From: Robert Klemme <shortcutter@...>
Date: 2010-02-04T19:55:08+09:00
Subject: Re: Where is the magic?

On 02/04/2010 11:33 AM, Ilya Lopatkin wrote:
> I'm learning Ruby and have found some interesting behaviour which I
> cann't understand myself yet.
> Here's some code and the output:
> class Foo
>   def self.new(*args)
>     obj = super
>     yield
>     obj
>   end
> end
> 
> Foo.new do
>   p self
> end
> 
> Struct.new("Foo", :foo, :bar) do
>   p self
> end
> 
> => main
> => Struct::Foo
> 
> The first output is quite understandable because self is bounded to the
> top-level class of the file and is sent to the closure.
> But in the second line it seems that scope of self has been changed
> somehow.
> I've had a fast look on the source file struct.c and haven't found
> something special in the new method.
> The same thing can also be found in the QtRuby bindings:
> require 'Qt4'
> 
> Qt::Application.new(ARGV) do
>     Qt::Widget.new do
> 
>         self.window_title = 'Hello QtRuby v1.0'
>         resize(200, 100)
> 
>         button = Qt::PushButton.new('Quit') do
>             connect(SIGNAL :clicked) { Qt::Application.instance.quit }
>         end
> 
>         label = Qt::Label.new('<big>Hello Qt in the Ruby way!</big>')
> 
>         self.layout = Qt::VBoxLayout.new do
>             add_widget(label, 0, Qt::AlignCenter)
>             add_widget(button, 0, Qt::AlignRight)
>         end
> 
>         show
>     end
> 
>     exec
> end
> 
> Please, could anybody explain me how does this work. Links will do too.
> :)

Struct.new uses instance_eval internally:

irb(main):001:0> o = Object.new
=> #<Object:0x9a93150>
irb(main):002:0> o.instance_eval { p self }
#<Object:0x9a93150>
=> #<Object:0x9a93150>
irb(main):003:0> p self
main
=> main
irb(main):004:0>

An example more close to what Struct does:

irb(main):001:0> class Foo
irb(main):002:1> def initialize(&b)
irb(main):003:2> printf "in init: %p\n", self
irb(main):004:2> instance_eval(&b)
irb(main):005:2> end
irb(main):006:1> end
=> nil
irb(main):007:0> Foo.new { printf "in block: %p\n", self }
in init: #<Foo:0x8e742e8>
in block: #<Foo:0x8e742e8>
=> #<Foo:0x8e742e8>

Kind regards

	robert

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