Newbie problem: ArgumentError when creating subclass

Ruby
1

Could someone explain why the following code raises ArgumentError.
Thanks.

class SuperClass

def initialize #Make sure you spell ‘initialize’ correctly!
@my_attr = 1
end

attr_reader :my_attr
end

class SubClass < SuperClass

def initialize(data1, data2)
super
@data1 = data1
@data2 = data2
end

attr_reader :data1
attr_reader :data2

end

test = SubClass.new(“one”, “two”)
puts test.my_attr
puts test.data1
puts test.data2

$ ruby inheritance_problem.rb
inheritance_problem.rb:13:in initialize': wrong number of arguments (2 for 0) (ArgumentError) from inheritance_problem.rb:13:ininitialize’
from inheritance_problem.rb:23:in `new’
from inheritance_problem.rb:23

2

On 4/2/07, Dan Stevens (IAmAI) [email protected] wrote:

class SubClass < SuperClass

    def initialize(data1, data2)
            super

This should be super(). Without the parenthesis it defaults to passing
the arguments given to the current method to the super class’s
implementation.

Ryan

3

Alle lunedì 2 aprile 2007, Dan Stevens (IAmAI) ha scritto:

    from inheritance_problem.rb:13:in `initialize'
    from inheritance_problem.rb:23:in `new'
    from inheritance_problem.rb:23

If you call super with no arguments, it will pass to the superclass
method all
the parameters given to the sublcass method. In your case,
SubClass#initialize takes 2 arguments, while SuperClass#initialize takes
no
arguments. Since you call super without arguments, ruby passes both
arguments
to SuperClass#initialize, then complains because the number of arguments
is
wrong. What you need to do is:

class SubClass < SuperClass
def initialize(data1, data2)
super()
…
end
…
end

If SuperClass#initialized required one argument, you’d do:

def initialize(data1, data2)
super(data1)
…

By the way, you can pass more than one argument to attr_reader, so you
can
write

attr_reader :data1, :data2

I hope this helps

Stefano

4

Hi,

Am Dienstag, 03. Apr 2007, 05:59:31 +0900 schrieb Ryan L.:

   def initialize(data1, data2)
           super

This should be super(). Without the parenthesis it defaults to passing
the arguments given to the current method to the super class’s
implementation.

Ah, parentheses do the trick. I always helped me calling

super *[]

what is not actually readable or beautiful.

Bertram

5

Thanks all of you for clearing that up. In ruby I’m used not using
parenthesis when calling methods without parameters, but I should be
able to remember this exception :slight_smile: