example money class in Ruby (GBP)

money.rb

class Money
  STANDARD_VAT_RATE = 0.2.freeze

  attr_reader :amount

  def self.in_pence(amount_in_pence)
    new(amount_in_pence)
  end

  def initialize(amount)
    @amount = amount.to_int
    freeze
  end

  def to_currency
    amount / 100.0
  end

  def to_s
    "£%.2f" % to_currency
  end

  def to_int
    amount
  end

  def to_i
    amount
  end

  def negative?
    amount < 0
  end

  def positive?
    amount > 0
  end

  def zero?
    amount.zero?
  end

  def ==(other_money)
    self.class == other_money.class &&
      amount == other_money.amount
  end
  alias :eql? :==

  def +(value)
    self.class.new(amount + value.to_int)
  end

  def vat_of(vat_rate = STANDARD_VAT_RATE)
    self.class.new to_vat(vat_rate)
  end

  private

  def to_vat(vat_rate)
    (amount * vat_rate).round
  end
end