Equivalent of attr_accessor for class variables?

ruby-talk
1

It is simple to set a class constant in ruby:

  class Foo
    CLASS_CONSTANT = "never to change"
  end

And accessed like this:
  Foo::CLASS_CONSTANT

However, I'd like to be able to create a class variable that is easily
accessible (read/write) by outside users without having to write class
accessor methods. How can I do this?

A really ugly way to set the variable without an accessor would be
this:
  class Foo; @@class_constant = "newvalue" end
but that is write only.

The syntax:
  Foo.class_constant = "newvalue"
would be preferable, but how to do this without writing accessor
methods?

Thanks

2

class C
     class << self
       attr_accessor 'class_constant'
     end
   end

-a

···

On Fri, 14 Jul 2006, bwv549 wrote:

It is simple to set a class constant in ruby:

class Foo
   CLASS_CONSTANT = "never to change"
end

And accessed like this:
Foo::CLASS_CONSTANT

However, I'd like to be able to create a class variable that is easily
accessible (read/write) by outside users without having to write class
accessor methods. How can I do this?

A really ugly way to set the variable without an accessor would be
this:
class Foo; @@class_constant = "newvalue" end
but that is write only.

The syntax:
Foo.class_constant = "newvalue"
would be preferable, but how to do this without writing accessor
methods?

Thanks

--
suffering increases your inner strength. also, the wishing for suffering
makes the suffering disappear.
- h.h. the 14th dali lama

3

Hi --

···

On Fri, 14 Jul 2006, bwv549 wrote:

It is simple to set a class constant in ruby:

class Foo
   CLASS_CONSTANT = "never to change"
end

And accessed like this:
Foo::CLASS_CONSTANT

However, I'd like to be able to create a class variable that is easily
accessible (read/write) by outside users without having to write class
accessor methods. How can I do this?

A really ugly way to set the variable without an accessor would be
this:
class Foo; @@class_constant = "newvalue" end
but that is write only.

The syntax:
Foo.class_constant = "newvalue"
would be preferable, but how to do this without writing accessor
methods?

One way or another, if you want a class variable set when you call the
method class_constant=, then that method has to set the variable.

See Ara's answer; you're almost certainly better off using a real
attr_accessor operation on the class object. (Yes, as I pointed out,
it doesn't match the question exactly, but it's an improvement :slight_smile:

David

--
http://www.rubypowerandlight.com => Ruby/Rails training & consultancy
http://www.manning.com/black => RUBY FOR RAILS (reviewed on
                                     Slashdot, 7/12/2006!)
http://dablog.rubypal.com => D[avid ]A[. ]B[lack's][ Web]log
dblack@wobblini.net => me

4

Here's the Rails implementation if you just want to see one working
implementation. Note that it allows you to access the class attribute
from the class and from instances (which is similiar to static members
in Java, actually).

http://dev.rubyonrails.org/svn/rails/trunk/activesupport/lib/active_support/core_ext/class/attribute_accessors.rb

- Rob

···

On 7/13/06, bwv549 <jtprince@gmail.com> wrote:

It is simple to set a class constant in ruby:

  class Foo
    CLASS_CONSTANT = "never to change"
  end

And accessed like this:
  Foo::CLASS_CONSTANT

However, I'd like to be able to create a class variable that is easily
accessible (read/write) by outside users without having to write class
accessor methods. How can I do this?

A really ugly way to set the variable without an accessor would be
this:
  class Foo; @@class_constant = "newvalue" end
but that is write only.

The syntax:
  Foo.class_constant = "newvalue"
would be preferable, but how to do this without writing accessor
methods?

