Ruby Variable Scopes

Short tech talk I did for my coworkers on Ruby variable scopes.

Further read:
- http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/
- http://www.tutorialspoint.com/ruby/ruby_variables.htm
- http://stackoverflow.com/questions/11495098/difference-between-various-variables-scopes-in-ruby

Transcript

1. RUBY VARIABLE SCOPES
2. SCOPES • Local Variables • Instance Variables • Class Variables • Class Instance Variables • Global Variables • Constants
3. LOCAL VARIABLES • “normal” • x = 25 • local to a method/block/class declaration • goes away when its scope closes
4. INSTANCE VARIABLES • prefixed with `@` • @name = ‘Miha’ • available inside an instance scope • each instance has a separate value
5. CLASS VARIABLES • prefixed with `@@` • @@gears = 5 • available inside a class and instance scope • same for all instances • same for superclass and subclasses
6. CLASS VARIABLES EXAMPLE class Polygon @@sides = 10 def self.sides @@sides end end puts Polygon.sides # => 10 class Triangle < Polygon @@sides = 3 end puts Triangle.sides # => 3 puts Polygon.sides # => 3
7. CLASS INSTANCE VARIABLES • classes are objects too • prefixed with `@` • @cards = 52 • available anywhere inside a class scope • NOT same for superclass and subclasses
8. CLASS INSTANCE VARIABLES EXAMPLE class Polygon @sides = 8 def self.sides @sides end def sides @sides end end puts Polygon.sides # => 8 puts Polygon.new.sides # => nil class Triangle < Polygon @sides = 3 end puts Triangle.sides # => 3 puts Polygon.sides # => 8
9. GLOBAL VARIABLES • prefixed with `$` • $HOME = ‘Ljubljana’ • should be used sparingly • available everywhere • can be traced
10. CONSTANTS • begin with a capital letter • upper snake case by convention • I_AM_A_CONSTANT = 25 • supposed to remain constant
11. CONSTANTS EXAMPLE [1] pry(main)> A_CONST = 10 => 10 [2] pry(main)> A_CONST = 20 (pry):50: warning: already initialized constant A_CONST (pry):49: warning: previous definition of A_CONST was here => 20 [3] pry(main)> A_CONST => 20
12. THANKS