From: Phrogz <phrogz@...>
Date: 2007-11-11T12:45:03+09:00
Subject: Re: composition (Dragon "has a" Trait)

Thufir wrote:
> C:\code\creat3>type Creature.rb
> class Creature
...
> def initialize ()
>         @location       =       Room.new
>         @traits         =       Traits.new
> end
...
> end
...
> class Dragon < Creature
> def initialize ()
>         @traits.life    =       1340
> end
> end

It seems you think that the initialize of Creature gets called because
Dragon is a subclass of it. This is not the case. For example:

irb(main):001:0> class Foo
irb(main):002:1>   def initialize
irb(main):003:2>     p "Foo.initialize"
irb(main):004:2>   end
irb(main):005:1> end
=> nil
irb(main):006:0> Foo.new
"Foo.initialize"

irb(main):007:0> class Bar < Foo
irb(main):008:1>   def initialize
irb(main):009:2>     p "Bar.initialize"
irb(main):010:2>   end
irb(main):011:1> end
=> nil
irb(main):012:0> Bar.new
"Bar.initialize"


If you want the initialize method from a superclass to be called, you
need to explicitly do so, and decide when to do it. For example:

irb(main):019:0> class Bar2 < Foo
irb(main):020:1>   def initialize
irb(main):021:2>     super # call this first
irb(main):022:2>     p "Bar2.initialize"
irb(main):023:2>   end
irb(main):024:1> end
=> nil
irb(main):025:0> Bar2.new
"Foo.initialize"
"Bar2.initialize"


Though it doesn't make a difference in my above simple example, note
that calling "super" without any parentheses automatically passes in
any arguments passed to the initialize function. This is convenient,
but can be a problem if your superclass's initialize method expects
different arguments than the subclass's.