Ruby – How to Ruby’s attr_accessor produce class variables or class instance variables instead of instance variables

class-instance-variablesclass-variablesinstance-variablesruby

If I have a class with an attr_accessor, it defaults to creating an instance variable along with the corresponding getters and setters. But instead of creating an instance variable, is there a way to get it to create a class variable or a class instance variable instead?

Best Solution

Like this:

class TYourClass
  class << self
    attr_accessor :class_instance_variable
  end
end

You can look at this as opening the metaclass of the class (of which the class itself is an instance) and adding an attribute to it.

attr_accessor is a method of class Class, it adds two methods to the class, one which reads the instance variable, and other that sets it. Here's a possible implementation:

class Class
  def my_attr_accessor(name)
    define_method name do
      instance_variable_get "@#{name}"
    end 
    define_method "#{name}=" do |new_val|
      instance_variable_set "@#{name}", new_val
    end
  end
end

Completely untested class attribute accessor:

class Class
  def class_attr_accessor(name)
    define_method name do
      class_variable_get "@@#{name}"
    end 
    define_method "#{name}=" do |new_val|
      class_variable_set "@@#{name}", new_val
    end
  end
end

Related Solutions

Python – Are static class variables possible in Python

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass:
...     i = 3
...
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have

>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can't be accessed using a reference to an instance.

See what the Python tutorial has to say on the subject of classes and class objects.

@Steve Johnson has already answered regarding static methods, also documented under "Built-in Functions" in the Python Library Reference.

class C:
    @staticmethod
    def f(arg1, arg2, ...): ...

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I'm still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn't matter.

Python – How to get instance variables in Python

Every object has a __dict__ variable containing all the variables and its values in it.

Try this

>>> hi_obj = hi()
>>> hi_obj.__dict__.keys()

Best Solution

Related Solutions

Python – Are static class variables possible in Python

Python – How to get instance variables in Python

Related Question