What is the difference between passing arguments to initialize and setting them within initialize?

user_12288523 0
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I would like to know the difference between passing arguments in parentheses to initialize vs setting those values manually in initialize?

class CashRegister
  attr_accessor :items, :discount, :total, :last_transaction

  def initialize(discount=0)
    @total = 0
    @discount = discount
    @items = []
  end
end
ruby
1 Answers

The difference is that by putting it in the arguments, you allow the caller to provide different values, versus hard-coding them. For example:

def initialize(discount=0, total=0, items=[])
  @total = total
  @discount = discount
  @items = items
end

Now you can call it with no arguments, or you can specify the arguments:

MyClass.new
# uses all defaults

MyClass.new(123)
# Uses custom value for discount only

MyClass.new(1, 2, ["item"])
# Uses custom values for all

The problem with this approach, is what happens when you want to give a custom value for total (the second argument) but use the default value for discount (the first)? With positional arguments, you can't. So it's preferred to use keyword arguments instead:

def initialize(discount: 0, total: 0, items: [])

Now you can specify custom values for any combination of the three arguments, for example:

MyClass.new
# uses all defaults

MyClass.new(total: 123)
# this uses default values for discount and items
user_29814290
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