From: Dave Thomas <Dave@...>
Date: 2000-05-11T00:06:19-05:00
Subject: [ruby-talk:02730] Re: Public/Private clarification

Albert Wagner <alwagner@tcac.net> writes:

> I have read the documentation and am now browsing sample code to learn
> more about ruby. The following code, export.rb, triggers some questions:
> 
> 1)  What does "public :printf" do in class Foo?

'printf' is a module function in Kernel, and the Kernel module is
included in class Object. The printf method is therefore available
throughout Ruby as either

   Kernel::printf "hello\n"        # module method
or
   printf "hello\n"                # instance method

However, printf is private, which means it must be called in
functional form (ie without a receiver). Something like

   self.printf ...

would raise an error.

In class Foo, printf is made public, which means it can then be called 
with an instance of Foo (or a subclass of Foo) as a receiver.

  class Foo
    public :printf
  end

  f = Foo.new
  f.printf "hello\n"

> 2)  I thought a module defined outside a class was, by default, attached
> to Object and was private, such as "def foobar" below.  Why is a
> redefinition of class Foo required to change foobar() to public?
> Why is the "public :foobar" statement not immediatly after the def
> of foobar?  Does this mean that public/private access to an given
> Object method be private in one subclass and public in another?

Maybe a different example might make it clearer.

class Base
  def aMethod
    puts "Got here"
  end
  private :aMethod
end

class Derived1 < Base
  public :aMethod
end

class Derived2 < Base
end

Derived1.new.aMethod   # -> Got here
Derived2.new.aMethod   # -> private method `aMethod' called...

# Now change the base class

class Base
  def aMethod
    puts "changed"
  end
end

Derived1.new.aMethod   # -> changed
Derived2.new.aMethod   # -> changed


If you're in class X and change the visibility of a method defined in
an ancestor class, Ruby effectively defines a proxy method in X which
calls the ancestor method. That proxy method is then given the
assigned visibility. That way, the same method can have different
visibilities in different subclasses.

Neat, eh?


Dave

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