Classes can include—or inherit methods fom—modules.

Ruby’s objects are strictly encapsulated: their state can be access only through the methods they define.

Any Ruby program can add methods to existing classes, and it is even possible to add “singleton methods” to individual objects.


Defining a Simple Class

class Point
  @@n = 0        # How many points have created
  @@totalX = 0   # The sum of all X coordinates
  @@totlalY = 0  # The sum of all Y coordinates

  def intialize(x,y)
    @x, @y = x, y

    @@n += 1
    @@totalX += x
    @@totalY += y

  ORIGIN =, 0)
  UNIT_X =, 0)
  UNIT_Y =, 1)

  def x

  def x=(value)
    @x = value

  def y

  def y=(value)
    @y = value

  def +(other)
    raise TypeError, "Point argument expected" unless other.is_a? Point + other.x, @y + other.y)

  def -(other)
    raise TypeError, "Point argument expected" unless other.is_a? Point - other.x, @y - other.y)

  def *(scalar)*scalar, @y*scalar)

  def [](index)
    case index
    when 0, -2: @x
    when 1, -1: @y
    when :x, "x": @x
    when :y, "y": @y
    else nil

  def each
    yield @x
    yield @y

  def ==(o)
    @x == o.x && @y == o.y

  # alias eql? == # Often, we want eql? to work just like == operator.

  def eql?(o)
    if o.instance_of? Point
      @x.eql?(o.x) && @y.eql?(o.y)

  def hash
    code = 17
    code = 37*code + @x.hash
    code = 37*code + @y.hash

  def <=>(other)
    return nil unless other.instance_of? Point
    @x**2 + @y**2 <=> other.x**2 + other.y**2

  def self.sum(*points) # def Point.sum(*points)
    x = y = 0
    points.each { |p| x += p.x; y += p.y }, y)

  def to_s
  • Accessors and Attributes
class Point
  def intialize(x,y)
    @x, @y = x, y

  def x   # The accessor (or getter) method for @x

  def y   # The accessor (or getter) method for @y

class MutablePoint
  def initialize(x,y); @x, @y = x, y; end

  def x; @x; end # The getter method for @x
  def y; @y; end # The getter method for @y

  def x=(value)  # The setter method for @x
    @x = value
  def y=(value)  # The setter method for @y
    @y = value

p =,1)
p.x = 0
p.y = 0
  • metaprogramming

    • attr, attr_reader, and attr_accessor

      The attr_reader and attr_accessor methods are defined by Module class. Both methods take any number of symbols naming attributes.

      attr_reader creates trivial getter methods for the instance variables with the same name.

      attr_accessor creates getter and setter methods.

        class Point
          attr_accessor :x, :y  # Define accessor methods for our instance variables
        class Point
          attr_reader :x, :y    # Define reader methods for our instance variable
          # attr_reader "x", "y" Equivalently.

      attr is a similar method with a shorted name as a synonym for attr_reader.

        attr :x       # Define a trivial getter method x for @x
        attr :y, true # Define getter and setter methods for @y

      The attr, attr_reader, and attr_accessor methods create instance method for us (metaprogramming).

Method Visibility: Public, Protected, Private

Methods are normally public unless they are explicitly declared to be private or protected.

The initialize method is always implicitly private.

Global funcitons are defined as private instance methods of Object.

A private method is internal to the implementation of a class, and it can only be called by other instance method of the class.

Private methods are implicityly invoked on self, and may not be explicitly invoked on an object.

A protected method is like a private method in that it can only be invoked from within the implementation of a calss or its subclasses.

Protected methods may be explicitly invoked on any instance of the class, and it is not restricted to implicit invocation on self.

Protected methods are the least commonly defined and also the most difficult to understand.

Method visibility is declared with three methods named public, private, and protected. These are instance method of the Module class.

All classes are modules, and inside a class definition (both outside method definitions), self refers to class being defined.

class Point
  # public methods go here

  # The following methods are protected

  # protected methods go here

  # The following methods are private

  # private methods go here

class Widget
  def x                   # Accessor method for @x
  protected :x            # Make it protected

  def utility_method      # Define a method
  private :utility_method # And make it private

Instance and class variables are encapsulated and effectively private, and constants are effectively public.

