Question about "protected" and "private"

[Fasun Lau]

[Note: parts of this message were removed to make it a legal post.]

Hi, all
I'm reading "Programming Ruby", and have some questions about "protected"
and "private" of class.

In "Programming Ruby" :

Protected methods can be invoked only by objects of the defining class and
its subclasses. Access is kept within the family.
Private methods cannot be called with an explicit receiver. Because you
cannot specify an object when using them, private methods can be called only
in the defining class and by direct descendents within that same object.

The difference between "protected" and "private" is
fairly subtle, *and is different in Ruby than in most common OO languages*. If
a method is protected, it may be called by any instance of the defining
class or its subclasses. If a method is private, it may be called only
within the context of the calling object---it is never possible to access
another object's private methods directly, even if the object is of the same
class as the caller.

I was tried it with the follow code:

C:\Users\v-fsliu>ruby -v
ruby 1.8.6 (2008-08-11 patchlevel 287) [i386-mswin32]

C:\Users\v-fsliu>irb
irb(main):001:0> class Demo
irb(main):002:1> def call1
irb(main):003:2> method1 + "Object ID: " + self.object_id.to_s
irb(main):004:2> end
irb(main):005:1>
irb(main):006:1* def call2
irb(main):007:2> method2 + "Object ID: " + self.object_id.to_s
irb(main):008:2> end
irb(main):009:1>
irb(main):010:1* protected
irb(main):011:1> def method1
irb(main):012:2> "Hello, Ruby! "
irb(main):013:2> end
irb(main):014:1>
irb(main):015:1* private
irb(main):016:1> def method2
irb(main):017:2> "Hi, Ruby! "
irb(main):018:2> end
irb(main):019:1> end
=> nil
irb(main):020:0>
irb(main):021:0* class NewDemo < Demo
irb(main):022:1> def newcall
irb(main):023:2> newmethod + "Object ID: " + self.object_id.to_s
irb(main):024:2> end
irb(main):025:1>
irb(main):026:1* protected
irb(main):027:1> def newmethod
irb(main):028:2> "Hello, new demo! "
irb(main):029:2> end
irb(main):030:1> end
=> nil
irb(main):031:0> demo = Demo.new
=> #<Demo:0x3ae9284>
irb(main):032:0> demo.call1
=> "Hello, Ruby! Object ID: 30886210"
irb(main):033:0> demo.call2
=> "Hi, Ruby! Object ID: 30886210"
irb(main):034:0> demo.method1
NoMethodError: protected method `method1' called for #<Demo:0x3ae9284>
from (irb):34
from :0
irb(main):035:0> newDemo = NewDemo.new
=> #<NewDemo:0x3ae2a24>
irb(main):036:0> newDemo.newcall
=> "Hello, new demo! Object ID: 30872850"
irb(main):037:0> newDemo.newmethod
NoMethodError: protected method `newmethod' called for #<NewDemo:0x3ae2a24>
from (irb):37
from :0
irb(main):038:0>

My questions are:
1. It seems that protected methodcalled by any instance of the defining
class or its subclasses.
2. what's the different in Ruby than in most common OO languages.

Thanks and best regards!

 

[Shaun Keller]

In most OO languages, private methods can only be called from within the
defining class.

ie

class MyClass

def method1
method2
end

private

def method2
end
end

A protected method can also be called from subclasses. So if method2 was
protected, and MySubClass < Class, then method2 could be called from
within MySubClass.


In ruby, private and protected methods can be called by any instance of
the defining class or its subclasses, but private methods cannot have an
explicit receiver.

In short, the difference is that protected methods can be called from
within a class on ANY OBJECT of that class or its subclasses.

See http://blog.zerosum.org/2007/11/22/ruby-method-visibility for more
details

 

[Fasun Lau]

[Note: parts of this message were removed to make it a legal post.]

Got it! Thanks very much!

