How to reference global variables and class variables?

ruby

I'm new to programming. Right now I'm studying Ruby. To my understanding, global variables are defined in the global namespace (so outside of any classes or functions). I'm reading something and it says global variables have a $ sign before them. What does that mean? Does it mean when I define a function or class and want to reference my global variable (let's say it is edmund = 123) I would have to reference it like this: $edmund?

so:

edmund = 123
def my_function()
  456 + $edmund
end

Also are class variables (the ones that begin with @@) like instance variables (@) where you can access them by calling them through Class.classvariable? What is their purpose?

Global scope is scope that covers the entire program. Global scope is enjoyed by global variables, which are recognizable by their initial dollar-sign ($) character. They’re available everywhere and creating your own global variables can be tempting, especially for beginning programmers. But they’re not always a good idea.

$gvar = "I'm a global!"
class C
    def examine_global
        puts $gvar
    end
end

c = C.new
c.examine_global # I'm a global!

Class variables begin with two at signs: @@var, for example. Despite their name, class variables aren’t class scoped. Rather, they’re class-hierarchy scoped. At its simplest, the idea behind a class variable is that it provides a storage mechanism that’s shared between a class and instances of that class, and that’s not visible to any other objects.

class Parent
    @@value = 100
end

class Child < Parent
    @@value = 200
end

class Parent
    puts @@value
end

What gets printed is 200. The Child class is a subclass of Parent, and that means Parent and Child share the same class variables—not different class variables with the same names, but the same actual variables. When you assign to @@value in Child, you’re setting the one and only @@value variable that’s shared throughout the hierarchy— that is, by Parent and Child and any other descendant classes of either of them.

And to give credit where its due - This explanation comes from "The Well Grounded Rubyist" by David A Black, one of the best resources to learn about Ruby.

Excellent question. Unfortunately, you just jumped down a rabbit hole, but it's one that you have to fall through eventually in ruby to start understanding the real intricacies.

For your first question, regarding the $-prefixed global variables. They are truly global:

def mk_foo() $foo ||= "foo"; end

$foo                # => nil
mk_foo              # => "foo"
$foo                # => "foo"
mk_foo.object_id    # => 70299647799620
$foo.object_id      # => 70299647799620

As you can see, when $foo is defined within the mk_foo method, it is defined in the global space, and you can access it anywhere:

class CanSeeFoo
  def see_foo() $foo; end
end
CanSeeFoo.new.can_see_foo
# => "foo"
CanSeeFoo.new.can_see_foo.object_id
# => 70299647799620

As for the class variable question, this is where the rabbit-hole begins. First, you are correct that @@-prefixed variables are referred to as "class variables" and @-prefixed variables are referred to as "instance variables".

Class variables are static across all subclasses (at all sub-levels of the inheritance tree) of the defining class. The implication here is that if any subclass changes the class variable, it will change in all related subclasses and up to the defining class.

class A; end
class B < A; @@foo = "foo";  end
B.class_variable_get(:@@foo)    # => "foo"
A.class_variable_get(:@@foo)
  # => raises NameError "uninitialized class variable @@foo in A"

class C < B; end
C.class_variable_get(:@@foo)    # => "foo"

class D < C
  def self.change_foo(); @@foo = "bar"; end
  def change_foo(); @@foo = "baz"; end
end
D.class_variable_get(:@@foo)    # => "foo"

class E < D; end
E.class_variable_get(:@@foo)    # => "foo"

D.change_foo                    # => "bar"
D.class_variable_get(:@@foo)    # => "bar"
E.class_variable_get(:@@foo)    # => "bar"
C.class_variable_get(:@@foo)    # => "bar"
B.class_variable_get(:@@foo)    # => "bar"

D.new.change_foo                # => "baz"
D.class_variable_get(:@@foo)    # => "baz"
E.class_variable_get(:@@foo)    # => "baz"
C.class_variable_get(:@@foo)    # => "baz"
B.class_variable_get(:@@foo)    # => "baz"
A.class_variable_get(:@@foo)
  # => raises NameError "uninitialized class variable @@foo in A"

As for accessing class and instance variables, neither is accessible without the use of #instance_variable_get or ::class_variable_get until an accessor is defined. At present, ruby only has methods for defining accessors on instance variables, but it is simple enough to define the appropriate methods for the class variables:

class A
  @@foo = "foo"

  # the second argument `true` adds the writer method `#bar=`
  attr :bar, true

  def self.foo(); @@foo; end
  def self.foo=(v); @@foo = v; end

  def initialize()
    @bar = "bar"
  end
end
class B < A; end

A.foo             # => "foo"
B.foo = "foobar"
A.foo             # => "foobar"
B.foo             # => "foobar"

a = A.new
a.bar             # => "bar"
a.bar = "baz"
a.bar             # => "baz"

a.foo
  # => raises NoMethodError: undefined method `foo' for #<A:0x ...

You can see the attribute accessor methods here in the ruby core docs: http://www.ruby-doc.org/core-1.9.3/Module.html#method-i-attr. Also, ActiveSupport (http://rubygems.org/gems/activesupport) has "cattr" methods for defining class variable accessors http://api.rubyonrails.org/v3.2.5/classes/Class.html#method-i-cattr_accessor.

That's the simple stuff. The next step is understanding the "singleton class" also known as the "eigenclass" or "metaclass" (Wikipedia: Metaclass) (remember, everything in ruby is an Object, including the Class and Module constructs). Here I will point you to an excellent post by Yehuda Katz: Metaprogramming in Ruby: It’s All About the Self, and another Stack Overflow question: class << self idiom in Ruby.

As a preview: The singleton class (not to be confused with the singleton design pattern) allows you to access methods and instance data for a specific class or module. For some related documentation, see the core docs: http://www.ruby-doc.org/core-1.9.3/Object.html#method-i-singleton_class

class A; end
class B < A;
  class << self
    def foo() @foo end
    def foo=(v) @foo = v; end
  end
end
B.foo = "foo"

class C < B; end

A.foo
  # => raises NoMethodError: undefined method `foo' for A:Class

B.foo         # => "foo"
C.foo         # => nil
B.foo = "baz"
B.foo         # => "baz"
C.foo         # => nil
C.foo = "foo"
C.foo         # => "foo"
B.foo         # => "baz"

Lastly, remember to make use of the Ruby-Core docs. Most useful for understanding the above are:

http://www.ruby-doc.org/core-1.9.3/Object.html
http://www.ruby-doc.org/core-1.9.3/Module.html
http://www.ruby-doc.org/core-1.9.3/Class.html

The dollar sign is part of the variable name, so it has to be declared like this:

$edmund = 123

This is the same things for the instance and class variables: their names begin with @ or @@.

How to reference global variables and class variables?

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