From: Robert Klemme <bob.news@...>
Date: 2003-04-25T00:10:33+09:00
Subject: Re: How to call an object instance's method?


"Rene Tschirley" <pooh@cs.tu-berlin.de> schrieb im Newsbeitrag
news:b88tvk$1jg$1@news.cs.tu-berlin.de...
Dear Ruby Experts,


I'm somewhat new to Ruby and have got a problem which seems quite strange
to me. I intend to use a class as container for an array of instances of
an other class.

class A
   def funA ()
     puts "I'm alive!"
   end
end

class B
   def initialize ()
     @aList = Array.new
   end
   def funB ()
     # this code follows below
   end
end

One time, my program creates an instance of B and calls funB:

b = B.new
b.funB

funB fills the list 'aList' with some instances of A:

   def funB ()
     # append a new element to the list
     if @aList.empty?
       # in fact, i check if array size is big enough and
       # do the append if it doesn't
       @aList = @aList + Array.new(1, A)
     end
     # now we should have at least one instance of A in @aList, eh?
     # let's call the instance method
     @aList[0].funA                                 # this is line 20!
   end
end

    The error is in this line:

@aList = @aList + Array.new(1, A)

    You add an array containing the class instance of A not an instance of
A. Better do

@aList = @aList + Array.new(1, A.new)

Even better:

  @aList << A.new

or

  @aList.push A.new

And I expected this to work properly and waited for the "I'm alive". But
it doesn't. I get this error:

test.rb:20:in `funB': undefined method `funA' for A:Class (NameError)
         from /home/pooh/bin/test.rb:33

and don't know what it is intended to tell me.

I tried to find out more about @aList and its type and replaced line 20
with several checks:

     puts @aList.type � Array

I understand, so far.

     puts (@aList).type � A

This is confusing. Why the difference? Did I miss some special meaning of
"()"?

    Yes, the brackets in the line above are parsed as function invocation
brackets of "puts":

     (puts (@aList)).type

     puts "#{(@aList).type}" � Array

Should produce the same as above, but produces what I expected first. Hum?
But things are getting even more strange:

     puts @aList.at(0).type � Class
     puts @aList[0].type � Class

Wtf...? The Ruby book says "arr.at( anInteger ) -> anObject or nil" so I
expected to get "A" as type and not "Class".

    You put A into the array not an instance of A so A's type is Class.

        robert