SINGLETONS in C++
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The Cherno make a data structure series bro
lmao "up to 91% off"... My price went from $138 to $129...
Can i have an English subtitle to help with my study？
@The Cherno, can you explain *Dependancy Injection* pattern in C++ ?
Please make a most used design pattern related series... much needed..
I'd really like more of such design pattern content. Big fan btw, great content.
That's a really good topic. I have messed with PHP and have seen patterns like MVC, event driven, modular.. And I'd like to know how it is in C++ world
There is one important functional difference between a namespace and a singleton class. In a namespace, all of the data is initialized/loaded into memory when the program first starts; this might be undesirable sometimes when we don't want all the data initialized until we actually need it. In contrast, singleton instances are only loaded during the first call to the "Get()" method.
Great content from an experienced developer. I would note that technically there's a little functional difference between static member variables and local static variables. When you moved the static inside the Get() function, you made the initialization order deterministic. Local static variables are initialized the first time control passes through the declaration. This makes it safe to call one singleton from another singleton. Also local static variables are thread-safe beginning with C++11 (except when --fno-threadsafe-statics is used in GCC). Plain old static member variables aren't thread safe, and the undefined initialization order causes a nightmare situation. This is less of an issue with ints and floats, but if a singleton contains a class, like a string, you're in a really big trouble. Especially when you're using one singleton/static from the constructor of another singleton/static, and you have no way of knowing which one gets initialized first, and if you're out of luck. You may be using a variable that hasn't even been constructed or assigned yet. The destruction order is an even bigger mess than the initialization order, so stay away from custom destructors that rely on other statics. The object being destructed may be using an object that has already been destructed earlier.
Tamas Demjen
We really need people like you in the comments section
Actually, the standard guarantees that destruction order will always be the reverse of the construction order, so that's not so horrible.
Thank you for your comment - it's what I would have written if you hadn't already posted.
Thanks for your comment, I had not used the first time of Singleton he showed , where he uses Static Member Variables, only with Local Static Variables. Now I know the differences. :D
If the statics are for making the variables have file-scope then it can save you some recompilation time when adding or removing data that is no longer part of the interface.
@Ralph Reilly If you add static before a global variable defined in a cpp it goes from being a global to having file scope. As in you can't access it outside the cpp. Means the compiler can rip out the symbol right away instead of waiting for the linker to do it. Moving the private statics in the class definition to the cpp would mean changing them wouldn't change the header at all so would avoid recompilation from header-changes.
Yes! Please, do cover more Design Patterns!
Side question: can you type and talk like that or is that edited? Regards!
I am so damn happy this series is back. Have been waiting for a c++ video for so long!!
Cherno, have you considered covering more design patterns in the C++ series? Thanks!
A Log Class is a good example of a singleton. You just need a way to log things so you instantiate the Log singleton and can do Log.warning("something"). Other examples are in plugin architectures where a singleton can act as an API interface. You want to provide a certain class of functionality, then there is a singleton for that and you bind to it by registering handlers etc.
One use case for C++ singleton that makes sense to me is the case where you have several classes with a common base class. Each of the derived classes could be a singleton, benefiting from reused code and encapsulation and they're similar enough to where it makes sense to derive them all from a single base virtual class. In this case, the base class takes care of most of the structure - making it reusable. Then the singleton derived classes have just a few minor differences for each of their use cases.
Thanks cherno for all these videos. I binge watched you whole series almost a week ago so that I can be comfortable with c++ again(even more now). It really helped me to understand c++ codebase of mlpack organization, I even made a PR on it which is almost accepted. Thanks a lot.
Loving the content! Could you possibly do a video on iterators? As far as what they are, how you can use them in a queue/linked list implementation, etc
Keep it up, i am a student that is c++ geek. I learnt a lot through you.
Great Video. First ran across singletons on a Java project (where, as you point out, everything is a class). Since then, used it occasionally if needed to reuse some object that was 'expensive' to create. One was where a lengthy connection to another server database had to be established. Each query on the object was stand-alone, but creation took time. So we created once in a singleton and then all the code could 'get' and 'query' without having to recreate the connection all the time.
