Understanding The Visitor Design Pattern

[@TheSimpleEngineer]

Let me know what design patterns you want to learn!

[@TheSimpleEngineer]

@@dg10mcdos cool 🙂 I'll do that one next

[@hinathahseen8643]

Thanks for the above one. Double dispatch couldn't be simplified more.Can you t explain examples of Extension object and Template method design pattern:)

[@mirazzaidi]

Can you do a decorator?

[@deepankarsingh7230]

Can you please make a video on strategy design pattern?

[@Nemecis313]

I would like to understand the Mediator design pattern if possible. Thank you for this video!!

[@mehmeterdem7581]

This is probably the best video on the net for this pattern with a perfect example.Thank you!

[@illosophycom]

Isn't this still tightly coupled though? If you get rid of the Bronze Card or add a Platinum card, you have to update your OfferVisitor interface and all of the classes that implement OfferVisitor right? That's still a maintenance nightmare. It seems like the problem wasn't solved but just relocated.

[@TheSimpleEngineer]

There will always be some coupling, but the amount is reduced as your class base grows large.

[@ruixue6955]

2:50 UML dia of Visitor Pattern 4:20 the original piece of hierarchy 6:50 why - single vs double dispatch 7:39 single dispatch 8:14 what single dispatch allows us to do 9:58 double dispatch: interaction of 2 objects 10:12 example 10:28 modify the interface: in Animal interface, add makeSound(Dog) and makeSound(Cat) 11:45 *compile error* due to not support on double dispatch

[@usamakhawaja8571]

Just half way through the video, and I would like to say "Good Job". (y)

[@maciejkowalski3019]

ok, but what if you have created hundreds of different visitors and you have to add a platinum card?

[@SylvainBrunerie]

Great explanation, thanks! My question is the following: we made CreditCard easy to extend without needing to modify it, but now OfferVisitor kinda is in the situation we wanted to avoid, right? If we create a new PlatinumCreditCard, the OfferVisitor will have to define visitPlatinumCreditCard, and all the visitors will have to implement it. It feels like we need to take a guess at which side (credit card or offer) is more likely to be extended in the future, and use a visitor pattern for that one. But we can’t have that for both of them. Is that right?

[@rolandsoftwareguy2515]

I think the idea is you might add a card once or twice a year but maybe 3 or 4 offers a month. I guess what we would normally to is have a GetCashBack(CreditCard, Offer) method but then there would be the overhead of maintaining a datastore

[@GiorGoS07031992]

I think so like the a scenario like the one described on this video. You don't come up with new credits cards every day, but you may get new offers every day/month.

[@genovo]

Problem: 12:56 Solution: 28:00 Fantastic video!

[@deepankarsingh7230]

Amazing explanation. I like discussions a lot when it cover the background as well like why something is needed, current limitations in some paradigm and ways to solve that. Thanks a lot for the video. :)

[@rahulsaxena9103]

He has made the pattern as easy to understand as hard it is going through other explanations on the web. Amazing!!

[@atanubanerjee3389]

Hi Ryan - so could you please tell how I apply Visit/Accept pattern to resolve the original problem of cat-bark or dog-meow you originally started with ? Should I create one more class like CATDOG from Animal, accept runner request and forward to makeSound Visitor - visit(CATDOG). Or simply type-check a => in Dog "makeSound(Animal a)" for Runner request of "dog.makeSound(cat)" and take action . Much appreciate and thanx.

[@ashrasmun1]

I would love to see constructive criticism from someone who disliked this video. I think it's unbelievably helpful, especially given that you explained the whole "dispatch" thing, which is not obvious at all for beginners. Thank you very much!

[@TheSimpleEngineer]

Thanks, glad you liked it!

[@piotrpieniak738]

I'm so grateful for it, good job! I always struggle with theoretical explanations and definitions, so combining them with actual examples made it so much easier to understand.

[@williamquintero3133]

Excelent bro... i've just spend a lot of time trying to understand this patterm. And you did it in just 30min :) thanks..

[@tinker7722]

Thanks for that brilliant tutorial! Actually, "accept" is calling the visitors visit function, isn't it?

[@ArchaeanDragon]

GoldCreditCard silver Had no effect on the running code, but boy did it trip my OCD! lulz :P I think it would have done better to drill into why you couldn't pass cat to itself. Basically, you can't pass ancestors references into descendant params and vars. The compiler gives you that hairy eyeball because the running app can't use a crystal ball to figure out which descendant you meant. I think it was part of your higher-level explanation, but for newbs, the underlying problem wasn't as clear.