There is no way to make an instance variable accessible from outside a class. And there is no way to define a constant that is inaccessible to outside use.

  • private_class_method and public_class_method

    You can make a private class method private with private_class_method.

      private_class_method :new

    You can make a private class method public again with public_class_method.

Subclassing and inheritance

In Ruby 1.9, Object is no longer the root of the class hierarchy. A new class named BasicObject serves that purpose, and Object is a subclass of BasicObject. BasicObject is a very simple class, with almost no methods of its own, and is useful as the superclass of delegating wrapper classes.

When you create a class in Ruby 1.9, you still extend Object unless you explicity specify the superclass, and most programmer will need to use or extend BasicObject.

The syntax for extending a class is to add < character and the name of the superclass to your class statement.

class Point3D < Point   # Define class Point3D as a subclass of Point

Subclassing a Struct

class Point3D <"Point3D", :x, :y, :z)
    # Superclass struct gives us acess method, ==, to_s, etc
    # Add point-specific methods here
  • Overriding Methods

      # Greet the World
      class WorldGreeter
          def greet           # Display a greeting
              puts "#{greeting} #{who}"
          def greeting        # What greeting to use
          def who             # Who to greet
      # Greet the world in Spanish
      class SpanishWorldGreeter < WorldGreeter
          def greeting        # Override the greeting
      # We call a mehtod defined in WorldGreeter, which calls the overriden
      # version of greeting in SpanishWorldGreeter, and prints "Hola World"
    • abstract vs concrete

        # This class is abstract; it doesn't define greeting or who
        # No special syntax is required: any class that invokes methods that are
        # intended for a subclass to implement is abstract.
        class AbstractGreeter
          def greet
            puts "#{greeting} #{who}"
        # A concrete subclass
        class WorldGreeter < AbstractGreeter
          def greeting; "Hello"; end
          def who; "World"; end
              # Displays "Hello World"
    • Overriding private methods

      Privates methods are inherited by subclass.

      Subclasses can invoke and override private methods.

      You should only subclass when you are familiar with the implementation of the superclass.

      Compositition > Inheritance

    • super

      super works like a special method invocation: it invokes a method with the same name as the current one, in the superclass of the current class.

      If you use super as a bare keyword—with no arguments and no parentheses—then all of the arguments that were passed to the current method are passed to the superclass method.

        class Point3D < Point
          def initialize(x,y,z)
            # Pass our first two arguments along to the superclass initialize method
            super(x , y)
            # And deal with the third argument ourself
            @z = z;
  • Inheritance of Class Methods

    Class methods may be inherited and overriden just as instance methods can be.

    As a stylistic matter, it is preferable to invoke class methods through the class object on which they are defined.

    Class methods can use super just as instance methods can to invoke the same-named method in the superclass.

      class Point
        def self.sum(*args)
          s = 0
          args.each { |x| s += x }
      class Point3D < Point
        def self.sum(*args)
          puts "Point3D"
  • Inheritance and Instance Variables

    Inheritance variables often appear to be inherited in Ruby.

      class Point3D < Point
        def initialize(x,y,z)
          super(x , y)
          @z = z;
        def to_s
          "(#@x, #@y, #@z)" # Variables @x and @y inherited?
  • Inheritance and Class Variables

    Class variables are shared by a class and all of its subclasses.

  • Inheritance of Constants

    Constants are inherited can can be overriden, much like instance methods can.

      class Point
        ORGIN = 0, 0
        def initialize(x, y)
          @x, @y = x, y
      class Point3D
        ORGIN = 0, 0, 0
        def initialize(x, y, z)
          super x, y
          @z = z

Object Creation and Intialization

  • new, allocate, and intialize

    Every class inerits the class method new.

    new method has two jobs: it must allocate a new object—actually bring the object into existence—and it must intialize the object.

    The new metho would look something like this:

      def new(*args)
        o = self.allocate     # Create a new object of this class
        o.intialize(*args)    # Call the object's intialize method with our args
        o                     # Return new object; ignore return value of intialize
    • allocate

      allocate is an instance method of Class, and it is inherited by all class objects.