This video is exactly what I needed, it helps me think of how to organize my code better.
Would love to see more design pattern videos!
Already knew about singletons, but super helpful that I learned how to circumvent the cumbersome declaration of the static instance in the translation unit.
YAY! More C++ :)
Thank you The cherno. This is great, just what I need. I wish you can look at more design patterns in future, I mean yuo realyy explain in a way that makes it easy to understand.
Thank you so much! Your videos are so good. Thanks to you I learned some little details that are important to know about C++. I would love to see more on idioms. The only idiom everyone seems to use for tutorials is the pimpl idiom.
This is HUGE in embedded programming because we have to often interface with C libraries. The singleton pattern allows us to interface with C callbacks but still use dependency injection for unit test.
I love your videos. You are doing this right. C++ is the best language ever made and you are making this accessible
thanks for this comment!
more design patterns please!! you're by far the best C++ teacher on yt
Me and my friend had to give a 40 minute presentation about the Design Pattern, specifically using C++ as well.
Singletons are, from what we've seen most of the time disliked and sometimes treated as an anti-pattern even. Now, Singletons do have their limited uses, so there's that. But how to construct one and especially how to destruct one or worse, doing that AND being in concurrent environment, that is a real piece of work.
What Cherno did in construction was a Meyers Singleton, i.e. the local static one. There's also the way of allocating the singleton on a heap and checking if the pointer is null and if so, creating the Singleton, otherwise just returning the pointer as a reference.
Threading usually involves something like double checked locking (you don't want to construct two singletons, that'd defeat their purpose) or something that ensures a single call of the constructor.
Destruction is pretty funny too - you can either ignore possible problems or think of the Keyboard-Display-Logger problem and think of the Phoenix Singleton (where you basically remake the Singleton anew) or the Longevity Singleton, which requires a lot more work.
If someone has a need to know more about Singletons, I recommend Modern Design Patterns in C++ or the original Design Patterns by GoF.
Tbh, once we did that presentation, I started to see Singletons a lot more. E.g. my browser uses files to lock the processes and guess what, that's a Singleton. Never seen it before the presentation.
I've seen a lot of hate towards Singletons, and never really understood why. I'm a self taught C++ programmer and I use C++ to automate my work as an engineer, which involves a lot of data processing, e.g. sucking in a lot of data files and extracting data etc. So for example, for each kind of data file I have to read I will create a class to handle doing it, and rather than everything that Cherno just described I create a class, sometimes the Constructor is empty and sometimes data needs to be passed. But it's a lot easier than everything described in this video. And I don't see any way around it for the work I do.
@@axelBr1 Singletons are hated by pure OOP devs who suck off big bob and his clean code principles. While anyone outside of the OOP cult know that clean code is in itself an anti pattern that creates extreme bloat and complexity with its constant abstraction and FactoryFactoryFactories. Singletons remove all that bloat and complexity so the SOLID fanboys hate it.
@@abuDA-bt6ei Thanks!
I've also being doing away with all the getters in my code and just making the variable public. If you want to F about with a variable then that's your problem. But filling the .hpp file with lots of .get... I don't think helps
Glad to hear you say you're just getting started! Can't wait!
I really like your explanation very much, the way you construct the solution makes things more easy, Please add video on all the other class Design Patterns
Good theoretical explanation while keeping it simple. Plus, a relevant example. Great video!
You C++ classes are simply brilliant. Thank you.
I recently used a singleton for a SDL wrapper. I wrote an initializer class in the protected field of the window class with a static instance of that initializer such that once the user instantiate a window, it will initialize SDL automatically in the constructor of the static initializer object and de-initialize it at the end of the main in the destructor of the initializer.
I've not yet used singletons in other ways than the one I described and I simply don't use them that much.
Excellent! I finally find some useful C++ tutorial on CZcams! Thanks!
Love this series, more design patterns please!!!!
You forgot to mention the most important thing.
It's a antipattern and should not be used except few rare exceptions.
It adds hidden dependencies into the whole codebase, makes unit testing harder and causes lots of pain and costs few years later.
sometimes it can make unit testing easier, because you can make your singleton inherit an interface and then use a mock instance for testing other stuff rather than the real singleton, actually anytime you want an essentially static module but you need it to inherit something then you basically need to make a singleton, i think thats the only time i can think of where it would be useful.