[@ben6]

This was the best resource I've found so far, but unsurprisingly, I am a bit confused. First: I guess we need to write a new `accept` method for each type of resource (e.g. offer) that Element can take (in this case, Bronze, SIlver, Gold credit cards.) We are not able to share `accept` methods between concepts: e.g. Not just offers (hotel, gas), but also other behaviour, like update state, get description, expiry. Could someone suggest a better name for accept? Second: The video author also wrote 'visitBronzeCreditCard' instead of just 'visit', so this actually goes around the problem of (not highlighting how visitor patterns overcomes lack of double dispatch)

[@sergeyt4118]

am I missing something? - this very much reminds me of a simple and well known MS Excel's LOOKUP() function - which solves to find an intersection of column and row in a 2d matrix based on input parameters (without any programming) - if I'm right, why would anyone want to re-invent the wheel and use such complex patterns if you could just refer to the respective MS assembly logic in c# (or maybe there are alternatives or code that does this thing outside excel boundaries) and just use this algorythm?= to me any code that cannot be logically and transparently walked-through step-by-step by a human is dangerous in its logical design; I hate OOP for such couter-intuitive constructs where the compiler does its magic and you just have to hope your tests will prove it brings the correct result and does not make mistakes in all possible input scenarios; I think in cases like this coders like mathematicians went a little over the edge

[@shekhar_2014]

Can you please make vdo on Builder, decorator, Abstract factory patterns ? Thank you

[@thesuperiorman8342]

You've appropriated this example without attributing it to the original creator. Pathetic Source: www.javagists.com/java-visitor-design-pattern

[@kinvain]

Really like your video. Especially that you extend fake Animal interface use case to something more realistic.

[@ksaisko]

I don’t understand how you can access attribut of the object accepting a visitor inside the visitor function, isn’t a problem?

[@kaustuvchatterjee4117]

This problem can be solved using decorator pattern also.. What is the subtle difference?

[@mysteriouse5891]

Very generic and common example. Does not attempt to show an actual interaction between the visitor work object and model data object. Printing lines is pedestrian

[@herrgrillparzer]

Such a clear explanation and great examples. Thank you so much for this 😊

[@coffeeandbytes9854]

Super load intro music followed by super quiet microphone.

[@houssemzaier]

I hate this DP for its non-semantic approach. You should fight against it if you are a kotlin user the language give you tools to skip this overhead, using the sealed classes you can do more with less code.

[@mrowox]

This is really fantastic. Is it possible to combine this pattern with the Factory pattern for example to dynamically determing which credit card to use in a certain scenario

[@yolokazinogantsho8111]

Great great great explanation. Totally understood! Big thanks

[@pm5828]

Thank you for the great video! can we use Async/Await with visitor Pattern since what i intend to do in visitor is make outgoing calls.

[@reallylordofnothing]

really good video. came here to understand rsql-parser visitor pattern. I can see that this pattern explains open closed principle very well

[@whatever6223]

Excellent explanation. Thank you very much!

[@AB-os3jy]

it would be a hit if you could do a series or video that puts together and deploys a multi platform mobile app. bonus if it has animation , audio and 3d capability.

[@chiranjeevisaride8655]

The best explanation anyone can find on this planet. Absolutely loved it!!

[@ilstam9942]

Awesome explanation and helpful example! I hadn't realize before about the double dispatch problem. I really liked that you didn't over-explain things keeping it short and to the point. Also thumbs up for cutting out the tedious parts where you just paste code. Thank you!

[@jonathankee9322]

At around 13:00, we could have used instanceof and casting to get the correct Cat or Dog object though?

[@FernandoNavarro_1]

Very clear presentation and illustrative code. Thanks

[@michaeljuergen4326]

Very good explenation, this is the Video I was searching for the whole time

[@kartech4592]

Can we say one drawback of this pattern is that the visitor needs to have a visit method for all card types even if it might not apply to that card? Is there a way to overcome this necessity?

[@mirazzaidi]

Great explanation! I am just wondering why you didn't overloaded the visit method in Visitor interface with Concrete element parameters and created 3 separate methods for each method.

[@lucykuo4769]

Finally found a great video for visitor pattern! Hope you can make more design pattern tutorials

[@rajeshtc8965]

If one more card type is introduced, then offervisitor interface need to be updated with one new function which support new card type right?? Is this violate solid design principle??

