What is the difference between class and instance methods

Question

What s the difference between a class method and an instance method   Are instance methods the accessors  getters and setters  while class methods are pretty much everything else

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Class methods are usually used to create instances of that class

For example, [NSString stringWithFormat:@"SomeParameter"]; returns an NSString instance with the parameter that is sent to it. Hence, because it is a Class method that returns an object of its type, it is also called a convenience method.

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In Objective-C all methods start with either a  -  or     character   Example    interface MyClass   NSObject    instance method -  void  instanceMethod      void  classMethod   end   The     and  -  characters specify whether a method is a class method or an instance method respectively   The difference would be clear if we call these methods  Here the methods are declared in MyClass   instance method require an instance of the class   MyClass  myClass     MyClass alloc  init    myClass instanceMethod     Inside MyClass other methods can call instance methods of MyClass using self   - void  someMethod        self instanceMethod       But  class methods must be called on the class itself    MyClass classMethod     Or   MyClass  myClass     MyClass alloc  init    myClass class  classMethod     This won t work      Error  myClass classMethod      Error  self classMethod

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So if I understand it correctly   A class method does not need you to allocate instance of that object to use   process it  A class method is self contained and can operate without any dependence of the state of any object of that class  A class method is expected to allocate memory for all its own work and deallocate when done  since no instance of that class will be able to free any memory allocated in previous calls to the class method   A instance method is just the opposite  You cannot call it unless you allocate a instance of that class  Its like a normal class that has a constructor and can have a destructor  that cleans up all the allocated memory    In most probability  unless you are writing a reusable library  you should not need a class variable

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Instances methods operate on instances of classes  ie   objects     Class methods are associated with classes  most languages use the keyword static for these guys

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Adding to above answers  Class method will work on class  we will use this for general purpose where like  stringWithFormat  size of class and most importantly for init  etc  NSString  str    NSString stringWithFormat     02f    someFloat      Instance Method will work on an instance of a class not on a class like we are having two persons and we want to get know the balance of each separately here we need to use instance method  Because it won t return general response  e g  like determine the count of NSSArray etc    johnson getAccountBalance    ankit getAccountBalance

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Note  This is only in pseudo code format  Class method     Almost does all it needs to do is during compile time  It doesn t need any user input  nor the computation of it is based on an instance  Everything about it is based on the class blueprint      which is unique ie you don t have multiple blueprints for one class  Can you have different variations during compile time  No  therefore the class is unique and so no matter how many times you call a class method the pointer pointing to it would be the same    PlanetOfLiving  return   Earth     No matter how many times you run this method   nothing changes    Instance Method    On the contrary instance method happens during runtime  since it is only then that you have created an instance of something which could vary upon every instantiation   initWithName    John  lastName    Doe Age 12   cool  initWithName    Donald  lastName    Drumpf Age 5 attitude   He started  initWithName    President  lastName    Obama Age 54 attitude    Awesome    As you can see the value can change for each instance      If you are coming from other languages Static methods are same as class methods  If you are coming from Swift  type methods are same as class methods

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Class methods can t change or know the value of any instance variable  That should be the criteria for knowing if an instance method can be a class method

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An instance method applies to an instance of the class  i e  an object  whereas a class method applies to the class itself   In C  a class method is marked static   Methods and properties not marked static are instance methods   class Foo     public static void ClassMethod             public void InstanceMethod

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I think the best way to understand this is to look at alloc and init  It was this explanation that allowed me to understand the differences   Class Method  A class method is applied to the class as a whole  If you check the alloc method  that s a class method denoted by the   before the method declaration  It s a class method because it is applied to the class to make a specific instance of that class   Instance Method  You use an instance method to modify a specific instance of a class that is unique to that instance  rather than to the class as a whole  init for example  denoted with a - before the method declaration   is an instance method because you are normally modifying the properties of that class after it has been created with alloc   Example  NSString  myString    NSString alloc     You are calling the class method alloc in order to generate an instance of that class  Notice how the receiver of the message is a class    myString initWithFormat   Hope this answer helps someone      You are modifying the instance of NSString called myString by setting some properties on that instance  Notice how the receiver of the message is an instance  object of class NSString

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Also remember  the same idea applies to variables  You will come across terms like static  member  instance  class and so on when talking about variables the same as you would for methods functions   It seems the common term in the Obj-C community is ivar for instance variable  but I am not an  Obj-C guy  yet

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Like most of the other answers have said  instance methods use an instance of a class  whereas a class method can be used with just the class name   In Objective-C they are defined thusly    interface MyClass   NSObject     void aClassMethod  -  void anInstanceMethod    end   They could then be used like so    MyClass aClassMethod    MyClass  object     MyClass alloc  init    object anInstanceMethod     Some real world examples of class methods are the convenience methods on many Foundation classes like NSString s  stringWithFormat  or NSArray s  arrayWithArray    An instance method would be NSArray s -count method

