Newbie question : private / public methods

Ruby
1

Hi,

I’m new both to this mailing list and to Ruby. Here’s an example,
question
will come later.

private.rb

class PrivateMethodTest
private
def privateMethod(the_string)
'haha! ’ + the_string
end

public
def publicMethod(the_string)
    self.privateMethod(the_string)
end

end

private_test.rb

require ‘private.rb’
require ‘test/unit’

class PrivateMethodTestTest < Test::Unit::TestCase

def test_publicMethod
    var = PrivateMethodTest.new()
    assert_equal('haha! test', var.publicMethod('test'))
end

end

Now the question: why is it, that when the self. (bolded in the code) is
there, the test fails, whereas if I remove it, the test is successful?
This
is the failure message:

  1. Error:
    test_publicMethod(PrivateMethodTestTest):
    NoMethodError: private method privateMethod' called for #<PrivateMethodTest:0x2b0876d805a0> ./private.rb:9:inpublicMethod’
    private_test.rb:8:in `test_publicMethod’

What am I missing?

Thanks for any help and sorry if this question is silly :slight_smile:
Cheers,
Chris

2

I’ve get some other error form this code. Check at:
http://img137.imageshack.us/img137/5754/errorxb0.jpg

2008/3/21, krzysieq Gazeta.pl [email protected]:

3

On Friday 21 March 2008, krzysieq Gazeta.pl wrote:

    'haha! ' + the_string

require ‘private.rb’
Now the question: why is it, that when the self. (bolded in the code) is
What am I missing?

Thanks for any help and sorry if this question is silly :slight_smile:
Cheers,
Chris

Ruby implements private methods as methods which can’t be called using
an
explicit receiver (i.e, using the form obj.method_name), but only using
the
implicit receiver (that is, method_name). Since the implicit receiver is
always self, this ensures that private instance methods can only be
called by
instance methods of that class (or of derived classes) and can’t be
accessed
by other instances of the same class. An unpleasant side effect of this
is
that you can’t call private methods using the explicit receiver even
when that
receiver is equal to the implicit one (that is, self), which is the
situation
you discovered.

4

Then it enforces habits, which I’ve been encouraged to develop anyways -
my
managers always told me not to use this.doSomething whenever there is no
risk of ambiguity. Don’t You think however, that this is quite harsh - I
mean, don’t You think this might get someone into bigger trouble
sometime? I
don’t know Ruby well enough to think of an example yet, but something
tells
me this “feature” could be tricky…

Mateusz: You changed the name of the test class, that’s why the error.

Cheers,
Chris

2008/3/21, Stefano C. [email protected]:

5

Well, my story is slightly different. I encountered this in a situation,
when I had a private method to do some operations, and a number of
public
methods to use that one. And these public methods can’t call the private
using self. It’s just something I’m used to doing, and perhaps now I
will
need to get unused to it :slight_smile: Maybe it’s not dangerous at all, it’s just
different from what I’ve seen so far. And people tend to fear what they
do
not understand…

Thanks for explanations.
Cheers,
Chris

2008/3/21, Stefano C. [email protected]:

6

On Friday 21 March 2008, Krzysieq wrote:

Then it enforces habits, which I’ve been encouraged to develop anyways - my
managers always told me not to use this.doSomething whenever there is no
risk of ambiguity. Don’t You think however, that this is quite harsh - I
mean, don’t You think this might get someone into bigger trouble sometime?
I don’t know Ruby well enough to think of an example yet, but something
tells me this “feature” could be tricky…

Do you refer to being unable to write self.a_private_method ? I don’t
think it
can cause any trouble. It does prevent you from writing code like the
following:

class C

def initialize
@x = 2
end

def a_method value
self.x = value
end

private

def x= value
#set the value of @x to something and do some other thing
end

end

What the code is trying to do is define a private method, x= which
changes the
value of the instance variable @x and does some extra processing, then
use
this method every time it needs to change the value of @x. However, this
kind
of setter methods (those ending in =) need to be called with an explicit
receiver, since otherwise ruby thinks we want to create the local
variable x.
But, since x= is a private method, it can’t be called with an explicit
receiver, either.

Of course, this is not a big problem: we have to call the method with
another
name (set_x, for example) and everything works again, even if it’s not
as
pretty as it would have been with x=.

Stefano

7

On Fri, 21 Mar 2008 23:29:08 +0900, Mateusz T. wrote:

I’ve get some other error form this code. Check at:
http://img137.imageshack.us/img137/5754/errorxb0.jpg

see:

http://support.microsoft.com/kb/317591

for how enable quick edit and copy from the command prompt in Windows :slight_smile:

-Thufir