Simple decorator pattern

audit_log.rb

class AuditLog
  def record(number, action, amount, balance)
    printf(
      "%<time>s #%<number>s %<action>s %<amount>.2f (%<balance>.2f)\n",
      time:    Time.now, 
      number:  number, 
      action:  action, 
      amount:  amount / 100, 
      balance: balance / 100)
  end
end

audited_account.rb

require 'delegate'
class AuditedAccount < SimpleDelegator
  def initialize(account, audit_log)
    super(account)
    @audit_log = audit_log
  end

  def deposit(amount)
    super
    @audit_log.record(number, :deposit, amount, balance_cents)
  end

  def withdraw(amount)
    super
    @audit_log.record(number, :withdraw, amount, balance_cents)
  end
end

bank_account.rb

class BankAccount
  def initialize(number)
    @number        = number
    @balance_cents = 0
  end

  def deposit(amount)
    @balance_cents += amount
  end

  def withdraw(amount)
    @balance_cents -= amount
  end

  def inspect
    "#<%s %s ($%0.2f)>" % 
      [self.class, @number, @balance_cents / 100.0]
  end
  alias_method :to_s, :inspect
end