classes.md

Classes

Class definition

Example

class A
  # A class method
  def self.foo -> Int
    1
  end

  # An instance method
  def bar -> Int
    2
  end
end

p A.foo #=> 1
p A.new.bar #=> 2

Instance variables

Name of an instance variable starts with @. All instance variables of a class must be initialized in the method initialize.

Example

class Book
  def initialize(title: String, price: Int)
    @title = title
    @price = price
  end
end

Syntax sugar:

class Book
  def initialize(@title: String, @price: Int); end
end

Instance variables are readonly by default. To make it reassignable, declare it with var.

Accessors

For each instance variable, accessor methods are automatically defined unless they are defined explicitly.

Example

class Person
  def initialize(name: String, age: Int)
    let @name = name
    var @age = age
  end
end

let taro = Person.new("Taro", 20)
p taro.name #=> "Taro"
p taro.age  #=> 20
taro.age += 1

Class hierarchy

^ ... superclass-subclass relationship
~ ... class-instance relationship

               Object       Object       Object
                  ^            ^            ^
                Class     ~ MetaClass  ~ MetaClass
                  ^
     Object ~ Meta:Object ~ MetaClass
        ^         ^ 
        |         |       
        |         |        
123 ~  Int ~   Meta:Int   ~ MetaClass

Example:

p 123                   #=> 123
p 123.class             #=> #<class Int>
p Int                   #=> #<class Int>
p 123.class == Int      #=> true

p Int.class             #=> #<class Meta:Int>
p Int.class.class       #=> #<class Metaclass>
p Int.class.class.class #=> #<class Metaclass>