Ruby，2008-02-19

########################## 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