Singleton class - instance or class variables?

Ruby
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 T. 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

3

Caleb T. 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.

4

On Tue, 20 Dec 2005, Caleb T. 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?

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