Q) about singleton methods and access control

ruby-talk
1

This is what I’d “like” to do:
class A

def initialize()
@varInternalOnly = ‘test’
end

def A.createSpecialA()
testObj = A.new
testObj.internalHelperMethod() # [Fails because internalHelperMethod is private - even though this is a class A method. :]
return testObj
end

private
def internalHelperMethod()
@varInternalOnly = ‘foo’
end
end

A.createSpecialA()

I would like to have my singleton methods (like in C++) have access to protected/private members of its own class. Without this, it’s not possible to have static (singleton) class methods
act as helpers (or in my case, alternative constructors) and access ‘private’ methods/variables of its own class type. To me at least, this reduces the usability of the access-control modifiers quite a bit.

···

-----Original Message-----
From: Christoph [mailto:chr_news@gmx.net]
Sent: Monday, August 12, 2002 3:09 PM
To: ruby-talk ML
Subject: Re: Q) about singleton methods and access control

“Patrick Bennett” wrote,
…

Recently, I was rather surprised to learn that singleton methods aren’t
able to access protected/private members of a class.

Hm I not quite sure to what access problem you are
refering to. Could illustruted this in an example. Afaikt
every thing seems to be oaky - for example, I get the
output

“fine”
“fine”
“fine”
protected method sometimes_fine' called for #<B:0x284ab10> protected method bad’ called for #<B:0x284ab10>
me everything seems to be okay - for example

when running the the test script

illustrate protected

class A
def okay(x) x.fine end
def sometimes_okay(x) x.sometimes_fine end
def never_okay(x) x.bad end

def sometimes_fine;   p "fine" end
protected
def fine;             p "fine" end

end

class B < A
protected
def bad
p “bad”
end
end

a = A.new
b = B.new

a.okay b

class << a
def okay(x)
x.fine
end
end

a.okay b
a.sometimes_okay b

class << b
protected
def sometimes_fine
super
end
end

begin
a.sometimes_okay b
rescue NameError => mes
puts mes
end

begin
a.never_okay b
rescue NameError => mes
puts mes
end

Unfortunately, I’d like to have a class with multiple singleton
‘constructor’ methods. Each would return a new instance of the class,
initialized properly. None of these actions would really make sense to
have in Initialize.
Well, if the singleton method can’t access protected/private
members/methods of the class, then how can I have protected portions of
the class (to avoid problems with normal users of the class), and still
have
a signleton method create intiailized instances of the class?

Hopefully I’m completely missing the boat on this one and there’s a
solution to my problem.

Hm, I am quite sure that this problem can be resolved (in the
worst case just use the “send(:name_of_private_method,*meth_args)
trick”- Again could illustrate your problem with a code example?

/Christoph

2

“Bennett, Patrick” Patrick.Bennett@inin.com wrote in message
news:58A40B582A73F54AB5D8739C0678F3A802320696@i3exchange.i3domain.inin.com…
…

This is what I’d “like” to do:
class A

def initialize()
@varInternalOnly = ‘test’
end

def A.createSpecialA()
testObj = A.new
testObj.internalHelperMethod()

internalHelperMethod is an ``instance method’’ not a Class method
(see below). Also note quite generally that private methods can only
be invoked in the implicit receiver ( = self ) form

      some_private_method(*args)

Note that even

     self.some_private_method(*args).

you will get you into trouble. Anyway, as the short term fix you can
simply write

    testObj.send(:internalHelperMethod)

[Fails because internalHelperMethod is private -

even though this is a class A method. :]
return testObj
end

private
def internalHelperMethod()
@varInternalOnly = ‘foo’
end
end

A.createSpecialA()

I would like to have my singleton methods (like in C++) have access to
protected/private members of its own class. Without this, it’s not possible
to have static (singleton) class methods
act as helpers (or in my case, alternative constructors) and access
‘private’ methods/variables of its own class type. To me at least, this
reduces the usability of the access-control modifiers quite a bit.

Note that C++ static (class) method are quite different to Ruby’s class
methods
(disregarding the fact that they share the same name and mainly due to the
fact that in C++ does not have Class objects). In C++ you can invoke
a class (== static) method in the receiver form

SomeClass::class_meth(…)

or instance form

any_class_instance.class_meth(…)

Possible receivers of a Ruby class method

def SomeClass.class_meth(…)
# body
end

(in Ruby terminology ``a singleton method of the Class instance
SomeClass’') is the Class instance SomeClass itself or a Class
descendant of SomeClass.

/Christoph

3

“Christoph” wrote
…

(disregarding the fact that they share the same name and mainly due to
the
fact that in C++ does not have Class objects). In C++ you can invoke
a class (== static) method in the receiver form

this should be ``receiverless’’ form

/Christoph