I love your video. Some explanations (e.g. the linkedin explanation ) are confusing because they use complicated classes as their demo. I really liked how you didn't bother making a working random generator function and focused on singletons!
I'm bagging you: more design patterns!
great video!
Would definitely to see some other design patterns video. Love your style of teaching and examples.
Dare I say, your playlist of C++ videos are the best in the universe so far, youtube aside. I can not thank you enough. If I had the financial ability, Id definitely join the patreon group, not for the extra access but just to show my support for the greatest C++ videos on the net. Who knows, maybe I will get there soon.
I particularly would like to see a video on the Command pattern, since I've been told its useful for game dev.
I agree, of course there are other resources for that, but I really like the way Yan explains things!
Thanks for your great videos, can you please make a tutorial about DESIGN PATTERNS
Note: function local static variables (depending on implementation) can have guards for thread safety, which make accesses take longer. This can be particularly problematic when it's a pointer because of the dependency stall.
Please do more design patterns! Your content is great and your explanation is very clear and elaborate!!
We use a lot of static function variables in methods especially for fixes that should not break ABI (Introducing variables or reordering may break ABI).
The best advantage from this approach is the guarantee for MT safety when initialization the variable - either better if the variable is a class.
Something you didn't mention (Or I missed it) is MT thread safety which is the crucial point for singleton initialization.
(Refers to the famous double-if-lock pattern).
This was pretty useful and easy to understand. Thank you!
Hey Cherno.. Great content and well explained. Expecting more videos on other design patterns as well..!!
I used them in ue4 as a game audio programmer and found them to be tricky to work with. The engine instantiates the Singleton as expected but tweaks to the Singleton data doesn't show until the entire engine is rerun. That makes iterating on designs a lengthy trial. Otherwise it's a handy way to get your world actors to survive between map transitions.
That's fantastic! Thank you a lot! More Patterns please
loved the video. Please make more Design pattern tutorials like observer pattern, Factory pattern, the decorator pattern etc.
Cherno you saved my life with this video (again) thx for the great tutorial
this is called Meyer's singleton and after c++11 it is thread safe too, but has an overhead cost of the Get functuion because of the internal locking mechanism
everything with locking has overhead
@@rblackmore1445 yeah but it is not obvious that there is a locking in this code and it was not before c++11, it was added to the standard later, as Meyers states:
- Static function objects are initialized when control flow hits the function for the first time.
- The lifetime of function static variables begins the first time the program flow encounters the declaration and ends at program termination.
- If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.
@@rblackmore1445 With the right memory model (which C++ didn't have as part of the language spec pre-C++11 though may now, I'm not sure), you can construct a safe "double checked" locking scheme that really doesn't suffer much.
Les goooooo. Hell yeah, more c++ content. 🔥🔥
Very very informational. Thank you
Great video, loved it
I think a big advantage of singletons is that you can put init code in the constructor and not worry about calling it manually. Advantage or disadvantage, depending on if the init code relies on some other singleton that may or may not have been initialized
Thanks for creating such informative videos. If possible please also tell us an efficient way to create singleton class with arguments and correct way to incorporate smart pointers in it.
Great vid,
One of my use cases for Singlton in real life is database connection,
for every request you get, it's better to establish 1 connection to the database, and use it throughout the code, instead of re-establishing connection for every query which will slow down your app.
Thank you a lot for the great content. Please don't stop making such great videos. I will certainly contribute to your Patreon account once I start a full-time job.
More c++ ep please!
We need more of this
Another great video. Thanks!
I make the pointer static and initialize that to NULL. The get_instance method checks the static pointer for NULL and will instantiate it if NULL, so one instance.
In general singletons have some overhead for the function calls and initialization when you need to access. In addition, it's hard to reason about initialization and shutdown order so I completely avoid them in my engine. Instead I have a class that contains static instances of all the engine components. I can explicitly control startup and shutdown this way. It's not perfect though as c++ guarantees initialization before access in multithreaded environments, however, I've found that even with this performance cost, it's about 50% faster than using singletons
thank you very much for making these great videos
YES. EDUCATIONAL C++. MORE OF THIS PLEASE.
