Scope of an @variable

ruby-talk
1

I'm also a newbie so others feel free to correct me as well :wink:

Your understanding is correct in that @name applies to the specific
instance of the class and the @@name applies to all instances of the
class.

Hope this example helps.

class Person
  @@lastname = "Doe"
  @name=""
  
  def setLastName(newName)
    @@lastname = newName
  end
  
  def changeName(newName)
    @name = newName
  end
  
  def sayName()
    "My name is " + @name
  end
  
  def fullName()
    "My full name is " + @name + " " + @@lastname
  end
end

puts "---Defining John---\n"
p1 = Person.new()
p1.changeName('John')
puts p1.sayName()
puts p1.fullName()

puts "\n"

puts "---Defining Jane---\n"
p2 = Person.new()
p2.changeName('Jane')
puts p2.sayName()
puts p2.fullName()

puts "\n"

puts "---Changing Last Name---\n"
p1.setLastName('Smith')

puts "John's full name -> " + p1.fullName()
puts "Jane's full name -> " + p2.fullName()

路路路

-----Original Message-----
From: list-bounce@example.com [mailto:list-bounce@example.com] On Behalf
Of Nathan Olberding
Sent: Tuesday, March 14, 2006 4:18 PM
To: ruby-talk ML
Subject: Re: Scope of an @variable

Adam Shelly wrote:

you need to use @@name.

I was under the impression (newbie alert) that @name was for instances
and @@name was for classes as a whole (ie, @@ variables change that
value in all instances of Class). Is there a way to have variables that
apply to all instances of a Class?

--
Posted via http://www.ruby-forum.com/\.

2

William (Bill) Froelich wrote:

I'm also a newbie so others feel free to correct me as well :wink:

Your understanding is correct in that @name applies to the specific
instance of the class and the @@name applies to all instances of the
class.

This may be wrong, but I like to think of @@ as a class variable and @ as a method variable. @@ is accessible by any method in the class and @ is accessible by the method only.

3

rtilley wrote:

This may be wrong, but I like to think of @@ as a class variable and @
as a method variable. @@ is accessible by any method in the class and @
is accessible by the method only.

You're close. @ are instance variables. Any instance method in your
class can
access them.

(Maybe you're thinking of local variables, which are defined within a
method and are accessible by that method only.)

Jeff

www.jeffcohenonline.com

路路路

--
Posted via http://www.ruby-forum.com/\.

4

Jeff Cohen wrote:

rtilley wrote:

This may be wrong, but I like to think of @@ as a class variable and @
as a method variable. @@ is accessible by any method in the class and @
is accessible by the method only.

You're close. @ are instance variables. Any instance method in your class can
access them.

(Maybe you're thinking of local variables, which are defined within a method and are accessible by that method only.)

Thanks Jeff! That makes sense. I was confusing @ vars with local vars.

5

You can also access @ variables from a class method - ie ClassName.function; @a_variable; end. They are local to each class within a hierarchy, rather than @@ which would be shared among classes in the hierarchy.

So:

class X
  def X.var;
    @var
  end

  def X.var=(v)
    @var=v
  end
end

class Y<X;end

X.var=10
Y.var=20
X.var => 10
Y.var => 20

Cheers,
  Benjohn

路路路

On 15 Mar 2006, at 00:54, Jeff Cohen wrote:

rtilley wrote:

This may be wrong, but I like to think of @@ as a class variable and @
as a method variable. @@ is accessible by any method in the class and @
is accessible by the method only.

You're close. @ are instance variables. Any instance method in your
class can
access them.

6

Hi --

rtilley wrote:

This may be wrong, but I like to think of @@ as a class variable and @
as a method variable. @@ is accessible by any method in the class and @
is accessible by the method only.

You're close. @ are instance variables. Any instance method in your
class can
access them.

You can also access @ variables from a class method - ie ClassName.function; @a_variable; end. They are local to each class within a hierarchy, rather than @@ which would be shared among classes in the hierarchy.

So:

class X
  def X.var;
    @var
  end

  def X.var=(v)
    @var=v
  end
end

class Y<X;end

X.var=10
Y.var=20
X.var => 10
Y.var => 20

Nice, isn't it? It's really just this:

   obj = Object.new
   def obj.var
     @var
   end

   def obj.var=(v)
     @var = v
   end

with obj being a Class object. Plus the special-cased thing where the
subclasses get to call it (because the singleton class of X is
considered the superclass of the singleton class of Y).

You can also do:

   class X
     class << self # singleton class of X
       attr_accessor :var
     end
   end

David

路路路

On Fri, 17 Mar 2006, Benjohn Barnes wrote:

On 15 Mar 2006, at 00:54, Jeff Cohen wrote:

--
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

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