From: Stefano Crocco <stefano.crocco@...>
Date: 2013-02-18T03:23:28+09:00
Subject: Re: What is Ruby's default constructor?

On Monday 18 February 2013 Love U Ruby wrote
> Hi,
> 
> I wrote the below class :
> >>  class Adder
> 
> ?>    def initialize(my_num)
> 
> >>      @my_num = my_num
> >>    
> >>    end
> 
> ?>  end
> => nil
> 
> >> ob = Adder.new
> 
> ArgumentError: wrong number of arguments (0 for 1)
>   from (irb):3:in `initialize'
>   from (irb):8:in `new'
>   from (irb):8
>   from /usr/bin/irb:12:in `<main>'
> 
> #~~~> Here it is not allowing the creation of object.Even does not
> allowing automatic call to its default constructor which it does when a
> class has been defined without "initialize" method.
> 
> >> ob = Adder.new(10)
> 
> => #<Adder:0x000000013df828 @my_num=10>
> 
> #~~~> here it is OK. and it should be.
> 
> #~~> then in the below class how object is created, what constructor it
> has called?
> 
> >> class A
> >> p "hi"
> >> end
> 
> "hi"
> => "hi"
> 
> >> ob = A.new
> 
> => #<A:0x0000000122ff50>
> 
> 
> --
> Posted via http://www.ruby-forum.com/.

What you call ruby's default constructor is, in fact, Object#initialize, which 
takes no argument and does nothing. When you define an #initialize method in 
your class, that method will be called from the class .new method and all the 
arguments passed to .new will in turn be passed to #initialize.

Since you defined Adder#initialize to take one argument, you have to pass 
Adder.new one argument (or give a default value to the argument of 
Adder#initialize). As for calling the default constructor when Adder.new is 
given no arguments, this is something which never happens in ruby. If you 
override a method in a derived class (#initialize is not special in any way), 
the overriding method is the one which will always be called, even if the 
arguments are wrong.

I hope this helps

Stefano