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Group ownership in object oriented programming
This post discusses the concept of ownership in oriented object programming, and more precisely, how to deal with instances shared by several instances of an other class.
Real world modeling
In oriented-object programming, we easily models concepts against real world things. I can create a Person class, to represent what is the abstraction of “person”. Then I define the person’s attributes, as gender, age, place, nutrition, etc. I continue with defining methods for the persons, like walking and eating.
In a ruby way, this will be something like this :
class Person
def initialize
@gender = 'M'
@age = 20
@place = 'home'
@nutrition = 5
end
def walk_to destination
@place = destination
end
def eat
@nutrition += 1
end
end
Then, I instantiate one Person, so say : john = Person.new .
ownership
But John like watches. No problem : I create a class Watch and I define a method for John to buy a watch.
Class Person
def buy_a_watch
@watch = Watch.new
end
end
All of this is straightforward in OO programming. There’s a class for persons, there’s a class for watches, and one particular person has one particular watch, so I can simply create the Watch within the Person class, in an instance method.
We can also imagine things that will be shared by everyone, like the date. I create the class World_date, and I instantiate it as singleton in a global variable. Any particular person can access it through this global variable.
group ownership
But now, how to deal with class with many objects instantiated that are, nevertheless, shared by groups of objects, as the weather, or TV programs?
With things as political_context, it will be easier. I may create a State class, with a @political_context attributes, and Persons will be created and manipulated inside a State instantiated object.
But will I say that weather includes persons (will I instantiate Persons in the Weather objects)? The weather is common to many people of the same area, but we can’t say that people are within weather. And this is even more difficult for we will have to consider that they may be in Weather and in TV_Program.
I could let the weather and programs in global scope. But will I have as many global variables as there are TV programs? And even if I put those in a global array, how will a person create a new TV program? This would be an instance method acting on the global focus…
suggestion
The problem I want to point is : where to put “instance = AClass.new” when instance is shared between many objects of an other class?
To solve this, I took the habit of creating such shared objects in a hash as class variable.
class Weather
def initialize( area )
@clouds = 10
@humidity = 5
@@objects = Hash.new unless defined? @@objects
@@objects[ area ] = self
end
# class methods
def self.list
@@objects.each_key { |key| puts key }
end
def self.objects
@@objects
end
end
Then, you create Weather.new “Paris”, Weather.new “London” and whatever you want from where you want, without needing to store it in a variable.
This way, you can easily access it in any method of the instances john or william simply like this :
Weather.objects[ @place ]
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