Inheritance in ruby

ruby-talk
1

Hi, all.
I'm really new to Ruby. Got this error:

class Page
    @url=""

    def initialize
        puts "going to "+@url
    end
end

class MainPage < Page
    @url = "Main"
end

class LoginPage < Page
    @url = "Login"
end

every time a page is created, I want it to go to its own url.

m = MainPage.new()
# should print going to Main
l = LoginPage.new()
# should print going to Login

But I tried @@, @ and found neither of them works.

Anyone knows what kind of method or variable I could use?

2

It the url is fixed for all instances of each class I wouldn't use a
variable at all:

class Page
   def initialize
       puts "going to "+ url
   end
end

class MainPage < Page
   def url
     "Main"
   end
end

class LoginPage < Page
   def url
     "Login"
   end
end

···

On Tue, May 12, 2009 at 6:47 AM, jarodzz <jarodzz@gmail.com> wrote:

Hi, all.
I'm really new to Ruby. Got this error:

class Page
@url=""

def initialize
puts "going to "+@url
end
end

class MainPage < Page
@url = "Main"
end

class LoginPage < Page
@url = "Login"
end

every time a page is created, I want it to go to its own url.

m = MainPage.new()
# should print going to Main
l = LoginPage.new()
# should print going to Login

But I tried @@, @ and found neither of them works.

Anyone knows what kind of method or variable I could use?

--
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

3

The problem is that the @url mentioned in the first line of each class
definition and the @url mentioned in the initialize method are not the
same. This is a common beginner mistake. The way to speak of an instance
variable is from inside an instance method (during code that runs inside
an instance). So:

class Page
    def initialize
        puts "going to "+@url
    end
end

class MainPage < Page
    def initialize
        @url = "Main"
        super
    end
end

class LoginPage < Page
    def initialize
        @url = "Login"
        super
    end
end

m.

···

jarodzz <jarodzz@gmail.com> wrote:

class Page
    @url=""

    def initialize
        puts "going to "+@url
    end
end

class MainPage < Page
    @url = "Main"
end

class LoginPage < Page
    @url = "Login"
end

--
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4

class Page
@url=""

def initialize
    puts &quot;going to &quot;\+@url
end

end

class MainPage < Page
@url = "Main"
end

class LoginPage < Page
@url = "Login"
end

The problem is that the @url mentioned in the first line of each class
definition and the @url mentioned in the initialize method are not the
same. This is a common beginner mistake. The way to speak of an instance
variable is from inside an instance method (during code that runs inside
an instance). So:

Right!

class Page
def initialize
puts "going to "+@url
end
end

class MainPage < Page
def initialize
@url = "Main"
super
end
end

class LoginPage < Page
def initialize
@url = "Login"
super
end
end

Keep in mind that super class initialization should be invoked
/before/ sub class initialization. That's the general pattern
enforced in other programming languages and there is a good reason to
do it that way: that way sub class functionality can rely on the super
class being properly constructed. This is important if you have sub
class methods which are invoked from within initialize as well as from
client code. If they cannot rely on the super class state to be
complete they will either break during initialization or have to be
made overly complex because they have to react to different
situations. Rick's solution does not have this property and is much
cleaner in this regard.

Kind regards

robert

···

2009/5/12 matt neuburg <matt@tidbits.com>:

jarodzz <jarodzz@gmail.com> wrote:

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

5

thanks all.
It's now working after
removing the first line @url,
and adding each class their own initialize
following Robert's suggestion.

···

On Tue, May 12, 2009 at 9:46 PM, Robert Klemme <shortcutter@googlemail.com> wrote:

2009/5/12 matt neuburg <matt@tidbits.com>:

jarodzz <jarodzz@gmail.com> wrote:

class Page
@url=""

def initialize
    puts &quot;going to &quot;\+@url
end

end

class MainPage < Page
@url = "Main"
end

class LoginPage < Page
@url = "Login"
end

The problem is that the @url mentioned in the first line of each class
definition and the @url mentioned in the initialize method are not the
same. This is a common beginner mistake. The way to speak of an instance
variable is from inside an instance method (during code that runs inside
an instance). So:

Right!

class Page
def initialize
puts "going to "+@url
end
end

class MainPage < Page
def initialize
@url = "Main"
super
end
end

class LoginPage < Page
def initialize
@url = "Login"
super
end
end

Keep in mind that super class initialization should be invoked
/before/ sub class initialization. That's the general pattern
enforced in other programming languages and there is a good reason to
do it that way: that way sub class functionality can rely on the super
class being properly constructed. This is important if you have sub
class methods which are invoked from within initialize as well as from
client code. If they cannot rely on the super class state to be
complete they will either break during initialization or have to be
made overly complex because they have to react to different
situations. Rick's solution does not have this property and is much
cleaner in this regard.

Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/