ruby/doc/syntax/modules_and_classes.rdoc

= Modules

Modules serve two purposes in Ruby, namespacing and mix-in functionality.

A namespace can be used to organize code by package or functionality that
separates common names from interference by other packages.  For example, the
Curses namespace provides functionality for curses that prevents a collision
for the common name "Window".

Mix-in functionality allows sharing common methods across multiple classes or
modules.  Ruby comes with the Enumerable mix-in module which provides many
enumeration methods based on the +each+ method and Comparable allows comparison
of objects based on the <code><=></code> comparison method.

Note that there are many similarities between modules and classes.  Besides the
ability to mix-in a module, the description of modules below also applies to
classes.

== Module Definition

A module is created using the +module+ keyword:

  module MyModule
    # ...
  end

A module may be reopened any number of times to add, change or remove
functionality:

  module MyModule
    def my_method
    end
  end

  module MyModule
    alias my_alias my_method
  end

  module MyModule
    remove_method :my_method
  end

Reopening classes is a very powerful feature of Ruby, but it is best to only
reopen classes you own.  Reopening classes you do not own may lead to naming
conflicts or difficult to diagnose bugs.

== Nesting

Modules may be nested:

  module Outer
    module Inner
    end
  end

Many packages create a single outermost module (or class) to provide a
namespace for their functionality.

You may also define inner modules using <code>::</code> provided the outer
modules (or classes) are already defined:

  module Outer::Inner::GrandChild
  end

Note that this will raise a +NameError+ if +Outer+ and
<code>Outer::Inner</code> are not already defined.

This style has the benefit of allowing the author to reduce the amount
of indentation.  Instead of 3 levels of indentation only one is necessary.
However, the scope of constant lookup is different for creating a namespace
using this syntax instead of the more verbose syntax.

== Scope

=== +self+

+self+ refers to the object that defines the current scope.  +self+ will change
when entering a different method or when defining a new module.

=== Constants

Accessible constants are different depending on the module nesting (which
syntax was used to define the module).  In the following example
the constant <code>A::Z</code> is accessible from B as A is part of the
nesting:

  module A
    Z = 1

    module B
      p Module.nesting #=> [A::B, A]
      p Z #=> 1
    end
  end

However, if you use <code>::</code> to define <code>A::B</code> without
nesting it inside +A+ a NameError exception will be raised because the nesting
does not include +A+:

  module A
    Z = 1
  end

  module A::B
    p Module.nesting #=> [A::B]
    p Z #=> raises NameError
  end

If a constant is defined at the top-level you may preceded it with
<code>::</code> to reference it:

  Z = 0

  module A
    Z = 1

    module B
      p ::Z #=> 0
    end
  end

=== Methods

For method definition documentation see the {syntax documentation for
methods}[rdoc-ref:syntax/methods.rdoc].

Class methods may be called directly.  (This is slightly confusing, but a
method on a module is often called a "class method" instead of a "module
method".  See also Module#module_function which can convert an instance method
into a class method.)

When a class method references a constant it uses the same rules as referencing
it outside the method as the scope is the same.

Instance methods defined in a module are only callable when included.  These
methods have access to the constants defined when they were included through
the ancestors list:

  module A
    Z = 1

    def z
      Z
    end
  end

  include A

  p self.class.ancestors #=> [Object, A, Kernel, BasicObject]
  p z #=> 1

=== Visibility

Ruby has three types of visibility.  The default is +public+.  A public method
may be called from any other object.

The second visibility is +protected+.  When calling a protected method the
sender must be a subclass of the receiver or the receiver must be a subclass of
the sender.  Otherwise a NoMethodError will be raised.

Protected visibility is most frequently used to define <code>==</code> and
other comparison methods where the author does not wish to expose an object's
state to any caller and would like to restrict it only to inherited classes.

Here is an example:

  class A
    def n(other)
      other.m
    end
  end

  class B < A
    def m
      1
    end

    protected :m

  end

  class C < B
  end

  a = A.new
  b = B.new
  c = C.new

  c.n b #=> 1 -- C is a subclass of B
  b.n b #=> 1 -- m called on defining class
  a.n b # raises NoMethodError A is not a subclass of B

The third visibility is +private+.  A private method may not be called with a
receiver, not even +self+.  If a private method is called with a receiver a
NoMethodError will be raised.

=== +alias+ and +undef+

You may also alias or undefine methods, but these operations are not
restricted to modules or classes.  See the {miscellaneous syntax
section}[rdoc-ref:syntax/miscellaneous.rdoc] for documentation.

= Classes

Every class is also a module, but unlike modules a class may not be mixed-in to
another module (or class).  Like a module, a class can be used as a namespace.
A class also inherits methods and constants from its superclass.

== Defining a class

Use the +class+ keyword to create a class:

  class MyClass
    # ...
  end

If you do not supply a superclass your new class will inherit from Object.  You
may inherit from a different class using <code><</code> followed by a class
name:

  class MySubclass < MyClass
    # ...
  end

There is a special class BasicObject which is designed as a blank class and
includes a minimum of built-in methods.  You can use BasicObject to create an
independent inheritance structure.  See the BasicObject documentation for
further details.

== Inheritance

Any method defined on a class is callable from its subclass:

  class A
    Z = 1

    def z
      Z
    end
  end

  class B < A
  end

  p B.new.z #=> 1

The same is true for constants:

  class A
    Z = 1
  end

  class B < A
    def z
      Z
    end
  end

  p B.new.z #=> 1

You can override the functionality of a superclass method by redefining the
method:

  class A
    def m
      1
    end
  end

  class B < A
    def m
      2
    end
  end

  p B.new.m #=> 2

If you wish to invoke the superclass functionality from a method use +super+:

  class A
    def m
      1
    end
  end

  class B < A
    def m
      2 + super
    end
  end

  p B.new.m #=> 3

When used without any arguments +super+ uses the arguments given to the
subclass method.  To send no arguments to the superclass method use
<code>super()</code>.  To send specific arguments to the superclass method
provide them manually like <code>super(2)</code>.

+super+ may be called as many times as you like in the subclass method.

= Singleton Classes

The singleton class (also known as the metaclass or eigenclass) of an object is
a class that holds methods for only that instance.  You can access the
singleton class of an object using <code>class << object</code> like this:

  class C
  end

  class << C
    # self is the singleton class here
  end

Most frequently you'll see the singleton class accessed like this:

  class C
    class << self
      # ...
    end
  end

This allows definition of methods and attributes on a class (or module) without
needing to write <code>def self.my_method</code>.

Since you can open the singleton class of any object this means that this code
block:

  o = Object.new

  def o.my_method
    1 + 1
  end

is equivalent to this code block:

  o = Object.new

  class << o
    def my_method
      1 + 1
    end
  end

Both objects will have a +my_method+ that returns +2+.