Global and Class Variable Scope

Constants

• start with a capital letter
• might be scoped inside modules e.g. Math::PI
• once set, cannot be changed
• turns out that all classes and modules are actually constants

Global Variables

• start with $
• available everywhere
• Danger!

Built-in Global Variables

• $: load path (also $LOAD_PATH)
• $* command-line args (also ARGV)
• $$ pid
• $! the most recent shell error
• for more, open English.rb in the Ruby source
  • $HOME/.rvm/src/ruby-1.9.2-*/lib/English.rb

Class Variables

• start with @@
• really "class and instance and descendants" variables
• available inside class definitions and instance methods
• used for counters, caches, etc.

Class Variable Problems

• must be initialized before access (unlike instance vars)
• very widely shared, so easily polluted
• can be replaced with class instance variables
  • single-@ vars inside class methods

Class Instance Variables

• start with @
• accessible only inside class scope
  • e.g. inside a class method
• clearer and more consistent semantics than @@ class variables

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