# 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 IRB namespace provides functionality for irb that prevents a collision for the common name “Context”.

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 `<=>` 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 `::` provided the outer modules (or classes) are already defined:

    module Outer::Inner::GrandChild
    end

Note that this will raise a `NameError` if `Outer` and `Outer::Inner` 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 `A::Z` 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 `::` to define `A::B` 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 `::` 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.

`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 `==` 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 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 `<` 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 `super()`. To send specific arguments to the superclass method provide them manually like `super(2)`.

`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 `class << object` 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 `def self.my_method`.

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`.