Member Avatar for arupface

class variables have a niche to fill: visibility to a class and its instances, and to no one else. Typically, this means being visible in class-method definitions and instancemethod definitions, and sometimes at the top level of the class definition.

I understood that. But didn't find any example of the part - sometimes at the top level of the class definition.

Could anyone help me here with example what it means by?

  • 3 Contributors
  • 6 Replies
  • 208 Views
  • 1 Week Discussion Span
  • Latest Post Latest Post by sepp2k

Dani AI

Generated

Short answer: "top level of the class definition" simply means the class body — the code that runs while Ruby is defining the class (not inside an instance method or a dedicated class method). A class variable (the @@ form) can be read or assigned in that body just like it can in instance and class methods; 's example demonstrates initialization at that spot. (rubytalk.org)

Example that shows a class variable used at class-definition time to affect how the class is built (this runs once, when the class is parsed):

class FeatureToggle
  @@enabled = ENV['FEATURE_FLAG'] == 'on'

  if @@enabled
    def self.status
      :enabled
    end
  else
    def self.status
      :disabled
    end
  end
end

Here @@enabled is set at the top level of the class and used immediately to choose which class method gets defined; that choice happens during class definition, not later at instance creation. This is the practical meaning of "sometimes at the top level of the class definition." (rubytalk.org)

Caution: @@ variables are shared across a class hierarchy, so a subclass or reopened ancestor can mutate them and affect other classes — this is a common source of surprise. Accessing a class variable that has never been assigned anywhere in the hierarchy will raise an error. For per-class (non-shared) state, prefer a class instance variable (@) and a small accessor; it keeps state local to that class and avoids inheritance collisions. Example pattern:

class Base
  @count = 0
  class << self; attr_reader :count; end

  def initialize
    self.class.instance_variable_set(:@count, self.class.instance_variable_get(:@count).to_i + 1)
  end
end

For the thread: correctly showed that class variables can be set at class top-level; @LastMitch's mention of an "error" likely comes from the uninitialized-case or from subclass collisions, not from the fact that top-level access itself is forbidden. (ruby-lang.org)

Recommended Answers

All 6 Replies

Member Avatar for Member #949455

Can class variable be accessible to the top level of the class definition?

If you want to find that error then post the code that is related to it. Which platform are you using?

Could anyone help me here with example what it means by?

You have to post the code where the error appears. I mean for you understanding the error is good but I feel you want to debug this issue.

Edited by Member #949455 because: grammer
Member Avatar for arupface

@LastMitch - thanks for your reply. But no error I have found. I wanted to see one example against the bold marked statement in my description. I don't have any valid example to digest that statement too.

Member Avatar for sepp2k

Here's one example of a class variable being used at the top level of the class definition:

class MyClass
  @@number_of_instances = 0

  def self.number_of_instances
    @@number_of_instances
  end

  def initialize
    @@number_of_instances += 1
  end
end

As you can see, the class variable @@number_of_instances is used in 3 places here: it is read in the class method number_of_instances, it is incremented in the instance method initialize and it is initialized to 0 at the top level of the class definition.

@LastMitch:

You want to see how the error looks like?

What error? No one mentioned any error except you.

Member Avatar for Member #949455

What error? No one mentioned any error except you.

What is your problem?

Member Avatar for sepp2k

I don't have a problem. I just pointed out that the OP did not ask about any error, so I was a bit a confused about what your replies were about. If I was rude about it, I apologize. I didn't mean to be.

Edited by sepp2k
Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.