In addition, are there any performance advantages to static methods over instance methods?
I came across the following recently: http://www.cafeaulait.org/course/week4/22.html :
When should a method be static?
Neither reads from nor writes to instance fields
Independent of the state of the object
Mathematical methods that accept arguments, apply an algorithm to those
arguments, and return a value
Factory methods that serve in lieu of constructors
I would be very interested in the feedback of the Stack Overflow community on this.
Make methods static when they are not part of the instance. Don't sweat the micro-optimisations.
You might find you have lots of private methods that could be static but you always call from instance methods (or each other). In that case it doesn't really matter that much. However, if you want to actually be able to test your code, and perhaps use it from elsewhere, you might want to consider making those static methods in a different, non-instantiable class.
Whether or not a method is static is more of a design consideration than one of efficiency. A static method belongs to a class, where a non-static method belongs to an object. If you had a Math class, you might have a few static methods to deal with addition and subtraction because these are concepts associated with Math. However, if you had a Car class, you might have a few non-static methods to change gears and steer, because those are associated with a specific car, and not the concept of cars in general.
Another problem with static methods is that it is quite painful to write unit tests for them - in Java, at least. You cannot mock a static method in any way. There is a post on google testing blog about this issue.
My rule of thumb is to write static methods only when they have no external dependencies (like database access, read files, emails and so on) to keep them as simple as possible.
Just remember that whenever you are writing a static method, you are writing an inflexible method that cannot have it's behavior modified very easily.
You are writing procedural code, so if it makes sense to be procedural, then do it. If not, it should probably be an instance method.
This idea is taken from an article by Steve Yegge, which I think is an interesting and useful read.
#jagmal I think you've got some wires crossed somewhere - all the examples you list are clearly not static methods.
Static methods should deal entirely with abstract properties and concepts of a class - they should in no way relate to instance specific attributes (and most compilers will yell if they do).
For the car example, speed, kms driven are clearly attribute related. Gear shifting and speed calculation, when considered at the car level, are attribute dependent - but consider a carModel class that inherits from car: at this point theyy could become static methods, as the required attributes (such as wheel diameter) could be defined as constants at that level.
Performance-wise, a C++ static method can be slightly faster than a non-virtual instance method, as there's no need for a 'this' pointer to get passed to the method. In turn, both will be faster than virtual methods as there's no VMT lookup needed.
But, it's likely to be right down in the noise - particularly for languages which allow unnecessary parameter passing to be optimized out.
Here is a related discussion as to why String.Format is static that will highlight some reasons.
Another thing to consider when making methods static is that anyone able to see the class is able to call a static method. Whereas when the mehtod is an instance method, only those who have access to an instance are able to call that method.
Related
This is admittedly a rather loose question. My current understanding of singletons is that they are a class that you set up in such a way that only one instance is ever created.
This sounds a lot like a static class to me. The main difference being that with a static class you don't / can't instance it, you just use it such as Math.pi(). With a singleton class, you would still need to do something like
singleton firstSingleton = new singleton();
firstSingleton.set_name("foo");
singleton secondSingleton = new singleton();
Correct me if i am wrong, but firstSingleton == secondSingleton right now, yes?
secondSingleston.set_name("bar");
firstSingleton.report_name(); // will output "bar" won't it?
Please note, I am asking this language independently, more about the concept. So I am not worried about actually how to code such a class, but more why you would wan't to and what thing you would need to consider.
The main advantage of a singleton over a class consisting of statics is that you can later easily decide that you need in fact more than one instance, e.g. one per thread.
However, in practice the main purpose of singletons is to make people feel less bad about having global variables.
A practical example for a good use of a singleton: you have an app that uses an SQL database and you need a connection pool. The purpose of such a pool is to reuse DB connection, so you definitely want all clients to use the same pool. Thus, having it as a singleton is the correct design. But one day you need the app to connect to a second DB server, and realize that you cannot have connections to different servers in the same pool. Thus your "one instance overall" singleton becomes "one instance per DB server".
why you would wan't to
I wouldn't because singletons usually are very bad way to solve your problems. My recommendation to you is to avoid them completely.
The main reasons are:
Singletons mostly represent global state (which is evil).
Correct dependency injection becomes impossible.
