Question

I have a class that should look something like this:

class Family_Type1     @people = Array.new(3)     @people[0] = Policeman.new('Peter', 0)     @people[1] = Accountant.new('Paul', 0)     @people[2] = Policeman.new('Mary', 0)      def initialize(*ages)         for i in 0 ... @people.length             @people[i].age = ages[i]         end     end end 

I want to be able to define a bunch of classes similar to this one at runtime (define them once at startup) where the size of the array and the type assigned to each parameter is defined at runtime from an external specification file.

I sort of got it to work using evals but this is really ugly. Any better way?

Answer 1

First off, part of the reason your example code isn’t working for you is that you have two different @people variables – one is an instance variable and the other is a class instance variable.

class Example   # we're in the context of the Example class, so    # instance variables used here belong to the actual class object,   # not instances of that class   self.class #=> Class   self == Example #=> true   @iv = 'I'm a class instance variable'    def initialize     # within instance methods, we're in the context     # of an _instance_ of the Example class, so     # instance variables used here belong to that instance.     self.class #=> Example     self == Example #=> false     @iv = 'I'm an instance variable'   end   def iv     # another instance method uses the context of the instance     @iv #=> 'I'm an instance variable'   end   def self.iv     # a class method, uses the context of the class     @iv #=> 'I'm a class instance variable'   end end 

If you want to create variables one time in a class to use in instance methods of that class, use constants or class variables.

class Example   # ruby constants start with a capital letter.  Ruby prints warnings if you   # try to assign a different object to an already-defined constant   CONSTANT_VARIABLE = 'i'm a constant'   # though it's legit to modify the current object   CONSTANT_VARIABLE.capitalize!   CONSTANT_VARIABLE #=> 'I'm a constant'    # class variables start with a @@   @@class_variable = 'I'm a class variable'    def c_and_c     [ @@class_variable, CONSTANT_VARIABLE ] #=> [ 'I'm a class variable', 'I'm a constant' ]   end end 

Even so, in the context of your code, you probably don’t want all your instances of Family_Type1 to refer to the same Policemen and Accountants right? Or do you?

If we switch to using class variables:

class Family_Type1     # since we're initializing @@people one time, that means     # all the Family_Type1 objects will share the same people     @@people = [ Policeman.new('Peter', 0), Accountant.new('Paul', 0), Policeman.new('Mary', 0) ]      def initialize(*ages)         @@people.zip(ages).each { |person, age| person.age = age }     end     # just an accessor method     def [](person_index)       @@people[person_index]     end end fam = Family_Type1.new( 12, 13, 14 ) fam[0].age == 12 #=> true # this can lead to unexpected side-effects  fam2 = Family_Type1.new( 31, 32, 29 ) fam[0].age == 12 #=> false fam2[0].age == 31 #=> true fam[0].age == 31 #=> true 

The runtime initialization can be done with metaprogramming, as Chirantan said, but if you are only initializing a few classes, and you know what their name is, you can also do it just by using whatever you read from the file:

PARAMS = File.read('params.csv').split('\n').map { |line| line.split(',') } make_people = proc do |klasses, params|   klasses.zip(params).map { |klass,name| klass.new(name, 0) } end class Example0   @@people = make_people([ Fireman, Accountant, Fireman ], PARAMS[0]) end class Example1   @@people = make_people([ Butcher, Baker, Candlestickmaker ], PARAMS[0]) end