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Folks, I am learning ruby and recently moved from JAVA. In JAVA I could make a member variable of a class static and that member variable would remain the same across instances of the class. How do I achieve the same in ruby. I did something like this in my ruby class:

class Baseclass

  @@wordshash

end

This seems to serve the purpose so far while I am testing this, that is @@wordhash remains the same across instances of Baseclass. Is my understanding correct.

Also, I wanted to have a member method in the class that is equivalent of a static method in JAVA(I do not require to have an instance of the class to access this). How can I do this? For example I want to have a method in the Baseclass like getwordshash() which returns @@wordshash and the user of that method should not have to make an instance of Baseclass().So something like this:

class Baseclass

  @@wordshash

  #static or class method
  def getwordhash()
    return @@wordshash
  end
end

and then I can use it like

#without making an instance
@hash = Baseclass.getwordhash()

I apologize if this is a very naive question, i am really new to ruby but very excited to learn.

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2 Answers 2

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13

Using class variables as static store has unintended consequences in sub classes. You can easily implement static store by declaring an instance variable at the class level.

class Foo

  # initialize a static variable
  @names = ['Tom', 'Dick', 'Harry']

  # class method (notice the self. prefix
  def self.names
    @names
  end

  # instance methods

  def names
    @names
  end

  def names=val
    @names = val
  end

end

Let us print the static names

>> Foo.names
=> ["Tom", "Dick", "Harry"]

Let us print the instance variable

>> f = Foo.new
>> f.names
=> nil

Let us set the instance variable and try to print it again

>> f.names=["Bob", "Danny"]
=> ["Bob", "Danny"]

>> f.names
=> ["Bob", "Danny"]

Let us print the static variable again

>> Foo.names
=> ["Tom", "Dick", "Harry"]

I use the term static loosely. In reality both variables are instance variables. The @names declared in the class context is an instance variable for the Foo class object and @names inside a method is the instance variable of an instance of Foo class. Since there will ever be one object representing the type Foo, we can consider @names declared in the class context to be a static variable.

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4 Comments

is it possible to use some variation of the shorthand: attr_accessor for access to these quasi class instance variables?
@DennisHodapp have you tried using cattr_accessor and mattr_accessor methods?
no I haven't, but it seems those accessors are extensions of rails and I would rather not have to import them for a Ruby application.
@DennisHodapp, you don't need the entire rails for the feature. You just need ActiveSupport gem(rubygems.org/gems/activesupport). Here is a another way to use attr_accessor to do the same: stackoverflow.com/a/895752/163203
6 
def self.getwordhash
  @@wordshash
end

With Ruby and Rails Naming Conventions:

class Baseclass

  @@words_hash = Hash.new

  def self.words_hash
    @@words_hash
  end
end
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1 Comment

Actually, convention doesn't usually have the type. It would be @@words rather than @@words_hash.

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