[@ihormay3418]

yes, you need to implement new method. But only ones. But your can add new visititors with new card withour changing new methods. So, yeah, it's kinda violite solid principle in a some way, that's why Common Lisp is definetly better in this current situation. But if we use java/c++ we should accept this drawback, nothing we can do, basically. (Not talking about c++23 code generation possibilities)

[@TheFrogfather1]

Nice clear explanation. Thank you!

[@TommyBoy7Heads]

Legend.. thanks for the great explanation man.

[@kankipatisaikumar]

@the simple engineer Great explanation, easy to memorize

[@LemmeTellMe]

Good job on the video, heads up though the intro music could be tuned down a bit since it feels a lot louder than the rest of the audio

[@ambarishkapil8004]

This was very nicely explained. Thanks

[@luyandamsomi8402]

great job man, really appreciated💪👏

[@ashrasmun1]

I recreated your examples in C++ and they just work without the visitor pattern... I am super confused. I have classes "Duck" and "Chicken", deriving from "Bird". They all have a method "greet" and this code compiles and works just fine. Bird* d = new Duck; Bird* c = new Chicken; d->greet(c); This seems contradictory to what you've said around 13:30.

[@MirekKrenc]

Very well explained. Thanks!

[@mireazma]

The way I see it visitor pattern doesn't make you write any less code but it gives the advantage of decoupling. But even so, it kind of does and then it doesn't. In the "multi-dimensional" case like cards/offers it all depends on which of the dimensions you're likely to have more classes added. Even if card types are issued occasionally, for every new card you have to make modifications in every _offerVisitor_ . And there may be an order of magnitude more _offerVisitor_ classes than cards because that's precisely the case that visitor pattern tries to mitigate - many offers, few cards.

[@lascau2918]

@mzma You are right but what if both cards and offers increase drastically what is the solution/pattern such that you obey the O from SOLID?

[@mireazma]

@@lascau2918 Not only O but I as well. Think about the case where a credit card only has certain offers or there's an offer that only applies to certain credit cards (it depends on how you look at it). The key is not enforcing all combinations, i.e. not enforce implementation of all those interface methods. I'm thinking we could modify the visitor part a bit. Abstract out individual card-offer combination into something like _Interaction_ that would have an _interact()_ method that does the calculation . _OfferVisitor_ would have a _List_ . I haven't thought this through but the idea behind this more complicated structure is: 1. Each new card doesn't imply modifying all offers. 2. Only have implementations for actual card-offer combinations, and not also for nonexistent combinations.

[@natanloterio]

Great video. What pen table + software are you using? Wacom?

[@TheSimpleEngineer]

Wacom Intuous

[@saurabhchaturvedi6982]

Awesommest post on the internet on Visitor Design Pattern !! Understood everything in such a simple way that its gonna stuck in my mind forever !! Subscribed !! Thank you..may you have many more "Visitors" on your channel 😅🤟!

[@sagaragrawal893]

One small question - The GasOfferVisitor has 3 methods for the 3 types of credit cards. What if GasOffer is not applicable to Bronze Credit card? So in the GasOfferVisitor class you may remove that method pertaining to Bronze OR throw an exception from there? Whereas from the main method, BronzeCard would still be okay to accept GasOfferVisitor object as an input param at compile to only get a runtime exception or empty response. How can this use case be handled?

[@andres5709]

i'm no kidding there's a job offer in play here, and I have to deliver a code using this very pattern and i couldn't get it before watching this amazing explanation of yours! thank you for time and help from Brazil!

[@256sks]

Thank you for good explanation. Your final implementation require small correction. Visitors classes should have overloaded method rather than different name for different CreditCard. Otherwise it will work without double dispatch.

[@Rastrigin]

great job man, love your setup as well

[@PrashantGangwar]

I've seen lot of videos on this pattern but this is most real life understanding video on Visitor pattern, Thanks for this amazing video... luckily found this

[@aelafdev]

by far much cleaner explanation. visitor is mystrio no more. thank you

[@MN7Editz]

Very good explanation. So far the best explanation and presentation on this pattern and finally I got it understood. Thank you very much.

[@eliyahuspeyer1696]

your wonderful! thank you!