Thanks

--
http://www.robsanheim.com


http://www.ajaxian.com

5

   class C
     class << self
       attr_accessor 'class_constant'
     end
   end

For an intermediate ruby user, could you explain what is happening in
the line: 'class << self'

Thanks

6

Hi --

···

On Fri, 14 Jul 2006, ara.t.howard@noaa.gov wrote:

On Fri, 14 Jul 2006, bwv549 wrote:

It is simple to set a class constant in ruby:

class Foo
   CLASS_CONSTANT = "never to change"
end

And accessed like this:
Foo::CLASS_CONSTANT

However, I'd like to be able to create a class variable that is easily
accessible (read/write) by outside users without having to write class
accessor methods. How can I do this?

A really ugly way to set the variable without an accessor would be
this:
class Foo; @@class_constant = "newvalue" end
but that is write only.

The syntax:
Foo.class_constant = "newvalue"
would be preferable, but how to do this without writing accessor
methods?

Thanks

class C
   class << self
     attr_accessor 'class_constant'
   end
end

That doesn't use class variables, though -- which is an asset, as far
as I'm concerned :slight_smile: but doesn't exactly match the question.

David

--
http://www.rubypowerandlight.com => Ruby/Rails training & consultancy
http://www.manning.com/black => RUBY FOR RAILS (reviewed on
                                     Slashdot, 7/12/2006!)
http://dablog.rubypal.com => D[avid ]A[. ]B[lack's][ Web]log
dblack@wobblini.net => me

7

require 'facet/functor'

  module Kernel

    # Provides access to an object's metaclass (ie. singleton)
    # by-passsing access provisions. So for example:

···

ara.t.howard@noaa.gov wrote:

   class C
     class << self
       attr_accessor 'class_constant'
     end
   end

    #
    # class X
    # meta.attr_accesser :a
    # end
    #
    # X.a = 1
    # X.a #=> 1

    def meta
      @_meta_functor ||= Functor.new do |op,*args|
        (class << self; self; end).send(op,*args)
      end
    end

  end

T.

8

search the archives - this comes up about once per week. for now, suffice it
do say that

     class C
       class << self

         # everthing here is at class scope

         @a_class_instance_variable = 42

         def a_class_method() @a_class_instance_variable end

         alias_method 'one_class_method', 'to_another'

         extend ABunchOfClassMethods

       end
     end

regards.

-a

···

On Fri, 14 Jul 2006, bwv549 wrote:

   class C
     class << self
       attr_accessor 'class_constant'
     end
   end

For an intermediate ruby user, could you explain what is happening in
the line: 'class << self'

--
suffering increases your inner strength. also, the wishing for suffering
makes the suffering disappear.
- h.h. the 14th dali lama

9

Hi --

   class C
     class << self
       attr_accessor 'class_constant'
     end
   end

For an intermediate ruby user, could you explain what is happening in
the line: 'class << self'

The expression:

   class << obj

puts you in a class definition block for the singleton class of obj.
The singleton class of obj is where obj's singleton methods live --
that is, the methods that only obj can call.

   obj = Object.new
   def obj.x
     "Singleton method on obj"
   end

   class << obj
     puts instance_methods(false) # methods defined in this class only
   end

Output: x

You can also define methods inside the class definition block, of
course:

   class << obj
     def y
       "Another singleton method on obj"
     end
   end

David

···

On Fri, 14 Jul 2006, bwv549 wrote:

--
http://www.rubypowerandlight.com => Ruby/Rails training & consultancy
http://www.manning.com/black => RUBY FOR RAILS (reviewed on
                                     Slashdot, 7/12/2006!)
http://dablog.rubypal.com => D[avid ]A[. ]B[lack's][ Web]log
dblack@wobblini.net => me

10

yeah - what he said :wink:

google the list for 'class instance variable' and 'class << self'

-a

···

On Fri, 14 Jul 2006 dblack@wobblini.net wrote:

That doesn't use class variables, though -- which is an asset, as far
as I'm concerned :slight_smile: but doesn't exactly match the question.

--
suffering increases your inner strength. also, the wishing for suffering
makes the suffering disappear.
- h.h. the 14th dali lama