Class

< Module 6 instance methods 1 class methods

Classes in Ruby are first-class objects—each is an instance of class Class.

Typically, you create a new class by using:

Example

class Name
 # some code describing the class behavior
end

When a new class is created, an object of type Class is initialized and assigned to a global constant (Name in this case).

When Name.new is called to create a new object, the new method in Class is run by default. This can be demonstrated by overriding new in Class:

Example

class Class
  alias old_new new
  def new(*args)
    print "Creating a new ", self.name, "\n"
    old_new(*args)
  end
end

class Name
end

n = Name.new

produces:

Example

Creating a new Name

Classes, modules, and objects are interrelated. In the diagram that follows, the vertical arrows represent inheritance, and the parentheses metaclasses. All metaclasses are instances of the class ‘Class’.

                         +---------+             +-...
                         |         |             |
         BasicObject-----|-->(BasicObject)-------|-...
             ^           |         ^             |
             |           |         |             |
          Object---------|----->(Object)---------|-...
             ^           |         ^             |
             |           |         |             |
             +-------+   |         +--------+    |
             |       |   |         |        |    |
             |    Module-|---------|--->(Module)-|-...
             |       ^   |         |        ^    |
             |       |   |         |        |    |
             |     Class-|---------|---->(Class)-|-...
             |       ^   |         |        ^    |
             |       +---+         |        +----+
             |                     |
obj--->OtherClass---------->(OtherClass)-----------...

Class methods 1

.new

Instance methods 6

#allocate #attached_object #inherited #new #subclasses #superclass

Ruby version changes 2

3.2

Class#attached_object, which returns the object for which the receiver is the singleton class. Raises TypeError if the receiver is not a singleton class. [Feature #12084]

Example

class Foo; end

Foo.singleton_class.attached_object        #=> Foo
Foo.new.singleton_class.attached_object    #=> #<Foo:0x000000010491a370>
Foo.attached_object                        #=> TypeError: `Foo' is not a singleton class
nil.singleton_class.attached_object        #=> TypeError: `NilClass' is not a singleton class

3.1

Class#subclasses, which returns an array of classes directly inheriting from the receiver, not including singleton classes. [Feature #18273]

Example

class A; end
class B < A; end
class C < B; end
class D < A; end
A.subclasses    #=> [D, B]
B.subclasses    #=> [C]
C.subclasses    #=> []