Private methods in Ruby

ruby-talk
1

##Won't work method #1

class Foobar
  private
  def foo; p "Foobar"; end
end

class Baz < Foobar
  def blah; foo; end
end

baz = Baz.new
baz.foo

## Work method #2

class Foobar
  private
  def foo; p "Foobar"; end
end

class Baz < Foobar
  def blah; foo; end
end

baz = Baz.new
baz.blah

PickAxe2:

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.

So what happens in Method#2, why foo can be easily called, when called
from inside the class Baz? I mean, that method foo should be still
bound to instance baz...so in which context foo gets called in
Method#2.

···

--
There was only one Road; that it was like a great river: its springs
were at every doorstep, and every path was its tributary.

2

Hi --

···

On Tue, 12 Dec 2006, hemant wrote:

##Won't work method #1

class Foobar
private
def foo; p "Foobar"; end
end

class Baz < Foobar
def blah; foo; end
end

baz = Baz.new
baz.foo

## Work method #2

class Foobar
private
def foo; p "Foobar"; end
end

class Baz < Foobar
def blah; foo; end
end

baz = Baz.new
baz.blah

PickAxe2:

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.

So what happens in Method#2, why foo can be easily called, when called
from inside the class Baz? I mean, that method foo should be still
bound to instance baz...so in which context foo gets called in
Method#2.

"In the context of" an object means: "when 'self' is that object". In
the first example, your call to foo takes place when baz is not self.
In the second example, inside blah, self is indeed baz, so it's
allowed to call foo.

David

--
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A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black\)
    aka The Ruby book for Rails developers!
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

3

I'll take a stab at this...

since the instance/object baz is of class Baz, inherited from Foobar, it
has the method foo. When you call baz.blah, baz internally calls its
own foo method, which it is allowed to do.

the "even if the object is of the same class..." bit is a little
confusing, but means that if you (somehow) link baz and baz2 (two
different instances) they can't call each others private methods, even
though they are the same class.

hope that helps

Brian Broom

···

On Tue, 2006-12-12 at 08:28 +0900, hemant wrote:

##Won't work method #1

class Foobar
  private
  def foo; p "Foobar"; end
end

class Baz < Foobar
  def blah; foo; end
end

baz = Baz.new
baz.foo

## Work method #2

class Foobar
  private
  def foo; p "Foobar"; end
end

class Baz < Foobar
  def blah; foo; end
end

baz = Baz.new
baz.blah

PickAxe2:

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.

So what happens in Method#2, why foo can be easily called, when called
from inside the class Baz? I mean, that method foo should be still
bound to instance baz...so in which context foo gets called in
Method#2.

4

So can i rephrase this as, private methods can't be called from
outside class definition, because self is not bound to anything
outside the class. If my assertion is correct, then ok..I get the
idea.

Thanks

···

On 12/12/06, dblack@wobblini.net <dblack@wobblini.net> wrote:

Hi --

On Tue, 12 Dec 2006, hemant wrote:

> ##Won't work method #1
>
> class Foobar
> private
> def foo; p "Foobar"; end
> end
>
> class Baz < Foobar
> def blah; foo; end
> end
>
> baz = Baz.new
> baz.foo
>
> ## Work method #2
>
> class Foobar
> private
> def foo; p "Foobar"; end
> end
>
> class Baz < Foobar
> def blah; foo; end
> end
>
> baz = Baz.new
> baz.blah
>
> PickAxe2:
>
> 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.
>
> So what happens in Method#2, why foo can be easily called, when called
> from inside the class Baz? I mean, that method foo should be still
> bound to instance baz...so in which context foo gets called in
> Method#2.

"In the context of" an object means: "when 'self' is that object". In
the first example, your call to foo takes place when baz is not self.
In the second example, inside blah, self is indeed baz, so it's
allowed to call foo.

--
There was only one Road; that it was like a great river: its springs
were at every doorstep, and every path was its tributary.