Ruby Variable Types

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variable.md

Variable Types

• Local
• Instance
• Class
• Constant
• Pseudo-global
• Global

Local

• Local variables live within a scope
• Useful for referencing values that are only needed within a scope
• Useful for setting temporary values in methods or other places
• Start with a lowercase letter and no symbols e.g. var

# local variable
x = 1

# even though we're scoped inside of a proc
# we can still access x since it was defined in
# a higher scope
proc { puts x }.() # 1

class Dummy
  def hi
    y = 2
  end

  def there
    puts y
  end
end

d = Dummy.new
d.hi
d.there # NameError

Instance

• Instance variables are tied to the instance scope they are created in
• Everything in Ruby is an object and therefore an instance of something
• Useful for tracking values for the lifetime of an object
• Start with an @ sign e.g. @var / @Var / @VAR

class Business
  attr_reader :name

  def initialize( name )
    @name = name
  end
end

b = Business.new "ACME Inc."
b.name # "ACME Inc."

Class

• Class variables are tied to the class scope they are created in
• Class variables are shared with and inherited by ancestor classes
• Not the reverse though!
• Useful for tracking data at the class level
• Typically not idiomatic ruby to use class vars
• Usually use instance variables at the class level -- no weird inheritance issues
• Start with two @s e.g. @@var / @@Var / @@VAR

class Business
  @@a = "word"

  @c = "word1"

  def self.hi
    puts @@a
  end
  hi # "word"

  def self.test
    puts @@b
  end
  test # NameError
end

class SuperBusiness < Business
  puts @@a # "word"
  @@b = "other"

  puts @c
  @c = :word2

  test # "other"
end

Constant

• Static values tied to the class scope they are created in
• Will receive warnings and errors attempting to change the value of a constant
• Visible in ancestors
• Class and Module names are constants
• Start with a capital letter and have no symbols except underscores

class Business
  ATTRIBUTES = [ :name , :address ]
end

class SuperBusiness < Business
end

Business::ATTRIBUTES
SuperBusiness::ATTRIBUTES

Pseudo-Global

• Result of Regex matches
• Result of forked processes
• Cannot set them on your own!
• Only the runtime sets them
• Look like globals but are thread local(?)
• Start with $ e.g. $1 / $2 / $3 / $@ / $$

"abc123" =~ /(\d+)\z/
puts $1

Global

• Accessible throughout the runtime including other processes and threads
• Not idiomatic to use them!
• Start with $ e.g. $stdout / $var / $Var / $VAR

$var = 1

fork do
  puts $var # 1
end