Class variables (@@) and their dangers in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class variable in Ruby?

A class variable in Ruby is a variable shared among a class and all its subclasses. It is prefixed with @@ and holds data common to the class hierarchy.

beginner

How do you declare a class variable in Ruby?

You declare a class variable by prefixing the variable name with @@ inside a class, for example: @@count = 0

intermediate

Why can class variables (@@) be dangerous in Ruby?

Class variables are dangerous because they are shared across the entire class hierarchy, so changes in one subclass affect others unexpectedly, leading to bugs that are hard to track.

intermediate

What is a safer alternative to class variables in Ruby?

Using class instance variables (e.g., @count inside class << self or self methods) is safer because they belong only to the specific class, not shared with subclasses.

advanced

Explain with an example how class variables can cause unexpected behavior.

If a superclass and its subclass both modify @@var, the changes affect each other. For example, if superclass sets @@count = 1 and subclass sets @@count = 5, both see the last value 5, which can cause confusion.

What symbol is used to declare a class variable in Ruby?

A @

B @@

C $

D #

Class variables in Ruby always start with @@.

What is a main risk of using class variables (@@) in Ruby?

A They are shared across subclasses, causing unexpected side effects.

B They cannot be accessed inside instance methods.

C They are slower than local variables.

D They are private by default.

Class variables are shared across the class hierarchy, so changes in one subclass affect others.

Which is a safer alternative to class variables for storing class-specific data?

A Class instance variables (@var)

B Global variables ($var)

C Local variables (var)

D Constants (VAR)

Class instance variables belong only to the class itself, not shared with subclasses.

If a superclass and subclass both modify @@var, what happens?

A An error is raised.

B Each has its own separate @@var.

C The subclass cannot access @@var.

D They share the same @@var, so changes affect both.

Class variables are shared across the class hierarchy, so both classes see the same variable.

How do you access a class instance variable inside a class method?

A Use $var inside the class method.

B Use @@var inside the class method.

C Use @var inside the class method.

D Use var without any prefix.

Class instance variables use a single @ and can be accessed inside class methods.

Describe what class variables (@@) are in Ruby and why they can cause problems in inheritance.

Think about how one change affects all related classes.

Explain the difference between class variables (@@) and class instance variables (@) in Ruby.

Focus on scope and sharing of data.