From: Joel VanderWerf <vjoel@...>
Date: 2005-06-14T09:32:40+09:00
Subject: Re: are these the same?

Lowell Kirsh wrote:
> I'm wondering about the difference between writing something like
> 
> @x = ...
> and
> self.x = ...
> 
> from within a class. Are they the same thing? To be more concrete, I 
> have given a fuller example below:

They are different, but the degree of difference depends on the context. 
The "@x =" construct is ruby's built-in way of assigning to an instance 
variable in the current object ("self"). It does not invoke any methods. 
The "self.x =" construct is a special syntax for calling the method 
whose name is "x=" on the object denoted by "self". This method may or 
may not assign to the instance variable @x, and it may do other things 
in addition to assigning to @x. For instance, you might also execute 
some debugging code:

class Foo
   def x=(val)
     puts "setting x = #{val.inspect}"
     @x = val
   end
end

Or you might not assign to @x at all. For example:

class Square
   def width=(val)
     self.height = val
   end
   def height=(val)
     @height = val
   end
end

Since width and height must be equal, there's no need to store them 
separately. But users of the Square class don't have to know that. Some 
people call this the "uniform access principle".