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CLASS METHODS    A class method typically either creates a new instance of the class or retrieves some global properties of the class  Class methods do not operate on an instance or have any access to instance variable     INSTANCE METHODS    An instance method operates on a particular instance of the class  For example  the accessors method that you implemented are all instance methods  You use them to set or get the instance variables of a particular object     INVOKE    To invoke an instance method  you send the message to an instance of the class   To invoke a class method  you send the message to the class directly     Source  IOS - Objective-C - Class Methods And Instance Methods

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Like the other answers have said  instance methods operate on an object and has access to its instance variables  while a class method operates on a class as a whole and has no access to a particular instance s variables  unless you pass the instance in as a parameter    A good example of an class method is a counter-type method  which returns the total number of instances of a class  Class methods start with a    while instance ones start with an -   For example   static int numberOfPeople   0    interface MNPerson   NSObject        int age     instance variable       int population    class method  Returns how many people have been made  -  id init    instance  Constructs object  increments numberOfPeople by one  -  int age    instance  returns the person age  end   implementation MNPerson -  id init      if  self    super init              numberOfPeople              age   0                return self        int population        return numberOfPeople     -  int age       return age      end   main m   MNPerson  micmoo     MNPerson alloc  init   MNPerson  jon     MNPerson alloc  init   NSLog   Age   d   micmoo age    NSLog    Number Of people   d   MNPerson population      Output   Age  0 Number Of people  2  Another example is if you have a method that you want the user to be able to call  sometimes its good to make that a class method  For example  if you have a class called MathFunctions  you can do this      int square  int num         return num   num      So then the user would call     MathFunctions square 34     without ever having to instantiate the class   You can also use class functions for returning autoreleased objects  like NSArray s      NSArray   arrayWithObject  id object   That takes an object  puts it in an array  and returns an autoreleased version of the array that doesn t have to be memory managed  great for temperorary arrays and what not   I hope you now understand when and or why you should use class methods

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class methods   are methods which are declared as static  The method can be called without creating an instance of the class  Class methods can only operate on class members and not on instance members as class methods are unaware of instance members  Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class   Instance methods   on the other hand require an instance of the class to exist before they can be called  so an instance of a class needs to be created by using the new keyword  Instance methods operate on specific instances of classes  Instance methods are not declared as static

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The answer to your question is not specific to objective-c  however in different languages  Class methods may be called static methods   The difference between class methods and instance methods are  Class methods   Operate on Class variables  they can not access instance variables  Do not require an object to be instantiated to be applied Sometimes can be a code smell  some people who are new to OOP use as a crutch to do Structured Programming in an OO enviroment    Instance methods   Operate on instances variables and class variables Must have an instanciated object to operate on

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All the technical details have been nicely covered in the other answers  I just want to share a simple analogy that I think nicely illustrates the difference between a class and an instance     A class is like the blueprint of a house  You only have one blueprint and  usually  you can t do that much with the blueprint alone     An instance  or an object  is the actual house that you build based on the blueprint  You can build lots of houses from the same blueprint  You can then paint the walls a different color in each of the houses  just as you can independently change the properties of each instance of a class without affecting the other instances

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Take for example a game where lots of cars are spawned   each belongs to the class CCar  When a car is instantiated  it makes a call to    CCar registerCar self    So the CCar class  can make a list of every CCar instantiated  Let s say the user finishes a level  and wants to remove all cars    you could either  1- Go through a list of every CCar you created manually  and do whicheverCar remove    or 2- Add a removeAllCars method to CCar  which will do that for you when you call  CCar removeAllCars   I e    allCars n  remove     Or for example  you allow the user to specify a default font size for the whole app  which is loaded and saved at startup  Without the class method  you might have to do something like    fontSize   thisMenu getParent   fontHandler getDefaultFontSize      With the class method  you could get away with  FontHandler getDefaultFontSize    As for your removeVowels function  you ll find that languages like C  actually have both with certain methods such as toLower or toUpper   e g  myString removeVowels    and  String removeVowels myString     in ObjC that would be  String removeVowels myString     In this case the instance likely calls the class method  so both are available  i e   public function toLower   String    return String toLower       public static function toLower  String inString  String     do stuff to string    return newString      basically  myString toLower    calls   String toLower ownValue   There s no definitive answer  but if you feel like shoving a class method in would improve your code  give it a shot  and bear in mind that a class method will only let you use other class methods variables

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An update to the above answers   I agree  instance methods use an instance of a class  whereas a class method can be used with just the class name    There is NO more any difference between instance method  amp  class method after automatic reference counting came to existence in Objective-C   For Example NS StringWithformat     a class method  amp    NSString alloc  initwihtformat     an instance method  both are same after ARC

[objective-c] What is the difference between class and instance methods?

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