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.
A module is created using the
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.
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
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.
self refers to the object that defines the current scope.
self will change when entering a different method or when
defining a new module.
Accessible constants are different depending on the module nesting (which
syntax was used to define the module). In the following example the
A::Z is accessible from B as A is part of the
module A Z = 1 module B p Module.nesting #=> [A::B, A] p Z #=> 1 end end
However, if you use
:: to define
nesting it inside
A, a NameError exception will be raised because
the nesting does not include
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
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
Ruby has three types of visibility. The default is
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
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
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 only be
called from inside the owner class without a receiver, or with a literal
self as a receiver. If a private method is called with a
receiver other than a literal
self, a NoMethodError will be raised.
class A def without m end def with_self self.m end def with_other A.new.m end def with_renamed copy = self copy.m end def m 1 end private :m end a = A.new a.without #=> 1 a.with_self #=> 1 a.with_other # NoMethodError (private method `m' called for #<A:0x0000559c287f27d0>) a.with_renamed # NoMethodError (private method `m' called for #<A:0x0000559c285f8330>)
You may also alias or undefine methods, but these operations are not restricted to modules or classes. See the miscellaneous syntax section for documentation.
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.
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
< 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.
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
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 may be called as many times as you like in the subclass
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
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
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