问题
I'm learning Ruby, and have come up to a point where I am confused.
The book I am using is talking about private, public, and protected methods, but I am still a bit confused. What are the differences between each?
回答1:
Public - can be called from anywhere
Private - The method cannot be called outside class scope. The object can only send the message to itself
ex: the baker has bake method as public but break_eggs is private
Protected - You can call an object's protected methods as long as the default object self is an instance of the same class as the object whose method you're calling
ex: with n protected method, c1 can ask c2 to execute c2.n, because c1 and c2 are both instances of the same class
And last but not least:
- Inheritance: Subclasses inherit the method-access rules of their superclass
if "class D < C", then D will exhibit the same access behaviour as instances of C
reference: http://www.amazon.com/Ruby-Rails-Techniques-Developers/dp/1932394699
回答2:
public methods are open to everyone. As for private versus protected, I refer to "Ruby Private Methods vs. Protected Methods":
What is the difference between 'private' and 'protected' methods in Ruby? In Ruby, the primary difference between a 'private' and 'protected' method is that a private method cannot be called with an explicit receiver, while a protected method can. What is an 'explicit receiver', you ask? An explicit receiver is the object that is receiving a message. In the following example, we have a receiver ('parent') and a method ('get_name'). The 'parent' object is receiving the instruction to perform the 'get_name' method.
回答3:
Check out "Ruby Programming/Syntax/Classes" for a detailed example and explanation.
Put simply, the differences between private, public, and protected methods are visibility of that method in the program, kinda like read-only, read and write, and near invisible.
Unlike some of the other languages, you can't completely hide a Ruby private method, you can only access private methods for your instance of object and not for any other object instance of a class.
Public, of course, is total accessibility and methods are usually defaulted to public with some exceptions.
Protected methods are accessible from objects of the same class or even children, which is not the case for a private method.
回答4:
Let me explain
Private and protected methods work a little differently in Ruby than in most other
programming languages. Suppose you have a class called Foo and a subclass SubFoo .
In languages like Java, SubFoo has no access to any private methods defined by Foo .
As seen in the Solution, Ruby provides no way to hide a class’s methods from its sub-
classes. In this way, Ruby’s private works like Java’s protected.
Suppose further that you have two instances of the Foo class, a and b. In languages
like Java, a and b can call each other’s private methods. In Ruby, you need to use a
protected method for that. This is the main difference between private and protected methods in Ruby.
class Foo
private
def pri
'hey I am private of Foo'
end
protected
def prot
'Hey I am protected of Foo'
end
end
Now subclass of Foo
class SubFoo < Foo
def call_pri_of_foo
pri
end
def call_prot_of_foo
prot
end
end
Now calling the accessors within SubFoo
> sub_foo = SubFoo.new
=> #<SubFoo:0x00000002b56ad8>
> sub_foo.call_pri_of_foo
=> "hey I am private of Foo"
> sub_foo.call_prot_of_foo
=> "Hey I am protected of Foo"
Up to here; there seem to be no difference
next_sub_foo = SubFoo.new
=> #<SubFoo:0x00000002b1a0b0>
def next_sub_foo.access_private(child_of_sub_foo)
child_of_sub_foo.pri
end
def next_sub_foo.access_protected(child_of_sub_foo)
child_of_sub_foo.prot
end
Now calling the accessor
> next_sub_foo.access_private(sub_foo)
# => NoMethodError: private method `pri' called for #<SubFoo:0x00000002b56ad8>
but it can access the protected methods of its siblings
> next_sub_foo.access_protected(sub_foo)
# => "Hey I am protected of Foo"
You can also see @tenderlove's blog for more clear picture http://tenderlovemaking.com/2012/09/07/protected-methods-and-ruby-2-0.html
回答5:
The difference will be on Visibility and how they are affected by Inheritance :
Visibility :
|| Anywhere || Public can be accessed from inside and outside the class.
|| Inside the class || Both Private and Protected can only be accessed from inside the class.
The similarity between Protected and Private :
- Both can be accessed from outside the class through a public method.
The differences between Protected and Private are :
Private method can not be called with a receiver (not even with #self). UNLESS ... calling a PRIVATE SETTER method. If you try to remove the receiver, Ruby will create a local variable. Self is a must in this case.
Protected may or may not use self.
Protected can access another object's protected method that comes from the same class, Private can't.
When it comes to Inheritance :
Private methods can only be called on subclasses implicitly (simply just the name of the method) but not explicitly (using #self).
Protected can be called both ways (with or without #self || implicitly or explicitly).
Example with code below :
class Dog
attr_accessor :name, :age
def initialize(n, a)
self.name = n
self.age = a
end
def accessing_private
"#{self.name} in human years is #{human_years}. This is secret!"
end
def accessing_protected
"Will this work? " + a_protected_method
end
def eat_more_than(other)
# accessing other instance's protected method from the same class
daily_diet < other.daily_diet
"#{name} eats more than #{other.name}"
end
def boy
gender_method("boy") # accessing private setter method
end
protected
def daily_diet
age * 2 # the younger, the more they have to eat
end
def a_protected_method
"Yes, I'm protected!"
end
private
attr_writer :gender
def gender_method(gender)
self.gender = gender # private setter method requires self
"#{name} is a #{gender}"
end
def human_years
age * 8
end
end
# Create the first object of Dog
blake = Dog.new("Blake", 5)
p blake.accessing_private # "Blake in human years is 16. This is secret!"
p blake.accessing_protected # "Will this work? Yes, I'm protected!"
# Create the second object of Dog
jackson = Dog.new("Jackson", 1)
# Below, protected methods from different objects of the same type/class
# are proven to share access
p jackson.eat_more_than(blake) # true -> "Jackson eats more than Blake"
# Below, accessing private setter method through a public method.
p blake.boy # Blake is a boy
回答6:
I think breaking down an explicit receiver is what is important if your having trouble grasping the concept.
An explicit receiver is an object that is accepting a message.
person.get_name
person is the receiver and the method "get_name" is giving instructions to the object "person" to perform the method "get_name".
class Person
attr_accessor :first_name, :last_name
def initialize(first_name, last_name)
@first_name = first_name
@last_name = last_name
puts "And #{phone_number}" # Private method called when initialized
end
private
def phone_number
return "XXX-XXX-XXXX"
end
end
p p1 = Person.new("mike", "jones")
p p1.phone_number # Not within the context of the object instance.
When a method is private, it can only be used by other methods inside the object in whose class it is defined.
回答7:
Studying the information I've taken from here, I extended explanations through errors, and for my opinion, helps to understand why and how to use protected and not private.
1) Protected:
The line num 12 crash because the parameter received is from another class, the error message is clear:
v.rb:12:in `==': undefined method `sku' for "Object of another class ==> crash":String (NoMethodError)
2) Private:
If remove self from line 8 and 12, and I change protected for private, crash because in line 12, other doesn't know what sku is:
v.rb:12:in `==': private method `sku' called for #<Product:0x00000001574e68 @name="Bread", @quantity=1> (NoMethodError)
The program:
class Product
attr_accessor :name, :quantity
def initialize(name)
@name = name
@quantity = 1
puts "The SKU is #{self.sku}"
end
def == (other)
self.sku == other.sku
end
protected
def sku
name.crypt("yo")
end
end
milk1 = Product.new("Milk")
milk2 = Product.new("Milk")
bread = Product.new("Bread")
puts milk1 == bread
puts milk1 == milk2
puts milk1 == "Object of another class ==> crash"
来源：https://stackoverflow.com/questions/9882754/what-are-the-differences-between-private-public-and-protected-methods