I suggest you read the rest (including thorough explanations) in this Google employee's blog:
http://misko.hevery.com/2008/08/17/singletons-are-pathological-liars/
http://misko.hevery.com/2008/08/21/where-have-all-the-singletons-gone/
http://misko.hevery.com/2008/08/25/root-cause-of-singletons/
http://misko.hevery.com/code-reviewers-guide/flaw-brittle-global-state-singletons/
Like others have said:
Singletons are global variables by another name.
Singletons are usually a bad idea.
Singletons could be replaced by "monostate" classes - classes that have apparently normal construction / destruction semantics but all share the same state.
Note that in my opinion "static classes" are usually also a bad idea, a hackish workaround for a language that does not allow free functions, or for sharing state between a bunch of functions without wanting to pass that state as a parameter.
In my experience nearly all designs with singletons or static classes can be turned into something better, more easily understood and more flexible by getting rid of those constructs.
Edit: By request, why most singletons are global variables by another name.
In most of the languages I know, most singleton classes are accessed through a static member function of that class. The single instance is available to all code that has access to the definition of the singleton class. This is a global variable - all code that includes the class could be making modifications to the single instance of your singleton.
If you do not use the static member function (or some static factory method which has the same implications), but instead pass the singleton object to all clients that need it, then you would have no need for the singleton pattern, just pass the same object to all clients.
Singletons are mostly useful when you want an interface to a singleton service, but you don't know until runtime which concrete class will be instantiated.
For instance, you might want to declare a central logging service, but only decide at runtime whether to hook in a file logger, stub logger, database logger, or message-queue logger.
A little knowledge is a dangerous thing and Singletons are dangerous entities. In addition to written things above, I can emphasize the life-time management of Singleton objects are also important. In ACE framework, it is handled successfully. You can find the paper here: http://www.cs.wustl.edu/~schmidt/PDF/ObjMan.pdf
Please also note that singletons should be non-copyable classes. This pattern may seem to be the easiest one, but, on the contrary it is one of the difficult. Therefore, I ask to candidates about this evil points in Singletons.
Not all languages have "static classes" (for example C++ doesn't have them).
Again with the C++ example, adding static variables to a class is a pain because you need to put them in both the header and the .cpp file, so a singleton in that case is very useful.
Every language is different. I guess in C# they are not very useful (and in fact, from what I know, they are not used very often)
Singleton is a very useful replacement of global variables, used all across the code.
Singletons are usually not "new"ed or "delete"d, they tend to be initialized on first use and deleted along with program scope
Singletons perfectly match for wrapping logging, configuration and other hardware-interfacing classes.
In addition to the other answers I'd have to say that Singletons can help you when you want a static class, but can't have it, because due to the design of your application it will be inheriting an instantiable class.
There's two ways to use singletons.
The way they should be used. Typically with immutable variables (C#'s String.Empty, classes in Smalltalk, etc.). This is approximately 1% of singleton usage.
As a replacement for global variables. This is bad. The root cause of this is people that want to share common objects without understanding how to properly use a Builder. Use of Singletons in this fashion is typically a sign of a lack of deep understanding of object-oriented design.
It is pretty much another word for "Global Variables", which has its pros and cons. However, the only thing that would make the Singleton worthy your time is it assures some sort of "maintainability" in the future for your code in case you decided that there is a actually a need for more than one "instance" of that class.
I'm refactoring some code and I'm looking at a class called HFile. HFile has all private constructors so that you can't actually create instances of it. Instead of creating instances of HFiles as follow:
var file = new HFile('filename')
file.Save()
all HFile interaction is handled via static methods. So if I wanted to save a file I would call:
HFile.save('filename')
and then internally an instance of HFile would be created and then saved. Obviously without knowing the whole story any reader must reserve judgment, but it seems like using static methods has become very fashionable at my place of work. So I'm wondering if there are good principles/best practices for usage of static methods that can helpful for a group of guys sitting down and reviewing their usage of static methods.
Lots of static methods/static classes is a symptom of proceduralitis -- writing procedural code in an object-oriented language. The best way that I know of to eliminate this kind of thinking is a thorough understanding of object-oriented principles and practices. Using test-driven development and forcing code to be testable will help, because it is much harder to write tests for static classes. Eventually, if you use TDD you naturally gravitate towards more decoupled, OO architectures, if only to ease the testing pain.
