Re: Question about "protected" and "private"

Fasun Lau wrote:
> 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".







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