What uses more memory, a Class object or an instance of that class?

Question

I am modeling dishes that a restaurant serves. I might make a Dish class something like this:

class Dish  
  attr_accessor :name, :price, :ingredients, etc...
  def initialize(dish_name, dish_price, etc...)
    name= dish_name
    price= dish_price
  end
end

With respect to memory usage, would it be better to create instances of Dish for each dish, Dish.new("Chicken Curry", ...), or create a new class that subclasses Dish, class ChickenCurry < Dish; ...; end;? Are there any other things to consider when choosing between these two methods with respect to hardware resources?

For clarification the ChickenCurry class would only contain a constructor where it sets the appropriate fields, like so:

class ChickenCurry < Dish
  def initialize
    super("Chicken Curry", ...) 
  end
end

Which uses more resources, Dish.new or ChickenCurry.new? Is the difference negligible? I plan to have thousands of these objects so even a 10 KB difference is worth considering.

I am using JRuby so please consider the JVM when answering but answers pertaining to Matz's Ruby are welcome too.

This is NOT a question about design. I'm only interested in resource usage.

Answer 1

Whole purpose of inheritance is that sub classes inheriting common things from super class and adding more specific things they need.

Why a Chicken Curry should have a field file sugarQuantity when it would not have any sugar. So, IMHO you should use inheritance to make a good OO design, so have a super class which will have common things and then each subclass will inherit the common and add more specific to it.

If you will put everything in one class then it would result in code abuse with so many unnecessary field.

In short, you need to use concept of Inheritance and Cohesion for make a good OO design, as per your case.

What uses more memory, a Class object or an instance of that class?

Definitely using inheritance will result in lesser memory usage when you will construct the objects of your dishes.

An object is the runtime representation of a class and it will occupy memory as per memory requirement of fields in the class. Try to minimize the number of fields (through inheritance and actual implementation) and you will have good memory footprint. If you will put everything in one class then definitely there will be a very bad memory foot print.

Consider you have 1000 fields in same dish class, and while creating object, all 1000 fields will occupy some memory space, as per their data types.

With inheritance and cohesion, you can have far more better memory footprint + OO design.

P.S.: I am a vegetarian :)

Answer 2

Don't worry about memory, the main question to ask yourself is: Does your ChickenCurry has any behaviour that extends Dish's behaviour? If your answer is no (like having only the constructor as you said) then use only one class.

Make your system clear to be understood.

Keep this in your heart:

"Premature optimization is the root of all evil" - DonaldKnuth