Forum: Ruby When are attr_reader type methods called?

Announcement (2017-05-07): www.ruby-forum.com is now read-only since I unfortunately do not have the time to support and maintain the forum any more. Please see rubyonrails.org/community and ruby-lang.org/en/community for other Rails- und Ruby-related community platforms.
6828ffc79486cd2442714bf32286a910?d=identicon&s=25 Vivek (Guest)
on 2005-12-21 05:14
(Received via mailing list)
I couldnt get a proper subject for the post but here is my question.
Its regarding methods like
attr_reader defined like below
You can do this in ruby

# file a.rb
class A
  call_this ("hello world")

  def  call_this
 ....
  end
end


The A::call_this is called immediately after I run the script.Becuase I
made the method print something and it printed it even though I hadnt
instantiated an object of the class with new.
What if the method uses some instance variables? wouldnt that be a
problem because its called even before an object is created?My question
is at what time during program execution are these methods called? If
there are more than one classes in a file then whats the order ? is it
defined by the language?
My guess is that these are like the equivalent of static member
variables in C, initialized before _main is called.?
Hope to get some clarification on this
vivek
2c51fec8183a5d21c4e11b430beabb47?d=identicon&s=25 Patrick Hurley (Guest)
on 2005-12-21 05:38
(Received via mailing list)
On 12/20/05, Vivek <krishna.vivek@gmail.com> wrote:
>  ....
> there are more than one classes in a file then whats the order ? is it
> defined by the language?
> My guess is that these are like the equivalent of static member
> variables in C, initialized before _main is called.?
> Hope to get some clarification on this
> vivek
>

Actually your code will generate an error. You can do this:

class Foo
  def self.bar
    puts 42
  end

  bar
end

This will work as the method bar exists and belongs to the class Foo,
not an instance of Foo. This will not work:

class Boo
  def hoo
    puts "Python is nice, but I like Ruby better :-)"
  end

  hoo      # this will raise a NoMethodError, unless hoo had been
defined further up the chain
end

Of course you can work around much of this with fun method_missing
maddness. In the particular case of attr_reader, they are actually
part of Module which is the superclass of Class, that is why you can
call them inside your class definitions -- they are methods available
in Class (and you are defining a Class).

pth
82e62c756d89bc6fa0a0a2d7f2b1e617?d=identicon&s=25 Ross Bamford (Guest)
on 2005-12-21 06:14
(Received via mailing list)
On Wed, 21 Dec 2005 04:09:33 -0000, Vivek <krishna.vivek@gmail.com>
wrote:

>  ....
> there are more than one classes in a file then whats the order ? is it
> defined by the language?

It's top to bottom, as you'd expect. Does the following surprise you?

   NewClass = Class.new(Array) do

     attr_accessor :myattr

     def this
       "that"
     end
   end

   puts (nc = NewClass.new).this
   nc.myattr = 16
   puts nc.myattr

   nc[3] = "Aha!"
   p nc

I just made a new nuby file trying out the attr_accessor and instance
var
stuff you asked about:

	http://roscopeco.co.uk/code/noob/class-def.html
6828ffc79486cd2442714bf32286a910?d=identicon&s=25 Vivek (Guest)
on 2005-12-22 10:10
(Received via mailing list)
Ok ..sorry for the wrong code.What I meant was do have a class
method.Thanks for the explanation ..so now I have this below (I learnt
the instance_eval from Ross' examples)

class X
  @ivar = 1
  def self.call_this
     puts "call this"
     @var = 1
  end
end
class A <X
  call_this
  def some_method(arg)
    puts arg
  end
end
puts X.instance_eval { @var}
puts X.instance_eval { @ivar}


So I execute the above program and call_this is called. So I guess
these methods which are called in a class but outside any function have
to be class methods and are called in a sequential order as they are
encountered by the interpreter.
This brings me to another question
I have  a statement like @var = 1 inside a class method 'call_this'
,but @var is an instance variable,so who come I dont get  an error?
Although when I try to print it out it gives 'nil' .Is this some thing
which ruby forgives ?

And second
I read the documentation of instance_eval at The practical programmer
site..which says.

instance_eval    obj.instance_eval(aString  [, file  [  line  ]  ] ) ->
anObject

But here and in Ross' examples we call it on a class ? Is the
documentation incorrect or am i missing something.

thanks again
vivek
2c51fec8183a5d21c4e11b430beabb47?d=identicon&s=25 Patrick Hurley (Guest)
on 2005-12-22 11:08
(Received via mailing list)
On 12/22/05, Vivek <krishna.vivek@gmail.com> wrote:
>     puts arg
>   end
> end
> puts X.instance_eval { @var}
> puts X.instance_eval { @ivar}
>
>
> I have  a statement like @var = 1 inside a class method 'call_this'
> ,but @var is an instance variable,so who come I dont get  an error?

You are asking the right questions, if you are feeling adventurous do a
quick:
ri Class

X is also an Object -- the variable @ivar is an instance variable in
that class. Similarly the instance variable @var you create in the
call_this method, becomes an instance variable in the class A. Try
this:

puts A.instance_eval { @var }    # -> 1

Also note that if you:

X.call_this
puts X.instance_eval { @var }    # -> 1

> And second
> I read the documentation of instance_eval at The practical programmer
> site..which says.
>
> instance_eval    obj.instance_eval(aString  [, file  [  line  ]  ] ) ->
> anObject
>
> But here and in Ross' examples we call it on a class ? Is the
> documentation incorrect or am i missing something.

You were (because I know you got it from my pitiful explanation :-)
were missing that a Class is an Object.

Good Luck and keep at it
pth
82e62c756d89bc6fa0a0a2d7f2b1e617?d=identicon&s=25 Ross Bamford (Guest)
on 2005-12-22 14:29
(Received via mailing list)
On Thu, 22 Dec 2005 10:05:44 -0000, Patrick Hurley <phurley@gmail.com>
wrote:

> On 12/22/05, Vivek <krishna.vivek@gmail.com> wrote:
>> But here and in Ross' examples we call it on a class ? Is the
>> documentation incorrect or am i missing something.
>
> You were (because I know you got it from my pitiful explanation :-)
> were missing that a Class is an Object.
>

(OP:) See also my 'Eureka' moment:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/... .

I have a feeling you're not far from discovering Ruby magic  - stick
with
it :)
This topic is locked and can not be replied to.