>("MyClass")
    .define_constructor(Constructor<MyClass>())
    .define_method("do_something", &MyClass::do_something);
}

The template parameter specifies the C++ type being wrapped. This creates a binding between the Ruby class and the C++ class, allowing Rice to automatically manage object lifetimes and method dispatch.

Data_Type vs Class

When wrapping C++ types, define_class<T>() returns a Data_Type<T> instead of a Class. The Data_Type<T> class provides the same interface as Class but includes additional type information that Rice uses for type conversions and safety checks.

Defining Classes Under Modules

To define a class within a module namespace, use define_class_under:

Module rb_mMyModule = define_module("MyModule");

Data_Type<MyClass> rb_cMyClass =
  define_class_under<MyClass>(rb_mMyModule, "MyClass");

This creates MyModule::MyClass in Ruby. You can nest classes arbitrarily deep:

Module rb_mOuter = define_module("Outer");
Module rb_mInner = define_module_under(rb_mOuter, "Inner");

Data_Type<MyClass> rb_cMyClass =
  define_class_under<MyClass>(rb_mInner, "MyClass");

This creates Outer::Inner::MyClass.

Inheritance

Inheriting from Ruby Classes

For Ruby-only classes, specify a parent class as the second parameter:

Class rb_cMySocket = define_class("MySocket", rb_cIO);

Inheriting from C++ Classes

When wrapping C++ class hierarchies, specify the parent C++ type as a second template parameter:

class Base
{
public:
  virtual void base_method();
};

class Derived : public Base
{
public:
  void derived_method();
};

extern "C"
void Init_test()
{
  Data_Type<Base> rb_cBase =
    define_class<Base>("Base")
    .define_constructor(Constructor<Base>())
    .define_method("base_method", &Base::base_method);

  Data_Type<Derived> rb_cDerived =
    define_class<Derived, Base>("Derived")
    .define_constructor(Constructor<Derived>())
    .define_method("derived_method", &Derived::derived_method);
}

The second template parameter tells Rice about the inheritance relationship, enabling proper type conversions and polymorphic behavior.

If you want to create Ruby classes that inherit from wrapped C++ classes and override virtual methods, see the Directors section.

Method Chaining

Most methods on Class and Data_Type return a reference to self, allowing you to chain method calls:

Data_Type<MyClass> rb_cMyClass =
  define_class<MyClass>("MyClass")
  .define_constructor(Constructor<MyClass>())
  .define_method("method1", &MyClass::method1)
  .define_method("method2", &MyClass::method2)
  .define_attr("value", &MyClass::value)
  .define_singleton_function("create", &MyClass::create);

Naming Conventions

When naming your C++ variables that hold Ruby classes, follow Ruby's conventions:

Prefix	Type	Example
rb_c	Class	rb_cMyClass
rb_m	Module	rb_mMyModule
rb_e	Exception	rb_eMyError

Complete Example

Here's a complete example showing various class definition features:

#include <rice/rice.hpp>
#include <rice/stl.hpp>

using namespace Rice;

class Shape
{
public:
  virtual ~Shape() = default;
  virtual double area() const = 0;
};

class Rectangle : public Shape
{
public:
  Rectangle(double width, double height)
    : width_(width), height_(height) {}

  double area() const override { return width_ * height_; }

  double width() const { return width_; }
  double height() const { return height_; }

private:
  double width_;
  double height_;
};

extern "C"
void Init_shapes()
{
  Module rb_mGeometry = define_module("Geometry");

  Data_Type<Shape> rb_cShape =
    define_class_under<Shape>(rb_mGeometry, "Shape")
    .define_method("area", &Shape::area);

  Data_Type<Rectangle> rb_cRectangle =
    define_class_under<Rectangle, Shape>(rb_mGeometry, "Rectangle")
    .define_constructor(Constructor<Rectangle, double, double>())
    .define_method("width", &Rectangle::width)
    .define_method("height", &Rectangle::height);
}

Usage in Ruby:

rect = Geometry::Rectangle.new(3.0, 4.0)
puts rect.area    # => 12.0
puts rect.width   # => 3.0
puts rect.height  # => 4.0