I was wondering when and when not to use class methods and instance methods. I need some practical examples. I am really confused. Another question: can't we do exactly the same things with instance methods that we can with class methods?
Class methods: The method isn't tied to any specific object. In a way it acts like a free function in the class's namespace. No 'self' pointer. For instance [UIScreen mainScreen] is a class method because there's only one screen and there's no need to care about multiple 'screen instances'.
Instance method: Tied to a specific object.
This applies to most OO languages, not just obj-C.
At the implementation level, an instance method call contains a hidden pointer to a data structure (the object), a class method does not.
The practical question to ask is whether your call requires sending the call some specific data which is or could best be encapsulated as instance data inside an object, or not.
You (usually) can do the same thing with class methods as instance methods, but then you have to explicitly pass the object as a visible parameter in the call, which is uglier looking and also potentially disables some method override features of the Objective C language.
Use class methods for utility functions and Instance methods for object oriented stuff.
Eg. For Mathematical calculation (eg sin x ) use class method. But for invoking a behavior specific to an object.. use instance method ..
A class method as the name implies is bounded to the class. You can invoke them just with the name of the particular class. These can be normally exposed methods of a class.
For example
NSArray
+ (id)arrayWithArray:(NSArray *)array;.
You call it with the class name NSArray. What you expect is just a creation of a object of the type of that particular class. This doesn't need an object to invoke. Also these are very basic method required so its better to make it as a class method.
On the other hand instance method as the name implies is very much bound to the instance. Object is an entity that encapsulates state (ivars) and behaviors (methods) of a class. This can be very specific to the object.
For example
- (NSUInteger)count;
Lets take NSArray *a and NSArray *b. If a contains 5 items whereas b contains 4, instance methods called upon these instances will produce different results. And thats why we need instances to be initialized while invoking instance method. They work on the context(or state) of the object they are been called upon. Also they are not exposed as the class methods are.
Hope this helps.
If you want to use instance objet or instance variable you have to go with instance Methods.
Bcz Inside the class you cant access the Instance instance objet or instance variable.
Class methods are static methods.
Related
"Instance" mean in Objective-C?
Kindly tell me where to use Class Method And where to use Instance Method,also tell me where we use (Instacetype) method?
why/where we use multi Parameters?
A class method is a method whose self parameter is a reference to the class's class object.
An instance method is a method whose self parameter is a reference to a specific instance of the class.
Those are the technical differences.
A more practical answer is that an instance method operates on a single instance of the class, while a class method operates at a more global, non-specific level. A class method can act as a factory method, such as NSString's stringWithFormat: method. It can also be used to configure behavior that will affect all instances of the class. It can also be used to operate on a collection of instances of the class, such as sorting or filtering.
instancetype is a keyword that can be used as a placeholder for the current class's type. It says to the compiler: pretend that I wrote <my class name> here, so if you see the result of this method assigned somewhere, you know what type it's supposed to be.
We always make singleton object through Class method.What will happened if i make that method instance(use - instead of +) and call that method through a nil object or simply through an object?
Conceptually a the instance method is not attached to any specific instance of a class.
This means that from a design point of view it doesn't make sense to invoke it on an existing object. It's like requiring that a +(int) sum:(int)a with:(int)b should be an instance method when it doesn't need to know anything about an object.
In addition usually a singleton class constructor shouldn't be accessible from clients, if you need to instantiate one to call -instance on it this would defeat the purpose.
Last thing, a singleton should be the only possible instance of an object, if you need to have one to instantiate one then you are in a deadlock: you need an instance to create an instance but you are no allowed to have more than an instance.
Regarding calling it on nil object, if I remember correctly a pointer returned from a message sent to nil will always be nil too.
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Objective-C: Class vs Instance Methods?
Objective-C - difference between class method and static method?
In ObjC, A single dash before a method name means it's a instance method. A plus before a method name means it's a class method. but what is the difference in programming?
The difference between a class method and an instance method is that an
instance method requires an instance of the class on which it will
(generally) operate. The message to invoke an instance method must be
sent to an instance of a class.
Probably the most common single use of class methods is object
factories; messages that you send to a class to create an instance
configured according to the parameters you've sent in. For example in
Cocoa the NSString class has several class methods named
stringWithSomethingOrOther: that will create a new NSString object and
hand it back to you.
On the other hand, NSString also has many instance methods -
operations which really have no meaning without an actual instance to
work with. A commonly-used one might be the length method, which tells
you how many characters are in the specific NSString instance to which
the message is sent.
Also see this.
What is the difference between class and instance methods?
An instance method is invoked on objects. A class method is invoked on class.
For example the line:
SomeClass *object = [[SomeClass alloc] init];
Here you can see that the "alloc" works on "SomeClass" and not on "object".
Whereas:
[object callMyFunction]; will act on "object" and not "class". This is an instance method.
The main difference with those two is the former one ie with single dash before it is only called by the instance of that class where it is declared ie one have to create one instance of that class means one object for that class and using . one can call the instance method
In class method, the later one can be called directly using the class name. To call class methods one dosen't need any object.
Please refer this link from apple developers documents
I have a custom ViewController class and many instances of it, and I want them all to be able to message the same Model (another custom class, only one instance). Passing pointers to the Model along to new instances of the ViewController seems impractical, especially since the model is lazily instantiated. What is the cleanest, most idiomatic, ARC way to do this?
Usually a singleton in ObjC will have a class method that serves as an accessor for the single instance. The convention is for this to be called either defaultX or sharedX. If your model class is indeed a singleton, you should already have such a method. Since class names are globally available, all you have to do to access the instance anywhere in your program is [MyModelClass sharedModel].
In Objective-C, while creating any class, how do we decide whether we need to mark a method as Class method or Instance Method ?
I know the difference between the 2, but my question is how to decide the marking (+/-) for any method ?
+ denotes a class method, - denotes an instance method. You create class or instance methods where your application needs them. Should you actually know the difference between the two, and your application, then you should have no problems understanding when to use which.
I believe you don't know the differences in how they apply to your application, so here's a small primer:
You use a class method when you need to access some behaviour globally through all instances of that class. i.e., [[self class] someSpecialThing];
You also use a class method when you need a factory method; and
Everywhere else, you use an instance method.