The indirection is a great idea. I have never thought of it like this 👍
FWIW, Singeltons are used quite a bit in embedded designs to control access to hardware.
Best learning. Please make such videos on as much Design patterns as possible to implement in C++. Awaiting
Although useful for begginers Singletons are an anti-pattern and end up coupling your code and make it difficult to test. Try creating an IoC (inversion of control), which is basically an object you inject into a constructor that has single references to you global objects. A framework that uses reflection to do this is super useful.
this. global state is just bad.
Best C++ tutorials on the internet. Hostinger better be giving you some good money.
OMG, You're even married, Lol, Its been 7 years man, since I followed your channel and waited for you to release more videos, CZcams recommendations have brought us back together lol.
@morph611 Lol
Hahaha lol
I've noticed that too his married that's for sure 😄
So I'm kind of learning the business at work, being introduced to embedded programming (from being a tech previously), and we commonly use classes for single instance objects... because it's embedded code on an MCU and there's no reason to have more of something like a Hardware class. The hardware class usually initializes a microcontroller and stuff. We do something like this to override the new operator:
template
T* static_allocate( void )
{
static type Object[ (sizeof( T ) + sizeof( type ) - 1U ) / sizeof( type ) ];
return reinterpret_cast( &Object[ 0U ] );
};
Then a class can be instantiated statically in memory with "new Hardware;" which will have a bunch of init functions in the constructor.
Hardware( void );
virtual ~Hardware( void ) { }
void* operator new( size_t ) { return static_allocate ( ); }
void operator delete( void * ) { }
I'd be really curious to see what your thoughts are on something like this. Is there any point in doing this and not using a namespace?
Functions are called in the application layer sometimes from the Hardware class, but it's really just a global instance this way...
something like " Hardware::uart_print("Hello"); "
I use singletons just if you must be able to reinit the object, for example restarting a server etc. Then you can just reassign the object and don't have to worry about those variables
yes. please make more such videos
hey Chemo been following ya since I got a interest on game devolopment past 5 years. recently been doing turotials on learning gamedev with c++ n defently looking forward to more content to advance my learning. :)
Superb, Superb , Superb.. Nicely explained..
1. create a private object inside a class..(mark it as static -> meaning that object is specific to an object //not shared with other objects)
2. mark constructor as private
3. make a public function to access that private object(return type is pass by reference because to refer an object that already being created not making a copy of it)
even though we declared constructor as private, there are still a default public copy constructor created by the compiler..so need to delete that too
@12:55 can be assigned to a another reference object/variable like so
auto& reference=Random::Get().Float();
What I missed in your video is the ability to control when the singleton instance is deleted again. This can be important, for example, when analyzing memory leaks. Without explicit control, the singleton instance is only deleted "too late", so that it is always detected as memory leak (provided that the singleton instance allocates dynamic memory, which of course is rather irrelevant for the random number generator from your example).
in short:
- declare a class with a public Get()-function, a private constructor and define the static singleton instance somewhere
- exec functions with Singleton::Get().function() OR alias Singleton first with Singleton& instance = Singleton::Get() and then use alias to call functions: instance.function()
- don't forget to delete the copy constructor: Singleton(const Singleton&) = delete;
you can also put the instantiating part into the Get() function of the class, as static variables will not be instantiated twice.
Thank you very much, you helped me a lot 😉😊
Very well explained, do more design pattern videos please
The very best video i'd seen. 🤯 Thanks for all
You are the real MVP man 💪🏻
This is the best C++ playlist on CZcams, Hope i know it earlier, but i think some topics are missing.
Singleton as class allows "optional singletons", so I can have singleton, but i can benefit to able create a new standalone instance for some special purpose. Singletons as class can also extend some interface.
Putting instance to static variable outside of function Get is slightly faster, then having it inside of the function Get because every-time the Get() it called, it must ask, whether the instance is already initialized. There is also some locking protocol to ensure MT safety. This doesn't applied to outside variable. However there is also benefit to having the static variable inside of the function. If the singleton is never used, it is also never initialized, which can help with performance in the such case.
