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comp.lang.ruby

accessor for Class Variable

Leonardo Francalanci

4/21/2005 2:03:00 PM

I know it's a stupid question, but I can't find the answer...

Isn't there another way to have an accessor for a class variable than a
"def"??? Something like "attr_reader"?

class Outing < ActiveRecord::Base
@@planingList = [ "YES", "NO", "YES/NO"]

def Outing.wind_directionList
return @@wind_directions
end
end


If the answer is somewhere in the docs, please point me to it.


Thank you
14 Answers

Robert Klemme

4/21/2005 2:13:00 PM

0


"Leonardo Francalanci" <lfrancalanci@simtel.it> schrieb im Newsbeitrag
news:4267B13A.20905@simtel.it...
> I know it's a stupid question, but I can't find the answer...
>
> Isn't there another way to have an accessor for a class variable than a
> "def"??? Something like "attr_reader"?

Not AFAIK.

> class Outing < ActiveRecord::Base
> @@planingList = [ "YES", "NO", "YES/NO"]
>
> def Outing.wind_directionList
> return @@wind_directions
> end
> end

In your case you could as well do

class Outing < ActiveRecord::Base
@planingList = [ "YES", "NO", "YES/NO"]

class<<self; attr_reader :planingList end
end

>> Outing.planingList
=> ["YES", "NO", "YES/NO"]

Class variables are really only useful if you want to access them from
instances and from the class in a similar way (via "@@var_name").
Otherwise they have more drawbacks than pros so I avoid to use them
whenever I can. (Can't really remember a case where I actually needed
them.)

Kind regards

robert

gabriele renzi

4/21/2005 2:19:00 PM

0

Leonardo Francalanci ha scritto:
> I know it's a stupid question, but I can't find the answer...
>
> Isn't there another way to have an accessor for a class variable than a
> "def"??? Something like "attr_reader"?
>
> class Outing < ActiveRecord::Base
> @@planingList = [ "YES", "NO", "YES/NO"]
>
> def Outing.wind_directionList
> return @@wind_directions
> end
> end
>
>
> If the answer is somewhere in the docs, please point me to it.

AFAIK there is not, I think you're supposed to access a class variable
directly from instances and class methods, and to generally use a
constant when possible, wich is visible from outside.
In general the use case for class attributes accessible from the outside
is not that much (in my own experience).

Anyway you should be able to write them easily with something like
(untested):

class Class
def class_attr sym
module_eval "def self.#{sym}() @@#{sym} end"
module_eval "def self.#{sym}=(x) @@#{sym}=x end"
end
end

class C
class_attr :foo
end

Leonardo Francalanci

4/21/2005 2:32:00 PM

0

> class<<self; attr_reader :planingList end


Aaargh... what is that? What does it mean?
Sorry, I'm coming from Java...

Leonardo Francalanci

4/21/2005 2:42:00 PM

0

> AFAIK there is not, I think you're supposed to access a class variable
> directly from instances and class methods, and to generally use a
> constant when possible, wich is visible from outside.
> In general the use case for class attributes accessible from the outside
> is not that much (in my own experience).

Ok, got it, I changed them into constants.


> Anyway you should be able to write them easily with something like
> (untested):
>
> class Class
> def class_attr sym
> module_eval "def self.#{sym}() @@#{sym} end"
> module_eval "def self.#{sym}=(x) @@#{sym}=x end"
> end
> end
>
> class C
> class_attr :foo
> end

Wow, looks like I have to learn a lot of stuff...

gabriele renzi

4/21/2005 2:50:00 PM

0

Leonardo Francalanci ha scritto:
<snip>
> Wow, looks like I have to learn a lot of stuff...

not that much, really, there are a few concepts in ruby with a lot of
usages :)
Anyway, assuming you're italian, I thought I'd point out that there is a
(low traffic) mailing list for italian people that you can subscribe from
http://ada2.uni...

ciao

Robert Klemme

4/21/2005 3:23:00 PM

0


"Leonardo Francalanci" <lfrancalanci@simtel.it> schrieb im Newsbeitrag
news:4267B7A4.6060708@simtel.it...
> > class<<self; attr_reader :planingList end
>
>
> Aaargh... what is that? What does it mean?
> Sorry, I'm coming from Java...

Then you'll never know... *evil grin*

Ok, class << x ... end is the singleton class of that instance. You can
use that to define methods for a certain instance only (in this case it's
the class). Some examples:

17:20:35 [robert.klemme]: irb
irb(main):001:0> obj = Object.new
=> #<Object:0x101949b8>
irb(main):002:0> def obj.foo() "foo" end
=> nil
irb(main):003:0> obj.foo
=> "foo"
irb(main):004:0> class <<obj
irb(main):005:1> def bar() "<<" + foo() + ">>" end
irb(main):006:1> end
=> nil
irb(main):007:0> obj.bar
=> "<<foo>>"

As a class is an object much like every other object, you can also define
singleton methods for them:

irb(main):008:0> def String.foo() "foo" end
=> nil
irb(main):009:0> String.foo
=> "foo"
irb(main):010:0> class <<String
irb(main):011:1> def bar() "<<" + foo() + ">>" end
irb(main):012:1> end
=> nil
irb(main):013:0> String.bar
=> "<<foo>>"
irb(main):014:0>

Does that help?

Kind regards

robert

Lyndon Samson

4/21/2005 3:26:00 PM

0

On 4/22/05, Robert Klemme <bob.news@gmx.net> wrote:
>
>
> Class variables are really only useful if you want to access them from
> instances and from the class in a similar way (via "@@var_name").
> Otherwise they have more drawbacks than pros so I avoid to use them
> whenever I can. (Can't really remember a case where I actually needed
> them.)
>
What about situations like the following ( taken from an attempt at an
old rubyquiz )

class Op
@@lookup = {}
def Op.register(newOp, newOpClass)
@@lookup[newOp] = newOpClass
end
end

class AddOp < Op
...
end
class SubOp < Op
...
end
class MulOp < Op
...
end
class DivOp < Op
...
end

Op.register("+", AddOp)
Op.register("-", SubOp)
Op.register("*", MulOp)
Op.register("/", DivOp)

They are not really OO 'clean' but better than globals surely?


--
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Leonardo Francalanci

4/21/2005 3:58:00 PM

0

>>Sorry, I'm coming from Java...
>
> Then you'll never know... *evil grin*

I knew I shouldn't have said that ;)


> Ok, class << x ... end is the singleton class of that instance. You can
> use that to define methods for a certain instance only (in this case it's
> the class).

> As a class is an object much like every other object, you can also define
> singleton methods for them

Ok, thanks!

Leonardo Francalanci

4/21/2005 4:00:00 PM

0

> Anyway, assuming you're italian, I thought I'd point out that there is a
> (low traffic) mailing list for italian people that you can subscribe from
> http://ada2.uni...

thanks,

ciao!

Nicholas Seckar

4/21/2005 4:14:00 PM

0

On 4/21/05, Leonardo Francalanci <lfrancalanci@simtel.it> wrote:
> I know it's a stupid question, but I can't find the answer...
>
> Isn't there another way to have an accessor for a class variable than a
> "def"??? Something like "attr_reader"?

Actually, Rails includes a mechanism to allow class attributes. I
believe the usage is

class Whatever
@@varname = "Default Value"
cattr_accessor :varname
end

I'm not sure if it is documented anywhere though. You can see some
examples here:
http://dev.rubyonrails.com/file/trunk/actionpack/lib/action_controll...