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.