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Variable Scope

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Slides

Ruby Scope

This section covers globals, constants, and class variables.

Ref.: WGR Ch. 5, The default object (self), scope, and visibility

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

Previous Lesson

Variable Scope