Good video! Only thing I missed is the downsides of the singleton pattern. For example, they are pretty much untestable in unit tests. And then there is threading issues. Also, the initialization order fiasco. In my opinion, this pattern has so many issues and too few benefits to balance it out.
Well designed singletons are very testable! What do you mean?
@@adamjcasey how do you test a function that make use of your singleton in an environment where your singleton is not present?
say you have that classical database singleton. while unit testing, you most likely want to swap your database connection with some sort of mock, which would be pretty easy with dependency injection, but is impossible with a singleton.
a singleton is global state. global state is evil. it is untestable in unit tests.
Please add seperate playlist and videos exclusively explaining design patterns in depth
I just made a tic tac toe c++ game and my main downside was that I had to call “TicTacToe main;” at the beginning of the file. I went on CZcams and this is the first thing I saw! Thanks man!
I made it using Object oriented programming by the way, hence the reason why I had to declare the instance
Hey, Nice content. Could you please cover all other design patterns as well
So singletons are basically the overcomplicated version of namespaces with public and private sections (yes, I know there is no pub, and priv sections for namspaces)...
I use a singleton for my data provider where I have to keep track of every request sent within a certain timeframe regardless of originating class
Thanks for the video 🤩Question: How would I implement it if the Singleton was a derived class? Would i need to delete the constructor of the base class as well, or wouldn't that matter?
Your fast typing is so satisfying
More design patterns would be great!
Thank you buddy it really helped me. Btw your english is quite understandable to me love from 🇮🇳
I use Singletons at work, and I learned something. Deleting the copy constructor. When I researched this topic I had not seen this part before.
You did not touch on the part of deleting the actual singleton, Im not sure if its actually an important thing to look into when using Singletons, but I have heard there is a particular way to actually delete the singleton in order to evade memory leaks?,
Watching for Thailand
Am waiting for a long time and teaching all 23 types
Huh, never thought about using namespace for implementing singletons. Need to try that way of creating it, maybe it have something special in it.
Yes! Amazing video,please do more of those on design patterns,id also enjoy some videos about multi-threading.
great videos man!
It would be nice to follow up this with a comparison to the Monostate design pattern.
17:25 cant you just have a class with everything static instead of a namespace so you can have private / public
If you do this, or use directly accessed namespaced data, you can run into something called the static initialization order fiasco. All global and class level static data gets initialized before the main function ever executes. Because there are no rules in the C++ standard dictating in what order translation units will be initialized you could end up with code depending on data that has not been initialized yet, causing a crash. Exceptions are also tricky to handle with "static classes", because you can't write exception handlers for code that runs before your main ever gets called. If a static object does throw on construction you end up with implementation defined behavior. That being said, you can use this pattern as long as you are entirely sure you aren't depending on any static variables outside your own translation unit and aware of the initialization order within your own translation unit. A singleton doesn't have this problem because the data is lazily initialized whenever you first need it.
I was thinking the same thing, and thank you @Ilendur for explaining
@@ilendur3810 This is correct and I wish the video mentioned this, for me the video didn't really show why singletons would be useful.
There are also some libraries that will not let you use certain functions during static initialization.
But it's important to point out that this all only applies to the pattern when lazy initialization is used, in other words the style where you put the static variable inside the function, if you just have it as a private static class variable there can still be problems.
@@ilendur3810 but you can just use a "Initialize" method to initialize later the class
At the start of the video I was unsure why not to just use a class - contributors to the code should understand that it's a singleton and shouldn't be instantiated more than once.
But at the end of the video I saw how nice the calling code inside main() is :)
Well explained thank you :)
osom...happy to c u back
i can see some chap on code interview ... " i can name function a "float" ... cherno does it ... "
Hi Cherno, Again a super nice video! You asked if there is interest in other patterns. I wonder what your opinion is about creating interface style (abstract) classes in combination with inversion of control in C++. I'm a big fan of unit testing and use this interface style programming (common in for example C#) to be able to mock out functionalities. I'm interested in your experience on this topic.