--- title: Class variables vs class instance variables in Ruby slug: class-variables-vs-class-instance-variables-in-ruby published_at: 2021-10-23 02:00:02 +0000 updated_at: 2026-03-04 20:13:53 +0000 summary: description: Class variables vs class instance variables in Ruby. tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, class variables, class instance variables, @@count] views: 3502 author: CJ Avilla url: https://www.cjav.dev/videos/class-variables-vs-class-instance-variables-in-ruby youtube_url: https://www.youtube.com/watch?v=Cf_4edpFMko youtube_id: Cf_4edpFMko embed_url: https://www.youtube.com/embed/Cf_4edpFMko thumbnail_url: https://i.ytimg.com/vi/Cf_4edpFMko/hqdefault.jpg type: video --- # Class variables vs class instance variables in Ruby *Published: October 23, 2021* *Views: 3502* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=Cf_4edpFMko) [![Class variables vs class instance variables in Ruby](https://i.ytimg.com/vi/Cf_4edpFMko/hqdefault.jpg)](https://www.youtube.com/watch?v=Cf_4edpFMko) ## Description Class variables vs class instance variables in Ruby. ## Transcript what's up welcome back in this episode we're going to talk about the difference between a ruby class variable and a ruby class instance variable and yeah the difference might surprise you a little bit actually maybe not so the difference is basically inheritance so for a class variable all inherited classes will have access to that same variable for class instance variables each class that inherits will have a different copy of the variable so let me show you what i'm talking about so if we have a class we'll use the classic example of animals so if we have an animal animal i don't know why my keyboard wants to type admin so if let's say we have a class animal and we have a class cat that inherits from animal okay this is in ruby right and if we wanted to set some variable at the class level that might track something about all of the instances one example might be that we want to track some count and so at the class like inside of the class definition we can use the double at sign which makes it look like you're looking at someone or maybe it's looking at you so this is a class variable and with the class variable we could define a class method here self.count and maybe this just returns count at count and we could also define a setter method for this so count equals some new number at count is equal to n okay so this is how we might interact with a class level variable and what we can do is we could write an initialize method that said every time we create a new animal increment count so here at the bottom we might just like print out p animal animal dot count and if we run this code we will see well we're going to see nothing because what's up oh because we haven't actually initialized any animals so if we say animal.new animal.new and then we run the code why isn't that doing anything okay animal.new we see one is printed out okay one is printed out down below now if we initialized two animals and then ran this again now we see two is being printed out now if we initialize two animals and a cat then we would expect three to print out because cat inherits from animal which means it also inherits the initialize method and so this will also increment this count so we are going to initialize an animal another animal and then a cat which is an animal so we should have the count of three right so now we see the number three so if you wanted to keep count of all the animals in your kingdom then this might be a way to do it right so let's say we have class dog and that inherits from animal also and if we were to initialize a new dog dog dot new and then run the same code now we see four so we're counting up all of the animals right in the animal kingdom of cats and dogs but oftentimes you might want a variable that is scoped to each class type so if we wanted to count how many cats we had so for instance if we tried to print out cat.count here now we're still seeing the number 4 because cat.count is still accessing that class variable count and so if we wanted to keep the count of cats and the count of dogs separate then what we might want to do is break out from a class variable and instead use a class instance variable so a class instance variable has just a single at sign but it's defined at the class level we can't actually just switch from at count here or to add count here because initialize is an instance method so in order to access that count variable we actually need to go through self.class.count and this is going to be in fact using the getter and setter methods of the class of the underlying class now here's the other thing when we initialize this class instance variable on the animal cat and dog will not actually get access to this new variable so those are going to be nil so when if we try to run this code it's going to fail because self.class.count when we try to increment this at count inside of cat is is nil so we're going to get a failure here so let's just try running it this is the error that we're getting the exception we're getting is initialize method undefined method plus for nil class and again that's because right now cat does not have a a default count set to zero so it's nil at count is nil inside of here and so this this uh increment does not work so what we can do is we can actually copy count down inside of both cat and dog and again those are starting out with just a single at sign and now if we run this we see that there is only one cat and that is correct so if we were to print out like let's just say like cat count is uh is this and then maybe uh dog count is this and then we can say animal count is animal and now we're gonna see that the cat count is one the dog count is one the animal count is two so if we maybe let's make three dogs or something and then we run this again so now we have three dogs two animals and one cat and now we're tracking the um the instance variable at the class level okay so that's the difference between class variables and class instance variables but let's just look just a tiny bit deeper at like what uh what might be going on sort of under the hood or maybe i don't know this is just one detail that might help you conceptualize a little bit more about what this is so if we print out cat.class what do you think we're gonna see cat itself is a class so what is the class of a class let's take a look so the class of a class is class it is the class class so a cat a ca the cat class is an instance of a class so let's look at this in more concrete in a more concrete example so let's say we have frog and this is going to be a new type of class we can say frog is equal to class.new do and you can even write methods in here def jump or something and and now we can say like puts i'm jumping and this is one way that you can define classes and so now frog is an instance of a class because here we're creating a new instance right and the type of the instance that we're creating is a class the class class and so what we can do is we can say now like like fred fred is a frog so we can say frog.new and now we can say fred.jump and this is totally valid so now we can see i'm jumping is printed out so each class that we define using this class syntax where we say class and the name and we can do the little carrot where we're inheriting from something this is actually creating a new instance of the class class and each of those instances can have their own instance variables and uh so yeah hopefully hopefully that helps a little bit what's what's really surprising is you can also say like um p uh frog.instancemethods.sort let's actually make that puts and here we can see all of the instance methods on frog including jump so that is one of the instance methods on the frog class that make sense i hope so uh similarly we could we could look at cat here we could say p cat dot instance variables and recall that cat is an instance of the class class and this one is going to be fun too so you can see that the instance variable at count is an instance variable of the cat class which is an instance of the class class cool super fun stuff so that is the difference between class variables and class instance variables in ruby thanks for thanks so much for watching and we'll see you next time [Music] --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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