In general, if your situation requires encapsulation of state or an organizational structure, then you will be using classes.
On the other hand, if you have something that is a cross-cutting concern, and can be used broadly across your application, you might consider methods in a static utility class. System.Math in the .NET framework (and Java) is an example of this.
In your example, HFile is probably an object with state, so I would not generally use a static method to save it. It's simpler just to make a method call on the specific HFile object, rather than having to pass the entire object to a static method for saving. Whether that makes sense or not in your particular application depends on whether your application's paradigm sees HFile objects as things to be passed around and acted on by outside methods, or as standalone objects capable of saving themselves.
Static methods are hard to test because you can't mock them out. For this reason we tend to avoid them at my place of work. Though we do use them for factory methods.
I wouldn't worry about the number of static methods so much as I would about what these methods are doing. A static method that returns a value, or modifies an object that you pass as an argument? No big deal. A static method that modifies private static fields? I worry. A static method that modifies public static fields belonging to other classes? I need to lie down in a darkened room with a damp washcloth on my brow.
This isn't very object oriented, is it. Now maybe your place doesn't really like OO code, and that's fine. But if you want to encapsulate data and methods, then you need instance methods to work on that data.
An abundance of static methods is probably associated with lots of global variables. Eg. the info that you're trying to save into the filename must be global if you're going to call a static HFile.save('filename'). Generally we try to reduce globals to keep things more manageable.
But if you want to write procedural code, then it's ok.
IMO static methods are not useful for the purpose you've described is common at your workplace.
Disadvantages of this method:
- what's the point of creating an object that represents a file in order essentially just to call one method on it?
- cannot use interface-based polymorphism
To answer your question, here are some cases where I would use a static method:
- a utility method that does something related to the functionality of the class, but not to any one object. Perhaps it takes an array of objects and compares them. Perhaps it does some generic data manipulation (conversions, etc.).
- where you need to do class-related stuff with class variables
- where you want to implement a singleton design pattern
I just realize some of my classes have no members. It has several public functions and maybe private functions and everything is passes through params. I realize functional programmers do this all the time but is this class considered special if it access nothing outside of the class and only uses its params for reading (except for out params and return values)?
I know this class can be static but static classes can modify outside variables. I want to know if this is a technique or maybe if a language may give additional benefits for writing it this way and etc.
-edit- This is looking like a wiki so lets make it one.
It is just called "stateless". Nothing really special about it.
There is nothing wrong with a class that has no members; controllers do this very frequently.
Yes, you could go static, but by not being static you allow for inheritance from your memberless class, which could add members along the way, if you so desired.
If there are no members, then there is no need for a class other than having a namespace. If you were programming in Python, then you would just put those methods into a module and don't bother with a class. If you are working in Java, then a a class is a must. If you are working in C++, it's up to you - but maybe you should consider using just a namespace, instead of a class, to make it less confusing.
Yes, this makes the class completely thread safe and thus no locking is required. To me, that is a fantastic attribute.
Sane programming languages allow you to define functions outside of classes when the functions do not require any persistent data - if you can define it in an appropriate namespace, all the better. If your language does not allow that, at least define them in a static class so it's clear that no state is accessed or mutated. Overall, though, this reminds me of an excellent article on the illogical abuse of classes in the name of "pure OOP".
I'd make the class static. I don't see what advantage it would give to keep it non-static. But then again - I'm a .NET developer, not Java. The languages are close, but there are many subtle differences I'm not aware of.
Remember that all methods of your class (even stateless) have one special variable -- pointer/reference to object - this (self, whatever) for which they are applied to.
Thus it is perfect sense in such stateless class if its methods could be overridden: in this case you have to have class to provide dispatching by this.
(Of course it's just emulating of first-class functions, so if your language already has ones it's no sense in this technique.)
At the moment I can't imagine why would you need stateless class without any virtual method.
Unless you want to have nice auto-complete in your IDE by typing objectName and dot :)
its simple class there is no specialty in it.
But i dont understand what is the use of having public or private methods when there in no member in it. because member methods are those which acts on particular instance's state.
