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 modules (or classes) is a very powerful feature of Ruby, but it is best to only reopen modules you own. Reopening modules 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 inherit the Class
or Module
which defines the method. 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.
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.
Just like modules, classes can also be reopened. You can omit its superclass when you reopen a class. Specifying a different superclass than the previous definition will raise an error.
class C end class D < C end # OK class D < C end # OK class D end # TypeError: superclass mismatch for class D class D < String end
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
.