########################## Singletons and other Constructors ##################
class MyLogger
private_class_method:new
@@logger = nil
def MyLogger.create
@@logger = new unless @@logger
@@logger
end
end
########################### Access Control ###########################
#Ruby gives you three levels of protection.
#Public methods can be called by anyone—no access control is enforced.Methods
#are public by default (except for initialize, which is always private).
#Protected methods can be invoked only by objects of the defining class and its
#subclasses. Access is kept within the family.
#Privatemethods cannot be called with an explicit receiver—the receiver is always
#self. This means that private methods can be called only in the context of the
#current object; you can’t invoke another object’s private methods.
#Ruby differs from other OO languages in another important way. Access control is
#determined dynamically, as the program runs, not statically. You will get an access
#violation only when the code attempts to execute the restricted method.
############################# Specifying Access Control #################
class MyClass
def method1 #default is 'public'
#...
end
protected
def method2
#...
end
private
def method3
#...
end
public
def method4
#...
end
end
#Alternatively, you can set access levels of named methods by listing them as arguments
#to the access control functions.
class MyClass
def method1
end
#...and so on
public :method1, :method4
protected :method2
private :method3
end
class Accounts
def initialize(checking, savings)
@checking = checking
@savings = savings
end
private
def debit(account, amount)
account.blance -= amount
end
def credit(account, amount)
account.blance += amount
end
public
def transfer_to_saving(amount)
debit(@checking, amount)
credit(@savings, amount)
end
end
class Account
attr_reader :blance
protected
:blance
def greater_balance_than(other)
return @blance > other.balance
end
end
###################### Variables #######################
person = "Tim"
puts person.id #936870
puts person.class #String
puts person #"Tim"
#is a variable an object? In Ruby, the answer is “no.”
#A variable is simply a reference to an object.
person1 = "Tim"
person2 = person1
person1[0] = "J"
person1 #"Jim"
person2 #"Jim"
#you could avoid aliasing by using the dup method of
#String, which creates a new String object with identical contents.
person1 = "Tim"
person2 = person1.dup
person1[0] = "J"
person1 #"Jim"
person2 #"Tim"
person1 = "Tim"
person2 = person1
person1.freeze #prevent modifications to the object
person2[0] = "j" #*****error