Yes you can have static methods in it.
This question already has answers here:
Why use getters and setters/accessors?
(37 answers)
Closed 7 years ago.
Allen Holub wrote the following,
You can't have a program without some coupling. Nonetheless, you can minimize coupling considerably by slavishly following OO (object-oriented) precepts (the most important is that the implementation of an object should be completely hidden from the objects that use it). For example, an object's instance variables (member fields that aren't constants), should always be private. Period. No exceptions. Ever. I mean it. (You can occasionally use protected methods effectively, but protected instance variables are an abomination.)
Which sounds reasonable, but he then goes on to say,
You should never use get/set functions for the same reason—they're just overly complicated ways to make a field public (though access functions that return full-blown objects rather than a basic-type value are reasonable in situations where the returned object's class is a key abstraction in the design).
Which, frankly, just sounds insane to me.
I understand the principle of information hiding, but without accessors and mutators you couldn't use Java beans at all. I don't know how you would follow a MVC design without accessors in the model, since the model can not be responsible for rendering the view.
However, I am a younger programmer and I learn more about Object Oriented Design everyday. Perhaps someone with more experience can weigh in on this issue.
Allen Holub's articles for reference
Why Extends Is Evil
Why Getter And Setter Methods Are Evil
Related Questions:
Java: Are Getters and Setters evil?
Is it really that wrong not using setters and getters?
Are get and set functions popular with C++ programmers?
Should you use accessor properties from within the class, or just from outside of the class?
I don't have a problem with Holub telling you that you should generally avoid altering the state of an object but instead resort to integrated methods (execution of behaviors) to achieve this end. As Corletk points out, there is wisdom in thinking long and hard about the highest level of abstraction and not just programming thoughtlessly with getters/setters that just let you do an end-run around encapsulation.
However, I have a great deal of trouble with anyone who tells you that you should "never" use setters or should "never" access primitive types. Indeed, the effort required to maintain this level of purity in all cases can and will end up causing more complexity in your code than using appropriately implemented properties. You just have to have enough sense to know when you are skirting the rules for short-term gain at the expense of long-term pain.
Holub doesn't trust you to know the difference. I think that knowing the difference is what makes you a professional.
Read through that article carefully. Holub is pushing the point that getters and setters are an evil "default antipattern", a bad habit that we slip into when designing a system; because we can.
The thought process should be along the lines; What does this object do? What are its responsibilities? What are its behaviours? What does it know? Thinking long and hard on these questions leads you naturally towards designing classes which expose the highest-level interface possible.
A car is a good example. It exposes a well-defined, standardised high-level interface. I don't concern myself with setSpeed(60)... is that MPH or km/h? I just accelerate, cruise, decelerate. I don't have to think about the details in setSteeringWheelAngle(getSteeringWheelAngle()+Math.rad(-1.5)), I just turn(-1.5), and the details are taken care of under the hood.
It boils down to "You can and should figure out what every class will be used for, what it does, what it represents, and expose the highest level interface possible which fulfills those requirements. Getters and setters are usually a cop-out, when the programmer is just to lazy to do the analysis required to determine exactly what each class is and is-not, and so we go down the path of "it can do anything". Getters and setters are evil!
Sometimes the actual requirements for a class are unknowable ahead of time. That's cool, just cop-out and use getter/setter antipattern for now, but when you do know, through experience, what the class is being used for, you'll probably want to comeback and cleanup the dirty low level interface. Refactoring based on "stuff you wish you knew when you write the sucker in the first place" is par for the course. You don't have to know everything in order to make a start, it's just that the more you do know, the less rework is likely to be required upon the way.
That's the mentality he's promoting. Getters and setters are an easy trap to fall into.
Yes, beans basically require getters and setters, but to me a bean is a special case. Beans represent nouns, things, tangible identifiable (if not physical) objects. Not a lot of objects actually have automatic behaviours; most times things are manipulated by external forces, including humans, to make them productive things.
daisy.setColor(Color.PINK) makes perfect sense. What else can you do? Maybe a Vulcan mind-meld, to make the flower want to be pink? Hmmm?
Getters and setters have their ?evil? place. It's just, like all really good OO things, we tend to overuse them, because they are safe and familiar, not to mention simple, and therefore it might be better if noobs didn't see or hear about them, at least until they'd mastered the mind-meld thing.