      Its purpose is to create a new instance of the class.

      You can call it yourself to create uninitialized instances of a class.

      But don’t try to override it; Ruby always inovkes this mehtod directly, ignoring any overriding versions you may have defined.

    • initalize

      initialze is an instance method and usually to create instance variables for the obect and set them to their initial values.

      Ruby implicitly makes the initialize method private.

      • Class::new and Class#new

        The class method Class::new is the Class class’ own version of the method, and it can be used to create new classes.

  • Factory Methods

      class Point
        # Define an initialize method as usual...
        def initialize(x,y) # Expects Cartesian coordinates
          @x,@y = x,y
        # But make the factory method new private
        private_class_method :new
        def Point.cartesian(x,y) # Factory method for Cartesian coordinates
          # We can still call new from other class methods
        def Point.polar(r, theta) # Factory method for polar coordinates
          new(r*Math.cos(theta), r*Math.sin(theta))
  • dup,clone, and intialize_copy

  • marshal_dump and marshal_load

  • The Singleton Pattern

      require 'singleton'         # Singleton module is not built-in
      class PointSats             # Define a class
        include Singleton         # Make it as singleton
        def intialize             # A normal initialization method
          @n, @totalX, @totalY = 0, 0.0, 0.0
        def record(point)         # Record a new point
          @n += 1
          @totalX += point.x
          @totalY += point.y
        def report                # Report point statistics
          puts "Number of points created: #@n"
          puts "Average X coordinate: #{@totalX/@n}"
          puts "Average Y coordinate: #{@totalY/@n}"
      class Point
          def initialize(x,y)
              @x, @y = x, y

    The Singleton module automatically creates the instance class method for us.


Like a class, a module is a named group of methods, constants, and class variables.

Modules stand alone; there is no “module hierarchy” of ineritance.

Modules are used as namespaces and as mixins.

Modules as Namespaces

Modules are a good way to group related methods when object-oriented programming is not necessary.

def base64_encode

def base64_decode

To define the two methods within a Base64 module to prevent namespace collisions.

module Base64   # Note the module names must begin with a capital letter.
def self.encode # def Base64.encode

def self.decode # def Base64.decode

Modules may also contain constants.

module Base64
             'abcdefghijklmnopqrstuvwxyz' \
             ' 0123456789+/ '

Outside the Base64 module, this constant can be referred to as Base64::DIGITS.

If the two methods had some need to share nonconstant data, they could use a class variable (with a @@ prefix), just as they could if they were defined in class.

  • Nested namespaces

    Modules, including classes, may be nested.

    This creates nested namespaces but has no other effect: a class or module nested within another has no special access to the class or module it is nested within.

      module Base64
        DIGITS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
        class Encoder
          def encode
        class Decoder
          def decode
        # A utility function for use by both classes
        def Base64.helper

    By structuring our code this way, we’ve defined two new classes, Base64::Encoder and Base64::Decoder.

    Inside the Base64 module, the two classes can refer to each other by their unqualified names, without the Base64 prefix.

    And each of the classes can use the DIGITS constant without a prefix.

  • Modules As Mixins

If a module defines instance methods instead of the class methods, those instance methods can be mixed in to other classes.

Enumerable and Comparable are well-known examples of mixin modules.

Enumerable defines useful iterators that are implemented in terms of an each iterator.

Enumerable doesn’t define the each mehtod itself, but any class that defines it can mix in the Enumerable module to instantly add many useful iterators.

And class that defines <=> can be mixed in Comparable to get <,<=,==,>=, and between? for free.

To mix a module into a class, use include.

class Point
  include Comparable

In fact, it is a private instance method of Module, implicitly invoked on self—the class into which the module is being included.

The inclusion of a module affects the type-checking method is_a? and the switch-equality operator ===.

"text".is_a? Comparable     # => true
Enumerable === "text"       # => true in Ruby 1.8, false in 1.9

Note the instance_of? only checks the class of its receiver, not superclasses or modules, so the following is false.

