Class static instance initializers (i.e. factory methods) in Ruby

Question

I have a class that I want to put factory methods on to spit out a new instance based on one of two construction methods: either it can be constructed from data in memory, or data stored in a file.

What I would like to do is to encapsulate the logic of how the construction is performed inside the class, so I would like to have static class methods that are set up like this:

class MyAppModel
    def initialize
        #Absolutely nothing here - instances are not constructed externally with MyAppModel.new
    end

    def self.construct_from_some_other_object otherObject
        inst = MyAppModel.new
        inst.instance_variable_set("@some_non_published_var", otherObject.foo)
        return inst
    end

    def self.construct_from_file file
        inst = MyAppModel.new
        inst.instance_variable_set("@some_non_published_var", get_it_from_file(file))
        return inst
    end
end

Is there no way to set @some_private_var on an instance of a class from the class itself without resorting to metaprogramming (instance_variable_set)? It seems like this pattern is not so esoteric as to require meta-poking variables into instances. I really don't intend to allow any class outside of MyAppModel to have access to some_published_var, so I don't want to use e.g. attr_accessor - it just feels like I'm missing something...

Holin · Accepted Answer · 2016-12-05 16:28:21Z

11

Maybe using a constructor is the better way to achieve what you want, just make it protected if you don't want to create instances from the "outside"

class MyAppModel
  class << self
    # ensure that your constructor can't be called from the outside
    protected :new

    def construct_from_some_other_object(other_object)
      new(other_object.foo)
    end

    def construct_from_file(file)
      new(get_it_from_file(file))
    end
  end

  def initialize(my_var)
    @my_var = my_var
  end
end

edited Dec 5, 2016 at 16:28

answered Feb 9, 2011 at 23:15

Holin

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2 Comments

Chuck Over a year ago

This is very nice and idiomatic.

Matt Over a year ago

Thanks, I think this is much cleaner than what I was doing - it still involves passing a bunch of stuff to the constructor, but since it's protected anyway, that's no big deal - and far better than instance_variable_set.

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