I think what Allen Holub tried to say, rephrased in this article, is the following.
Getters and setters can be useful for variables that you specifically want to encapsulate, but you don't have to use them for all variables. In fact, using them for all variables is nasty code smell.
The trouble programmers have, and Allen Holub was right in pointing it out, is that they sometimes use getters/setters for all variables. And the purpose of encapsulation is lost.
(note I'm coming at this from a .NET "property" angle)
Well, simply - I don't agree with him; he makes a big fuss about the return type of properties being a bad thing because it can break your calling code - but exactly the same argument would apply to method arguments. And if you can't use methods either?
OK, method arguments could be changed as widening conversions, but.... just why... Also, note that in C# the var keyword could mitigate a lot of this perceived pain.
Accessors are not an implementation detail; they are the public API / contract. Yup, if you break the contracft you have trouble. When did that become a surprise? Likewise, it is not uncommon for accessors to be non-trivial - i.e. they do more than just wrap fields; they perform calculations, logic checks, notifications, etc. And they allow interface based abstractions of state. Oh, and polymorphism - etc.
Re the verbose nature of accessors (p3?4?) - in C#: public int Foo {get; private set;} - job done.
Ultimately, all of code is a means to express our intent to the compiler. Properties let me do that in a type-safe, contract-based, verifiable, extensible, polymorphic way - thanks. Why do I need to "fix" this?
Getters and setters are used as little more than a mask to make a private variable public.
There's no point repeating what Holub said already but the crux of it is that classes should represent behaviour and not just state.
Some opposing views are in italics:
Though getIdentity starts with "get," it's not an accessor because it doesn't just return a field. It returns a complex object that has reasonable behavior
Oh but wait... then it's okay to use accessors as long as you return objects instead of primitive types? Now that's a different story, but it's just as dumb to me. Sometimes you need an object, sometimes you need a primitive type.
Also, I notice that Allen has radically softened his position since his previous column on the same topic, where the mantra "Never use accessors" didn't suffer one single exception. Maybe he realized after a few year that accessors do serve a purpose after all...
Bear in mind that I haven't actually put any UI code into the business logic. I've written the UI layer in terms of AWT (Abstract Window Toolkit) or Swing, which are both abstraction layers.
Good one. What if you are writing your application on SWT? How "abstract" is really AWT in that case? Just face it: this advice simply leads you to write UI code in your business logic. What a great principle. After all, it's only been like at least ten years since we've identified this practice as one of the worst design decisions you can make in a project.
My problem is as a novice programmer is sometimes stumbling onto articles on the internet and give them more credence then I should. Perhaps this is one of those cases.
When ideas like these are presented to me, I like to take a look at libraries and frameworks I use and which I like using.
For example, although some will disagree, I like the Java Standard API. I also like the Spring Framework. Looking at the classes in these libraries, you will notice that very rarely there are setters and getters which are there just to expose some internal variable. There are methods named getX, but that does not mean it is a getter in the conventional sense.
So, I think he has a point, and it is this: every time you press choose "Generate getters/setters" in Eclipse (or your IDE of choice), you should take a step back and wonder what you are doing. Is it really appropriate to expose this internal representation, or did I mess up my design at some step?
I don't believe he's saying never use get/set, but rather that using get/set for a field is no better than just making the field public (e.g. public string Name vs. public string Name {get; set; }).
If get/set is used it limits the information hiding of OO which can potentially lock you into a bad interface.
In the above example, Name is a string; what if we want to change the design later to add multiple Names? The interface exposed only a single string so we can’t add more without breaking existing implementation.
However, if instead of using get/set you initially had a method such as Add(string name), internally you could process name singularly or add to a list or what not and externally call the Add method as many times as you want to add more Names.
The OO goal is to design with a level of abstraction; don’t expose more detail than you absolutely have to.
Chances are if you’ve just wrapped a primitive type with a get/set you’ve broken this tenet.
Of course, this is if you believe in the OO goals; I find that most don't, not really, they just use Objects as a convienient way to group functional code.