[@rolandsoftwareguy2515]

Excellent explanation especially of the why. A lot of tutorials just go straight into the how. I think given the extensibility problem, I might have initially gone down the passing of delegate route or maybe another interface - but I guess this seems somewhat tidier. Note to any other C# Developers reading before watching, the video is still easy to follow. Well Done 👍🏾

[@atamanson]

Thanks for the awesome videos. Would you say Visitor pattern is suitable for basket/cart system ?

[@peacefulwater9689]

very good video, explaining single vs double dispatch really cleared it up. The only thing I didn't like was the intro

[@fakhrshaheen3394]

very well explained.it's really one of the most underrated patterns

[@gunjanchaudhary9716]

The best Explanation !

[@dhapategeetanjali741]

I wonder all over the internet to find the best to learn visitor design pattern and finally landed here and writing comments middle of the video, This is an awesome video so far I came across. Buddy keep it up you are the best.. !!!!

[@ruixue6955]

1:08 visitor pattern 2:48 UML

[@kushal800]

best video. love from india

[@zizimouad8291]

THE only video explain this pattern in the right way ! explination, diagramme and exemple with code all are perfect

[@samitabbakh8409]

Really nice explanation. I hope you would do a complete playlist of all design patterns. It is a shame that you made only 3 design patterns.

[@kitkarson4226]

This is good explanation. But how does it support open closed principle? You need to update the OfferVisitor interface to include if we introduce a new CreditCard type - say Platinum card. Basically we move this from one place to another. The original problem is still there.

[@ShaunStruwig]

There's essentially a trade off between type extensibility and extensibility of operations. So this pattern assumes that your types will be the closed set and you will only add more operations as time goes by as your open set. If you wanted to add more types it isn't the best choice of design pattern. So it supports Open Closed principle only in extensibility of operations but not types.

[@kitkarson4226]

@@ShaunStruwig It took some time for me to realize that.

[@dispatch3499]

He is using vim inside intellji😍

[@sv3167]

Wow, pretty legit!! I understand!! Thanks a lot!

[@MrJonnis13]

Simply excellent explanation of this design pattern. As you said, this one is underestimated but it is super useful. Please consider doing some other design patterns, especially the "not easy" ones Thanks again for this video

[@jaydenhooper]

Excellent explanation!

[@deltaphilip8611]

Finally make sense. Knowing what to accomplish makes it easier to understand how to do it

[@ruixue6955]

14:07 example of double dispatch (faking) - visitor 14:23 - 17:46 18:57 the concrete visitor: GasOffer, HotelOffer ... 19:32 CreditCard interface code 20:32 OfferVisitor 23:37 GasOfferVisitor

[@atulsaxena4286]

Very nice videos probably the best on the internet. Please do other patterns as well with the same kind of explanations you have done in these videos. Thanks.

[@DigitomProductions]

Thanks for this video. Life saver :)

[@allen0hu]

WTF this is so well explained! Thanks, saved my finals :)

[@AlbertoGS1990]

You are the best explaining design patterns. You should make a single video for every single pattern in the book. Thanks a lot for your teach.

[@andrewprotasenya7469]

thank you very much, I finally understood the essence of this pattern

[@simoncatteau2879]

Very clear video!

[@KeillaSellay]

Intro music sucks

[@adamblance3346]

Thank you! So many other videos on this feel rushed and don't go into enough detail.

[@martinx3856]

Good job, nice voice

[@rezasaidafkan8698]

Crisp! Good job!

[@Sss222ddd]

Wow thanks man

[@billuchoudhary4436]

What app u r using?

[@bimarshsharma7556]

Thanks for the clear and straightforward explanation.

[@rohitupadhyay9964]

Awesome tutorial, thanks very much.

[@agatalach2855]

This is a great video. Super clear explanation, great example, and the whole video was really well done. Thanks so much for your work, it's really helped me understand the pattern and I am now going to go check out your channel for more content, which I'm sure will be as good :) thumbs up!

[@HassanBadir]

damn, this is well explained, when I look for a new topic to study I usually run through multiple videos until I understand it completely, this time i watched the whole video once and got it on first time this was amazing thanks a lot man, big like!

[@dattatreykulkarni6472]

Good Job Bro!!!

[@ivannav]

Great job!

[@amitsamratmaurya3170]

Best content so far!! You won't need to read anything else after watching this.

[@ArvindDevaraj1]

very clear explanation of visitor pattern with real world examples

[@XxTonys555xX]

thanks

[@leepoking]

Too good