Singleton class - instance or class variables?

ruby-talk
1

When using a singleton class (one that includes Singleton, not an eigenclass), can anyone think of any advantage/disadvantage to using class variables over instance variables (or visa versa) to access data in the class?

Thanks,
Caleb

2

Caleb Tennis wrote:

When using a singleton class (one that includes Singleton, not an
eigenclass), can anyone think of any advantage/disadvantage to using
class variables over instance variables (or visa versa) to access
data in the class?

I would recommend using class instance variables,
but the only reason I can think relates to resource
control and instance instantiation. For the typical
case one could just as well use classes and class
instance methods.

Thanks,
Caleb

E

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3

Caleb Tennis wrote:

When using a singleton class (one that includes Singleton, not an eigenclass), can anyone think of any advantage/disadvantage to using class variables over instance variables (or visa versa) to access data in the class?

In answer to this question and the one about thread-specific singletons: another approach to consider is using a dependency injection framework. They all support singleton services, and at least some of them support services that return one unique object per thread.

One disadvantage to using class variables (@@var) is that they are (in the current ruby) per-hierarchy, not per class. So a subclass will share the value of @@var. This is, I've heard, going to change in ruby 2.0.

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4

inheritence will break with either.

with @@var then

   class Singleton
     @@var = 42
   end

   class SingletonII < Singleton
     @@var = 'forty-two'
   end

   class Singleton
     p @@var #=> 'forty-two'
   end

but with @var

   class Singleton
     @var = 42
   end

   class SingletonII < Singleton
   end

   class SingletonII
     p @var #=> nil
   end

my traits lib solves this - if inheritence of class data is important. it may
not be.

regards.

-a

···

On Tue, 20 Dec 2005, Caleb Tennis wrote:

When using a singleton class (one that includes Singleton, not an
eigenclass), can anyone think of any advantage/disadvantage to using class
variables over instance variables (or visa versa) to access data in the
class?

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