Public variables make sense when the class is nothing more than a bundle of data with no real coherency, or when it's really, really elementary (such as a point class). In general, if there's any variable in a class that you think probably shouldn't be public, that means that the class has some coherence, and variables have a certain relation that should be maintained, so all variables should be private.
Getters and setters make sense when they reflect some sort of coherent idea. In a polygon class, for example, the x and y coordinates of given vertices have a meaning outside the class boundary. It probably makes sense to have getters, and it likely makes sense to have setters. In a bank account class, the balance is probably stored as a private variable, and almost certainly should have a getter. If it has a setter, it needs to have logging built in to preserve auditability.
There are some advantages of getters and setters over public variables. They provide some separation of interface and implementation. Just because a point has a .getX() function doesn't mean there has to be an x, since .getX() and .setX() can be made to work just fine with radial coordinates. Another is that it's possible to maintain class invariants, by doing whatever's necessary to keep the class consistent within the setter. Another is that it's possible to have functionality that triggers on a set, like the logging for the bank account balance.
However, for more abstract classes, the member variables lose individual significance, and only make sense in context. You don't need to know all the internal variables of a C++ stream class, for example. You need to know how to get elements in and out, and how to perform various other actions. If you counted on the exact internal structure, you'd be bogged down in detail that could arbitrarily vary between compilers or versions.
So, I'd say to use private variables almost exclusively, getters and setters where they have a real meaning in object behavior, and not otherwise.
Just because getters and setters are frequently overused doesn't mean they're useless.
The trouble with getters/setters is they try to fake encapsulation but they actually break it by exposing their internals. Secondly they are trying to do two separate things - providing access to and controlling their state - and end up doing neither very well.
It breaks encapsulation because when you call a get/set method you first need to know the name (or have a good idea) of the field you want to change, and second you have to know it's type eg. you couldn't call
setPositionX("some string");
If you know the name and type of the field, and the setter is public, then anyone can call the method as if it were a public field anyway, it's just a more complicated way of doing it, so why not just simplify it and make it a public field in the first place.
By allowing access to it's state but trying to control it at the same time, a get/set method just confuses things and ends up either being useless boiler-plate, or misleading, by not actually doing what it says it does by having side-effects the user might not expect. If error checking is needed, it could be called something like
public void tryPositionX(int x) throws InvalidParameterException{
if (x >= 0)
this.x = x;
else
throw new InvalidParameterException("Holy Negatives Batman!");
}
or if additional code is needed it could be called a more accurate name based on what the whole method does eg.
tryPositionXAndLog(int x) throws InvalidParameterException{
tryPositionX(x);
numChanges++;
}
IMHO needing getters/setters to make something work is often a symptom of bad design.
Make use of the "tell, don't ask" principle, or re-think why an object needs to send it's state data in the first place. Expose methods that change an object's behaviour instead of it's state. Benefits of that include easier maintenance and increased extensibility.
You mention MVC too and say a model can't be responsible for it's view, for that case Allen Holub gives an example of making an abstraction layer by having a "give-me-a-JComponent-that-represents-your-identity class" which he says would "isolate the way identities are represented from the rest of the system." I'm not experienced enough to comment on whether that would work or not but on the surface it sounds a decent idea.
Public getters/setters are bad if they provide access to implementation details. Yet, it is reasonable to provide access to object's properties and use getters/setters for this. For example, if Car has the color property, it's acceptable to let clients "observe" it using a getter. If some client needs the ability to recolor a car, the class can provide a setter ('recolor' is more clear name though). It is important to do not let clients know how properties are stored in objects, how they are maintained, and so on.
Ummmm...has he never heard of the concept of Encapsulation. Getter and Setter methods are put in place to control access to a Class' members. By making all fields publicly visible...anybody could write whatever values they wanted to them thereby completely invalidating the entire object.
Just in case anybody is a little fuzzy on the concept of Encapsulation, read up on it here:
Encapsulation (Computer Science)
...and if they're really evil, would .NET build the Property concept into the language? (Getter and Setter methods that just look a little prettier)
EDIT
The article does mention Encapsulation:
"Getters and setters can be useful for variables that you specifically want to encapsulate, but you don't have to use them for all variables. In fact, using them for all variables is nasty code smell."
