# 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
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
gives me
NoMethodError: undefined method `call_this' for A:Class
from (irb):3
So you're obviously doing something more than just what you posted -
what's in the ...?
class A
attr_reader :monkeys
end
is roughly equivalent to writing:
A = Class.new
A.define_method :monkeys {
instance_variable_get(:@monkeys)
}
Is that what you're asking? Try posting a short piece of code with
actual and expected outputs.
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> 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?
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
--
Ross Bamford - ro...@roscopeco.remove.co.uk
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
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
(OP:) See also my 'Eureka' moment:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/167586 .
I have a feeling you're not far from discovering Ruby magic - stick with
it :)