Dynamically creating class in Ruby

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         


        

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

Answer 2

Assuming you want to create different classes per type/array size at runtime:

If (like in Python) a Ruby class is defined when executed (I think it is), then you can do this:

Define your class inside a function. Have the function recieve array size and type as parameters and return the class in its result. That way, you have a sort of class factory to call for each definition in your spec file :)

If on the other hand you want to just initialize @params based on actual data, keep in mind, that Ruby is a dynamically typed language: Just reassign @params in your constructor to the new array!

Answer 3

From what I understand, you need meta-programming. Here is a snippet of code for creating classes dynamically (on the fly) with initialize method that initializes instance variables-

class_name = 'foo'.capitalize
klass = Object.const_set(class_name,Class.new)

names = ['instance1', 'instance2'] # Array of instance vars

klass.class_eval do
  attr_accessor *names

  define_method(:initialize) do |*values|
    names.each_with_index do |name,i|
      instance_variable_set("@"+name, values[i])
    end
  end
  # more...
end

Hope you can tweak it to suit your requirements.