Using this method will lead to extremely hard to maintain code in the long run. If you find out half way through a project that spans years that a field needs to be Encapsulated, you're going to have to update EVERY REFERENCE of that field everywhere in your software to get the benefit. Sounds a lot smarter to use proper Encapsulation up front and safe yourself the headache later.
I think that getters and setters should only be used for variables which one needs to access or change outside a class. That being said, I don't believe variables should be public unless they're static. This is because making variables public which aren't static can lead to them being changed undesirably. Let's say you have a developer who is carelessly using public variables. He then accesses a variable from another class and without meaning to, changes it. Now he has an error in his software as a result of this mishap. That's why I believe in the proper use of getters and setters, but you don't need them for every private or protected variable.
I was confused when I first started to see anti-singleton commentary. I have used the singleton pattern in some recent projects, and it was working out beautifully. So much so, in fact, that I have used it many, many times.
Now, after running into some problems, reading this SO question, and especially this blog post, I understand the evil that I have brought into the world.
So: How do I go about removing singletons from existing code?
For example:
In a retail store management program, I used the MVC pattern. My Model objects describe the store, the user interface is the View, and I have a set of Controllers that act as liason between the two. Great. Except that I made the Store into a singleton (since the application only ever manages one store at a time), and I also made most of my Controller classes into singletons (one mainWindow, one menuBar, one productEditor...). Now, most of my Controller classes get access the other singletons like this:
Store managedStore = Store::getInstance();
managedStore.doSomething();
managedStore.doSomethingElse();
//etc.
Should I instead:
Create one instance of each object and pass references to every object that needs access to them?
Use globals?
Something else?
Globals would still be bad, but at least they wouldn't be pretending.
I see #1 quickly leading to horribly inflated constructor calls:
someVar = SomeControllerClass(managedStore, menuBar, editor, sasquatch, ...)
Has anyone else been through this yet? What is the OO way to give many individual classes acces to a common variable without it being a global or a singleton?
Dependency Injection is your friend.
Take a look at these posts on the excellent Google Testing Blog:
Singletons are pathologic liars (but you probably already understand this if you are asking this question)
A talk on Dependency Injection
Guide to Writing Testable Code
Hopefully someone has made a DI framework/container for the C++ world? Looks like Google has released a C++ Testing Framework and a C++ Mocking Framework, which might help you out.
It's not the Singleton-ness that is the problem. It's fine to have an object that there will only ever be one instance of. The problem is the global access. Your classes that use Store should receive a Store instance in the constructor (or have a Store property / data member that can be set) and they can all receive the same instance. Store can even keep logic within it to ensure that only one instance is ever created.
My way to avoid singletons derives from the idea that "application global" doesn't mean "VM global" (i.e. static). Therefore I introduce a ApplicationContext class which holds much former static singleton information that should be application global, like the configuration store. This context is passed into all structures. If you use any IOC container or service manager, you can use this to get access to the context.
There's nothing wrong with using a global or a singleton in your program. Don't let anyone get dogmatic on you about that kind of crap. Rules and patterns are nice rules of thumb. But in the end it's your project and you should make your own judgments about how to handle situations involving global data.
Unrestrained use of globals is bad news. But as long as you are diligent, they aren't going to kill your project. Some objects in a system deserve to be singleton. The standard input and outputs. Your log system. In a game, your graphics, sound, and input subsystems, as well as the database of game entities. In a GUI, your window and major panel components. Your configuration data, your plugin manager, your web server data. All these things are more or less inherently global to your application. I think your Store class would pass for it as well.
It's clear what the cost of using globals is. Any part of your application could be modifying it. Tracking down bugs is hard when every line of code is a suspect in the investigation.
But what about the cost of NOT using globals? Like everything else in programming, it's a trade off. If you avoid using globals, you end up having to pass those stateful objects as function parameters. Alternatively, you can pass them to a constructor and save them as a member variable. When you have multiple such objects, the situation worsens. You are now threading your state. In some cases, this isn't a problem. If you know only two or three functions need to handle that stateful Store object, it's the better solution.
