# Where is the magic?

**URL:** <https://www.ruby-forum.com/t/where-is-the-magic/182886>\
**Category:** Ruby\
**Created:** [February 4, 2010, 11:33am UTC](https://www.ruby-forum.com/t/where-is-the-magic/182886 "2010-02-04T11:33:11Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![uriel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/53a042/32.png) [@uriel](https://www.ruby-forum.com/u/uriel)\
**Post date:** [February 4, 2010, 11:33am UTC](https://www.ruby-forum.com/t/where-is-the-magic/182886/1 "2010-02-04T11:33:11Z")

</div>

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.  
🙂

---

<div class="post-metadata">

**Author:** ![uriel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/53a042/32.png) [@uriel](https://www.ruby-forum.com/u/uriel)\
**Post date:** [February 4, 2010, 11:58am UTC](https://www.ruby-forum.com/t/where-is-the-magic/182886/2 "2010-02-04T11:58:30Z")

</div>

On 02/04/2010 11:33 AM, Ilya Lopatkin wrote:

> ```
> Qt::Widget.new do
> self.layout = Qt::VBoxLayout.new do
> 
> ```
> 
> 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

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<div class="post-metadata">

**Author:** ![uriel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/53a042/32.png) [@uriel](https://www.ruby-forum.com/u/uriel)\
**Post date:** [February 4, 2010, 4:26pm UTC](https://www.ruby-forum.com/t/where-is-the-magic/182886/3 "2010-02-04T16:26:10Z")

</div>

Robert K. wrote:

> Struct.new uses instance\_eval internally:

Thanks for your response. I’ve almost grasped the idea. 🙂

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<div class="post-metadata">

**Author:** ![uriel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/53a042/32.png) [@uriel](https://www.ruby-forum.com/u/uriel)\
**Post date:** [February 4, 2010, 10:42pm UTC](https://www.ruby-forum.com/t/where-is-the-magic/182886/4 "2010-02-04T22:42:56Z")

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Ilya Lopatkin wrote:

> Robert K. wrote:
> 
> > Struct.new uses instance\_eval internally:
> 
> Thanks for your response. I’ve almost grasped the idea. 🙂

Here’s an explanation is other words:

When you create a block you create a parcel that contains three things:

- The code itself.
- The “binding” (links to the surrounding variables).
- The value of ‘self’ (this is how ruby knows which object to  
invoke methods on).

When you normally ‘yield’ to a block, the block uses the value of ‘self’  
saved in that parcel. On the other hand, when you do  
‘somebody.eval\_instance()’, the ‘self’ in that parcel is set to  
‘somebody’.

It is common in GUI frameworks to use eval\_instance() to make the code  
look like DSL (google for “DSL”).