# Newbie question : private / public methods

**URL:** <https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570>\
**Category:** Ruby\
**Created:** [March 21, 2008, 3:15pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570 "2008-03-21T15:15:21Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 21, 2008, 3:15pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/1 "2008-03-21T15:15:21Z")

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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:in`publicMethod’  
private\_test.rb:8:in `test\_publicMethod’

What am I missing?

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

---

<div class="post-metadata">

**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 21, 2008, 3:30pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/2 "2008-03-21T15:30:09Z")

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

2008/3/21, krzysieq Gazeta.pl [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

---

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**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 21, 2008, 3:37pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/3 "2008-03-21T15:37:15Z")

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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 🙂  
> 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.

---

<div class="post-metadata">

**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 21, 2008, 4:04pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/4 "2008-03-21T16:04:02Z")

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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. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 21, 2008, 4:30pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/5 "2008-03-21T16:30:32Z")

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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 🙂 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. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 21, 2008, 4:18pm UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/6 "2008-03-21T16:18:09Z")

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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

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**Author:** ![krzysieq\_Gazeta.pl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/97f17d/32.png) [@krzysieq\_Gazeta.pl](https://www.ruby-forum.com/u/krzysieq_Gazeta.pl)\
**Post date:** [March 28, 2008, 8:00am UTC](https://www.ruby-forum.com/t/newbie-question-private-public-methods/132570/7 "2008-03-28T08:00:31Z")

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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](http://img137.imageshack.us/img137/5754/errorxb0.jpg)

see:

[http://support.microsoft.com/kb/317591](http://support.microsoft.com/kb/317591)

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

-Thufir