 

[7stud --]

irb(main):020:0>
irb(main):021:0* class NewDemo < Demo
irb(main):022:1> def newcall
irb(main):023:2> newmethod + "Object ID: " + self.object_id.to_s
irb(main):024:2> end
irb(main):025:1>
irb(main):026:1* protected
irb(main):027:1> def newmethod
irb(main):028:2> "Hello, new demo! "
irb(main):029:2> end
irb(main):030:1> end
=> nil

irb(main):035:0> newDemo = NewDemo.new
=> #<NewDemo:0x3ae2a24>
irb(main):036:0> newDemo.newcall
=> "Hello, new demo! Object ID: 30872850"
irb(main):037:0> newDemo.newmethod
NoMethodError: protected method `newmethod' called for

Your second example would have worked the same if you hadn't inherited
from Demo, so it doesn't demonstrate anything new.


Here are some examples that may prove more illustrative:

class Demo

def democall(obj)
protected_meth #d1 is calling this method
private_meth #d1 is calling this method

obj.protected_meth
obj.private_meth
end

protected
def protected_meth
puts "hello from protected_meth"
end

private
def private_meth
puts "hello from private_meth"
end
end


d1 = Demo.new
d2 = Demo.new
d1.democall(d2)

--output:--
hello from protected_meth
hello from private_meth
hello from protected_meth
r1test.rb:7:in `call': private method `private_meth' called for
#<Demo:0x24dec> (NoMethodError)
from r1test.rb:23


==========


Here is an example with some inheritance thrown in:


class Demo
protected
def protected_meth
puts "hello from protected_meth"
end

private
def private_meth
puts "hello from private_meth"
end
end


class Bird < Demo
def birdcall(obj)
protected_meth #d1 is calling this method
private_meth #d1 is calling this method

obj.protected_meth
end
end

b1 = Bird.new
b2 = Bird.new
b1.birdcall(b2)

--output:--
hello from protected_meth
hello from private_meth
hello from protected_meth

In a language like C++, private methods are not accessible in a
subclass, so the call to private_meth in Bird would cause an error.

Note that self is always the object that called the method. For
example, in this example b1 is calling the method 'call', so inside the
call method self=b1. And when methods are not called with an object
name in front of the method, the implicit caller is self. In ruby, they
call the object that is calling the method "the receiver".

 

[7stud --]

Here is an example with some inheritance thrown in:


class Demo
protected
def protected_meth
puts "hello from protected_meth"
end

private
def private_meth
puts "hello from private_meth"
end
end


class Bird < Demo
def birdcall(obj)
protected_meth #d1 is calling this method
private_meth #d1 is calling this method

obj.protected_meth
end
end

b1 = Bird.new
b2 = Bird.new
b1.birdcall(b2)

--output:--
hello from protected_meth
hello from private_meth
hello from protected_meth

In a language like C++, private methods are not accessible in a
subclass, so the call to private_meth in Bird would cause an error.

Note that self is always the object that called the method. For
example, in this example b1 is calling the method 'call', so inside the
call method self=b1. And when methods are not called with an object
name in front of the method, the implicit caller is self. In ruby, they
call the object that is calling the method "the receiver".

Oh boy. Corrections:

class Demo
protected
def protected_meth
puts "hello from protected_meth"
end

private
def private_meth
puts "hello from private_meth"
end
end


class Bird < Demo
def birdcall(obj)
protected_meth #b1 is calling this method
private_meth #b1 is calling this method

obj.protected_meth
end
end

b1 = Bird.new
b2 = Bird.new
b1.birdcall(b2)

--output:--
hello from protected_meth
hello from private_meth
hello from protected_meth

In a language like C++, private methods are not accessible in a
subclass, so the call to private_meth in Bird would cause an error.

Note that self is always the object that called the method. For
example, in this example b1 is calling the method birdcall, so inside
the
birdcall method self=b1. And when methods are not called with an object
name in front of the method, the implicit caller is self. In ruby, they
call the object that is calling the method "the receiver".