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 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.
There are three ways to define method’s visibility, the first one being most used:
# 1. Define visibility for several methods at once
private
def private_method1
# ...
end
def private_method2
# ...
end
public
def public_again
# ...
end
# 2. Postfactum definition
def private_method1
# ...
end
def private_method2
# ...
end
private :private_method1, :private_method2
# 3. Inline definition
private def private_method1
# ...
end
private def private_method2
# ...
end
The third one is in fact the same as the second (passing method name to private), just utilizing the fact that def is an expression returning method name.
Note also that public, private and protected are not, in fact, keywords or some special syntax, they are just regular methods of Module class.
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>)
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.