Is using attr_accessor for class variables possible?

[Gabriel Dragffy]

I know that attr_accessor works for accessing instance variables, but
it doesn't seem to work for class ones. What is the best way to set/
retrieve the value of a class variable?

Many thanks

Gabriel

 

[David A. Black]

Hi --

I know that attr_accessor works for accessing instance variables, but it
doesn't seem to work for class ones. What is the best way to set/retrieve the
value of a class variable?

@@var = 1
@@var



If you want to wrap them in methods:

class C
def C.var
@@var
end

def C.var=(x)
@@var = x
end
end

In Rails, there's a thing called "cattr" that does this automatically:

class C
cattr_acccessor :var
end

It's a little misleading, though. The term "attribute" (or attr)
refers to an attribute or property of an object. Class variables are
very promiscuous: they're shared among many objects (a class, its
descendants, all instances of all of those classes), so a class
variable is not really the right choice for an "attribute", and "attr"
is not the best name for wrappers around class variables.

If you want to represent state per class, the best way is to give your
class an instance variable or accessor:

class C
class << self # C's singleton class
attr_accessor :var
end
end


David

--
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* Intro to Ruby on Rails, Edison, NJ, October 23-26
* Advancing with Rails, Edison, NJ, November 6-9
Both taught by David A. Black.
See http://www.rubypal.com for more info!

 

[Gabriel Dragffy]

Hi --



@@var = 1
@@var

Good point, I mean access the class variable from outside the class
(above the class).

If you want to wrap them in methods:

class C
def C.var
@@var
end

def C.var=(x)
@@var = x
end
end

I was wondering if I should do this. I just remember attr_accessor
was a short cut for this, but only for instance variables.

In Rails, there's a thing called "cattr" that does this automatically:

class C
cattr_acccessor :var
end

It's a little misleading, though. The term "attribute" (or attr)
refers to an attribute or property of an object. Class variables are
very promiscuous: they're shared among many objects (a class, its
descendants, all instances of all of those classes), so a class
variable is not really the right choice for an "attribute", and "attr"
is not the best name for wrappers around class variables.

If you want to represent state per class, the best way is to give your
class an instance variable or accessor:

class C
class << self # C's singleton class
attr_accessor :var
end
end

I read about singletons in Pragmatic Programming, not entirely sure
how to make use of it though

Thank you very much for your advice.

Regards

Gabe

 

[Bertram Scharpf]

Hi,

Am Sonntag, 07. Okt 2007, 21:39:24 +0900 schrieb David A. Black:

If you want to represent state per class, the best way is to give your
class an instance variable or accessor:

class C
class << self # C's singleton class
attr_accessor :var
end
end

I do this all the time. But:

class D < C ; end

Always keep in mind that `D.var' will be something different
than `C.var'.

Bertram

 

[David A. Black]

Hi --

Hi,

Am Sonntag, 07. Okt 2007, 21:39:24 +0900 schrieb David A. Black:

I do this all the time. But:

class D < C ; end

Always keep in mind that `D.var' will be something different
than `C.var'.

That's the point of attr_accessor: to give easy access to per-object
state.


David

--
Upcoming training from Ruby Power and Light, LLC:
* Intro to Ruby on Rails, Edison, NJ, October 23-26
* Advancing with Rails, Edison, NJ, November 6-9
Both taught by David A. Black.
See http://www.rubypal.com for more info!

 

[Bertram Scharpf]

Hi,

Am Sonntag, 07. Okt 2007, 22:56:54 +0900 schrieb David A. Black:

That's the point of attr_accessor: to give easy access to per-object
state.

About a month ago I found myself doing even this:

class C
@x = "x-default"
class <<self
def x ; @x or superclass.x ; end
end
end

class D < C ; end
class E < D ; @x = "x-special" ; end

puts C.x
puts D.x
puts E.x

I still don't know what is better: using stable instance
methods (same value on every call) or doing it this way.

Bertram