But in practice, that's not always the case. If every part of your app touches your Store, you will be threading it to a dozen functions. On top of that, some of those functions may have complicated business logic. When you break that business logic up with helper functions, you have to -- thread your state some more! Say for instance you realize that a deeply nested function needs some configuration data from the Store object. Suddenly, you have to edit 3 or 4 function declarations to include that store parameter. Then you have to go back and add the store as an actual parameter to everywhere one of those functions is called. It may be that the only use a function has for a Store is to pass it to some subfunction that needs it.
Patterns are just rules of thumb. Do you always use your turn signals before making a lane change in your car? If you're the average person, you'll usually follow the rule, but if you are driving at 4am on an empty high way, who gives a crap, right? Sometimes it'll bite you in the butt, but that's a managed risk.
Regarding your inflated constructor call problem, you could introduce parameter classes or factory methods to leverage this problem for you.
A parameter class moves some of the parameter data to it's own class, e.g. like this:
var parameterClass1 = new MenuParameter(menuBar, editor);
var parameterClass2 = new StuffParameters(sasquatch, ...);
var ctrl = new MyControllerClass(managedStore, parameterClass1, parameterClass2);
It sort of just moves the problem elsewhere though. You might want to housekeep your constructor instead. Only keep parameters that are important when constructing/initiating the class in question and do the rest with getter/setter methods (or properties if you're doing .NET).
A factory method is a method that creates all instances you need of a class and have the benefit of encapsulating creation of the said objects. They are also quite easy to refactor towards from Singleton, because they're similar to getInstance methods that you see in Singleton patterns. Say we have the following non-threadsafe simple singleton example:
// The Rather Unfortunate Singleton Class
public class SingletonStore {
private static SingletonStore _singleton
= new MyUnfortunateSingleton();
private SingletonStore() {
// Do some privatised constructing in here...
}
public static SingletonStore getInstance() {
return _singleton;
}
// Some methods and stuff to be down here
}
// Usage:
// var singleInstanceOfStore = SingletonStore.getInstance();
It is easy to refactor this towards a factory method. The solution is to remove the static reference:
public class StoreWithFactory {
public StoreWithFactory() {
// If the constructor is private or public doesn't matter
// unless you do TDD, in which you need to have a public
// constructor to create the object so you can test it.
}
// The method returning an instance of Singleton is now a
// factory method.
public static StoreWithFactory getInstance() {
return new StoreWithFactory();
}
}
// Usage:
// var myStore = StoreWithFactory.getInstance();
Usage is still the same, but you're not bogged down with having a single instance. Naturally you would move this factory method to it's own class as the Store class shouldn't concern itself with creation of itself (and coincidentally follow the Single Responsibility Principle as an effect of moving the factory method out).
From here you have many choices, but I'll leave that as an exercise for yourself. It is easy to over-engineer (or overheat) on patterns here. My tip is to only apply a pattern when there is a need for it.
Okay, first of all, the "singletons are always evil" notion is wrong. You use a Singleton whenever you have a resource which won't or can't ever be duplicated. No problem.
That said, in your example, there's an obvious degree of freedom in the application: someone could come along and say "but I want two stores."
There are several solutions. The one that occurs first of all is to build a factory class; when you ask for a Store, it gives you one named with some universal name (eg, a URI.) Inside that store, you need to be sure that multiple copies don't step on one another, via critical regions or some method of ensuring atomicity of transactions.
Miško Hevery has a nice article series on testability, among other things the singleton, where he isn't only talking about the problems, but also how you might solve it (see 'Fixing the flaw').
I like to encourage the use of singletons where necessary while discouraging the use of the Singleton pattern. Note the difference in the case of the word. The singleton (lower case) is used wherever you only need one instance of something. It is created at the start of your program and is passed to the constructor of the classes that need it.
class Log
{
void logmessage(...)
{ // do some stuff
}
};
int main()
{
Log log;
// do some more stuff
}
class Database
{
Log &_log;
Database(Log &log) : _log(log) {}
void Open(...)
{
_log.logmessage(whatever);
}
};
Using a singleton gives all of the capabilities of the Singleton anti-pattern but it makes your code more easily extensible, and it makes it testable (in the sense of the word defined in the Google testing blog). For example, we may decide that we need the ability to log to a web-service at some times as well, using the singleton we can easily do that without significant changes to the code.
By comparison, the Singleton pattern is another name for a global variable. It is never used in production code.