Ruby Variables and More

Variable Type	Naming Rule (Start)	Naming Rule (Characters)	Scope
Local Variable	Lowercase letter or _	Alphanumeric or _	Valid only inside methods or blocks
Global Variable	$	Alphanumeric or _	Accessible from anywhere
Class Variable	@@	Alphanumeric or _	Accessible from all instances of the class
Instance Variable	@	Alphanumeric or _	Accessible within the instance
Constant	Uppercase letter	Alphanumeric or _	Accessible in the same way in both instance and class methods

Local Variables

Valid only inside methods or blocks.

You must assign a value to a local variable with = before referencing it.

takada = "Local variable, come out!"
def sengen
  takada
end
p sengen
Traceback (most recent call last):
	1: from exam.rb:5:in `<main>'
exam.rb:3:in `sengen': undefined local variable or method `takada' for main:Object (NameError)
# takada is not declared, so this is an error

def sengen
  takada = "Local variable, come out!"
end
p sengen
"Local variable, come out!"

Global Variables

Accessible from anywhere. Generally, global variables should be avoided.

$takada = "Global variable, come out!"
def sengen
  $takada
end
p sengen
"Global variable, come out!"

Class Variables

Shared in both class methods and instance methods, and also shared between superclasses and subclasses.

class Takada
  @@sakebu = 'Class variable, come out!'

  def self.sakebu
    @@sakebu
  end

  def initialize(sakebu)
    @@sakebu = sakebu
  end

  def sakebu
    @@sakebu
  end
end

class Koriki < Takada
  @@sakebu = 'Not angry!'

 def self.sakebu
   @@sakebu
 end
end

p Takada.sakebu
p Koriki.sakebu

"Not angry!"
"Not angry!"

Takada should shout “Class variable, come out!”, but when the Koriki class is defined, @@sakebu is changed to “Not angry!”.

If you use a class instance variable instead, the result is different.

class Takada
  @sakebu = 'Class variable, come out!'

  def self.sakebu
    @sakebu
  end

  def initialize(sakebu)
    @sakebu = sakebu
  end

  def sakebu
    @sakebu
  end
end

class Koriki < Takada
  @sakebu = 'Not angry!'

 def self.sakebu
   @sakebu
 end
end

p Takada.sakebu
p Koriki.sakebu

"Class variable, come out!"
"Not angry!"

Instance Variables

Accessible within the instance.

Constants

Accessible in the same way in both instance and class methods.

Reference

プロを目指す人のためのRuby入門[改訂2版] 言語仕様からテスト駆動開発・デバッグ技法まで

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