what is a different? @name and name on ruby? sometime I confuse these..
@name is an "instance variable"
name is a local variable.
In a class, like the one below, @birthdate is used in two places: in getBirthdate and in ageNow. In order to use it inside more than one method it needs to be an instance variable (a var with an @). However, in ageNow, we create a local var called currentDate (it has no @). It is used in the formula to calculate age. We also need the current date in dateToday, but we cannot reuse the 'currentDate' var because it is local to only the ageNow method. So, in dateToday we create another local var dateOfToday.
class Person
def initialize @birthdate = '1950-06-15' end
def getBirthday return @birthdate end
def ageNow currentDate = Date.now ageInYears = Integer.new
currentDate - @birthdate = ageInYears # fake formula
return ageInYears end
def dateToday dateOfToday = Date.now return dateOfToday end
end
-- gw
Thank you, gw!. I got it!!
Does that mean that @birthdate will be accessible to other classes outside this one?
Elle
No, that is an instance variable of that class so it will only be available to a particular instance (not entirely true because of the rather rude instance_variable_get method). You can write an accessor method to allow access to it. That is the whole point of encapsulation, to provide an interface to the implementation and shield the user of your class from the gory details. I would suggest reading up on basic object oriented principals (Ruby specific or otherwise) as since everything in Ruby is an object, a good understanding of these principles will be a major help. You should look at the book “Programming Ruby” aka the “Pickaxe” book. It is very well written and has all the information you could want on this subject.
-Bill
elle wrote:
Does that mean that @birthdate will be accessible to other classes outside this one?
Not different classes: the "scope" of a variable determines how visible it is to other parts of the application. A local variable (name, with no @ or @@) is visible/accessible/usable only within the method in which it is defined. An instance variable, one defined with the @ sign, like @name is visible anywhere within the class, or a subclass ... but not, directly to any other class.
Now this gets a little confusing in Rails because Rails does some special magic for you. In specific, when you define a model, for example Employee, and that model is tied to a database with a table called employees, all of the columns of that database table, like "name", "salary", "start_date" and so on become magically defined as instance variables of the model. You won't see anything in the model that defines @name as anything, but if the employee table has a column called name, you have a free instance variable which you can reference as @name.
Cool, eh?
Tom
In plain Ruby, no. To make it accessible to other classes, you'd have to use
attr_reader birthdate # read only attribute birthdate # read and write (be careful)
or write an explicit method like the getBirthday in the previous message's example code.
However, as tharrison wrote, Rails takes some liberties with this in some of its innards. If you write your own class though (i.e. not a Rails controller or model), then you'll be following the rules of plain Ruby.
-- gw
Thank you guys for explaining that. It's been very helpful and by the way I have "Programming Ruby" on order.
Just one more question: views can see the instance variable, is that right? and if so, how do they know which instance variable they can see?
Elle
Thank you guys for explaining that. It's been very helpful and by the way I have "Programming Ruby" on order.
Just one more question: views can see the instance variable, is that right? and if so, how do they know which instance variable they can see?
ActionController magically copies its instance variables into the
object representing the views. There is a method on
ActionController#Base defining those ivars not to copy accross.
Fred
