Newbie problem: ArgumentError when creating subclass

[Dan Stevens (IAmAI)]

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

 

[Ryan Leavengood]

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

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

 

[Stefano Crocco]

Alle luned=EC 2 aprile 2007, Dan Stevens (IAmAI) ha scritto:

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

class SuperClass

def initialize #Make sure you spell 'initialize' correctly!
@my_attr =3D 1
end

attr_reader :my_attr
end

class SubClass < SuperClass

def initialize(data1, data2)
super
@data1 =3D data1
@data2 =3D data2
end

attr_reader :data1
attr_reader :data2

end

test =3D 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: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=20
the parameters given to the sublcass method. In your case,=20
SubClass#initialize takes 2 arguments, while SuperClass#initialize takes no=
=20
arguments. Since you call super without arguments, ruby passes both argumen=
ts=20
to SuperClass#initialize, then complains because the number of arguments is=
=20
wrong. What you need to do is:

class SubClass < SuperClass
def initialize(data1, data2)
super()
...
end
...
end
=20
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=
=20
write

attr_reader :data1, :data2

I hope this helps

Stefano

 

[Bertram Scharpf]

Hi,

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

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

 

[Dan Stevens (IAmAI)]

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

Hi,

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

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