"text".instance_of? Comparable  # => false

The normal way to mix in a module is with the `Module.include` method. Another way is with `Object.extend`. This method makes the instance methods of the specified module or modules into singleton methods of the receiver object. (And if the receiver object is a Class instance, then the methods of the receiver become class methods of that class.)

    countdown =          # A plain old object
    def countdown.each              # The each iterator as a singleton mehtod
      yield 3
      yield 2
      yield 1
    countdown.extend(Enumerable)    # Now the object has all Enumerable methods
    print countdown.sort            # Prints "[1, 2, 3]"
  • Includable Namespace Modules

    It is possible to define modules that define a namespace but still allow their methods to be mixed in.

      Math.sin(0)     # => 0.0: Math is a namespace
      include 'Math'  # The Math namespace can be included
      sin(0)          # => 0.0: Now we have easy access to the functions

    The Kernel module also works like this: we can invoke its methods through the Kernel namespace, or as private method of Object, into which it is included.

    If you want to create a module like Math or Kernel, define your methods as instance methods of the module. Then use module_function to convert those methods to “module functions”.

    module_funciton is a private instance method of Module, much like the public, protected, and private methods. It accepts any number of method names (as symbols or strings) as arguments.

    The primary effect of calling module_function is that it makes class method copies of the specified methods.

    A secondary effect is that it makes the instance methods private.

    Like the public, protected, and private methods, the module_funciton mehtod can also be invoked with no arguments. When invoked in this way, any instance methods subsequently defined in the module will be modue funciton: they will become public class methods and private instance methods.

Loading and Requiring Modules

Ruby programs may be broken up into multiple files, and the most natural way to partition a program is to place each nontrivial class or module into a separate file.

These separate files can then be reassembled into a single program (and, if well-designed, can be reused by other programs) using require or load which are global functions defined in Kernel, but are used like language keywords.

If the file to load is specified with an absolute path, or is relative to ~ (the user’s home directory), then that specific file is loaded.

Usually, however, the file is specified as a relative path, and load and require search for it relative to the directories of Ruby’s load path.

  • load, require andrequire_relative

    • In addition to loading source code, require can also load binary extensions to Ruby.

      Binary extensions are, of course, implementation-dependent, but in C-based implementations, they typically take the form of shared library files with extension like .so or .dll.

    • load expects a complete filename including an extension.

      require is usually passed a library name, with no extension, rather than a filename. In that case, it searchs for a file that has the library name as its base name and an approriate source or native library extension. If a direcotry contains both an .rb source file and a binary extension file, require will load the source file instead of the binary file.

    • load can load the same file multiple times.

      require tries to prevent multiple loads of the same file.

      require keeps track of the files that have been loaded by appending them to the global array $" (also known as $LOAD_FEATURES).

    • load loads the specified file at the current $SAFE level.

      require loads the specified library with $SAFE set to 0, even if the code that called require has a higher value for that variable.

    • require_relative, a special version of require, searchs and loads file from the current direcotry.

  • The Load Path

    Ruby’s load path is an array that you can access using either of the global variables $LOAD_PATH or $:.

    Each element of the array is the name of a direcotry that Ruby will search for files to load.

    Direcotories at the start of the array are searched before direcotories at the end of the array.

    The elements of $LOAD_PATH must be strings in Ruby 1.8, but in Ruby 1.9, they may be strings or any object that has a to_path method that returns a string.

  • Executing Loaded Code

    load and require execute the code in the specified file immediately.

    Files loaded with load or require are executed in a new top-level scope that is different from the one in which load or require was invoked.

    The loaded file can see all global variables and constants that have been defined at the time it is loaded, but it does not have access to the local scope from which the load was initiated.

    • The local variables defined in the scope from which load or require is invoked are not visible to the loaded file.

    • Any local variables created by the loaded file are discarded once the load is complete; they are never visible outside the file in which they are defined.

    • At the start of the loaded file, the value of self is always the main object, just as it is when the Ruby interpreter starts running.

    • The current module nesting is ignored within the loaded file. You cannot, for example, open a class and then load a file of method definitions. The file will be processed in a top-level socpe, not inside any class or module.

  • Autoloading Modules

    The autoload methods of Kernel and Module allow lazy loading of files on an as-needed basis. The global autoload function allows you to register the name of an undefined constant (typically a class or module name) and a name of the library that defines it. When that constant is first referenced, the named library is loaded using require. For example:

      # Require 'socket' if and when the TCPSocket is first used
      autoload :TCPSocket, "socket"

    The Module class defines its own version of autoload to work with constants nested within another module.

