Local, Global, Instance, and Class Variables
Ruby has different kinds of variables that can be seen in different places: some just in one spot, some inside one object, some in the whole class, and some everywhere. It is like having a desk drawer, a bedroom shelf, a family cupboard, and a town notice board.
In this page:
Syntax
local_var = value
$global_var = value
@instance_var = value
@@class_var = valueLocal, Global, Instance, and Class Variables
Ruby distinguishes variable scope by naming convention: local_var is local to its block/method, $global_var is accessible everywhere, @instance_var belongs to a specific object, and @@class_var is shared across all instances of a class.
This prefix-based naming is unique to Ruby compared to most other languages, which use separate declaration keywords instead. Global variables are rarely used in practice since they make code harder to reason about.
Warning:
Global variables ($var) are almost always considered bad practice in Ruby -- prefer instance or class variables scoped to an object or class.
Example: Local, Global, Instance, and Class Variables
$global_count = 0
class Counter
@@total_counters = 0
def initialize
@count = 0
@@total_counters += 1
$global_count += 1
end
end
Counter.new
Counter.new
puts "Total counters created: #{Counter.class_variable_get(:@@total_counters)}"
puts "Global count: #{$global_count}"Login to try C/C++/Java/PHP code in the editor
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Using a local variable inside a method that was defined outside it, which raises
NameErrorbecause methods do not see outer locals. - Using
@@countclass variables and being surprised that subclasses share them. - Reading an uninitialised
@countin an instance, which givesnilinstead of an error.
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