From: Stefano Crocco <stefano.crocco@...>
Date: 2007-06-29T16:24:23+09:00
Subject: Re: Calling private method in ruby

Alle venerd狸 29 giugno 2007, Alin Popa ha scritto:
> Hi there,
>
> I have a little problem regarding private methods in Ruby.
> I have the following case:
>
> class MyClass
>    private
>    def my_method
>       puts "something from my method"
>    end
> end
>
> myClass = MyClass.new
> begin
>    myClass.my_method
> rescue
>    puts "Method cannot be called because is private"
> end
>
> myClass.send("my_method")
>
> So, can anyone help me by explaining in a logical way, what is the point
> of private methods in ruby since I can call it using "send" ?
>
> Thanks in advance,
>
> Alin

In general, the idea behind private methods is that they can't be called from 
outside the object they belong to. In ruby, this is achieved by allowing 
calling them only without an explicit receiver. This means that the you can 
call them when the receiver of the method is the implicit receiver (self), 
that is, only from within instance methods:

class MyClass
  
  def private_method
    puts "This is private_method"
  end
  private :private_method

  def test_method
    puts "This test method is abouto to call private_method"
    private_method
  end

end

obj = MyClass.new
obj.test_method
puts "---"
obj.private_method

The output is:
This test method is abouto to call private_method
This is a private method
---
./script.rb:20: private method `private_method' called for 
#<MyClass:0xb7c171d8> (NoMethodError)

Note that no receiver is allowed for private method, not even self. So, 
test_method couldn't have been written as:

  def test_method
    puts "This test method is abouto to call private_method"
    self.private_method
  end

Doing this would again result in a NoMethodError exception.

I hope this helps

Stefano