Singleton Methods and the Eigenclass

To define a singleton method sum on an object Point, we’d write:

def Point.sum
  # Method body goes here

The singleton methods of an object are instance methods of the anonymous eigenchass associated with that object.

“Eigen” is a German word meaning (roughly) “self”, “own”, “particular to,” or “characteristic of.”

The eigenclass is also called the singleton class or the metaclass.

Ruby defines a syntax for opening the eigenclass of an object and adding methods to it thats provides an alternative to defining singleton methods one by one.

To open the eigenclass of the object o, use class « o. For example, we can define class methods of Point like this:

class << Point
  def class_method1     # This is an instance method of the eigenclass.

  def class_method2

Method Lookup

When Ruby evaluates a method invocation expression, it must first figure out which method is to be invoked. The process for doing this is called method lookup or method name resolution.

For the method invocation expression o.m, Ruby performs name resolution with the following steps:

  1. First, it checks the eigenclass of o for singleton methods named m.

  2. If no method m is found in the eigenclass, Ruby searchs the class of o for an instance method named m.

  3. If no method m is found in the class, Ruby searchs the instance methods of any modules included by the class of o. If that class includes more than one module, then the most recently included module is searched first.

  4. If no instance method m is found in the class of o or in its modules, then the search moves up the inheritance hierarchy to the superclass.

  5. If no method named m is found after completing the search, then a method named method_missing is invoked instead. In order to find an appropriate definitions of this method, the name resolution algorith starts over at step 1.

The Kernel module provides a default implementation of method_missing. What it does is raise an exception of NoMethodError.

The name resolution algorithm for class methods is exactly the same as it is for instance method.

Constant Lookup

When a constant is referenced without any qualifying namespace, the Ruby interpreter must find the appropriate definition of the constant.

  • Ruby first attempts to resolve a constant reference in the lexical scope of the reference.

    This means that it first checks the class or module that encloses the constant reference to see if that class or module defines the constant. If not, it checks the next enclosing class or module.

    This continues untils there are no more enclosing classes or modules.

    Note that top-level or “global” constants are not considered part of the lexical scope and are not considered during this part of constant lookup.

    The class method Module.nesting returns the list of clases and modules that are searched in this step, in the order they are searched.

  • If no constant definition is foud in the lexically enclosing scope, Ruby next tries to resolve the constant in the inheritance hierarchy by checking the ancesstors of the class or module that referred to the constant.

    The ancestors method of the containing class or module returns the list of classes and modules searched in this step.

  • If no constant definition is found in the inheritance hierarchy, then top-level constant definitions are checked.

  • If no definition can be found for the desired constant, then the const_missing method—if there is one—of the containing class or module is called and given the opportunity to provide a value for the constant.

A few points about this constant lookup algorithm.

  • Constants defined in enclosing modules are found in preference to constants defined in included modules.

  • The modules included by a class are searched before the superclass of the class.

  • The Object class is part of the inheritance hierarchy of all classes.

  • The Kernel module is an ancestor of Object.

module Kernel
  # Constants defined in Kernel
  A = B = C = D = E = F = "defined in Kernel"

# Top-level or "global" constants defined in Object
A = B = C = D = E = "defined at Top-level"

class Super
  A = B = C = D = "defined in superclass"

module Included
  # Constants defined in an included module
  A = B = C = "defined in included module"

module Enclosing
  # Constants defined in an enclosing module
  A = B = "defined in enclosing module"

  class Local < Super
    include Included

    # Locally defined constant
    A = "defined locally"

    # The list of modules searched, in the order searched
    # [Enclosing::Local, Enclosing, Inclued, Super, Object, Kernel]

    search = (Module.nesting + self.ancestors + Object.ancestors).uniq
    puts A # Prints "defined locally"
    puts B # Prints "defined in enclosing module"
    puts C # Prints "defined in included module"
    puts D # Prints "defined in superclass"
    puts E # Prints "defined at toplevel"
    puts F # Prints "defined in kernel"