Difference between class methods and instance methods?

I always confusing to when i used of instance method and class method in programming. Please tell me difference between instance meth开发者_高级运维od and class methods and advantages of one another.

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All the other answers seem to have been caught out by the incorrect tag that has now been fixed.

In Objective-C, an instance method is a method that is invoked when a message is sent to an instance of a class. So, for instance:

id foo = [[MyClass alloc] init];
[foo someMethod];
//   ^^^^^^^^^^   This message invokes an instance method.

In Objective-C, classes are themselves objects and a class method is simply a method that is invoked when a message is sent to a class object. i.e.

[MyClass someMethod];
//       ^^^^^^^^^^   This message invokes a class method.

Note that, in the above examples the selector is the same, but because in one case it is sent to an instance of MyClass and in the other case it is sent to MyClass, different methods are invoked. In the interface declaration, you might see:

@interface MyClass : NSObject
{
}

+(id) someMethod;  // declaration of class method
-(id) someMethod;  // declaration of instance method

@end

and in the implementation

@implementation MyClass

+(id) someMethod
{
    // Here self is the class object
}
-(id) someMethod
{
    // here self is an instance of the class
}

@end

Edit

Sorry, missed out the second part. There are no advantages or disadvantages as such. It would be like asking what is the difference between while and if and what are the advantages of one over the other. It's sort of meaningless because they are designed for different purposes.

The most common use of class methods is to obtain an instance when you need one. +alloc is a class method which gives you a new uninitialised instance. NSString has loads of class methods to give you new strings, e.g. +stringWithForma

Another common use is to obtain a singleton e.g.

+(MyClass*) myUniqueObject
{
    static MyUniqueObject* theObject = nil;
    if (theObject == nil)
    {
        theObject  = [[MyClass alloc] init];
    }
    return theObject;
}

The above method would also work as an instance method, since theObject is static. However, the semantics are clearer if you make it a class method and you don't have to first create an instance.

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If we don't want to create the object of class then we use the class method if we want call the method through object of a class then we use the instance method

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I don't know if we can talk of any advantage, this is rather a matter of what you are implementing.

Instance methods apply on instances of classes, so they need an object to be applied on and can access their caller's members:

Foo bar;
bar.instanceMethod();

On the other hand class methods apply on the whole class, they don't rely on any object:

Foo::classMethod();

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Static member functions are informally called class methods (incorrectly). In C++ there are no methods, there are member functions.

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Read up on the static keyword, that pretty much covers it.

MSDN: http://msdn.microsoft.com/en-us/library/s1sb61xd.aspx

Google search: http://www.google.ch/search?aq=f&sourceid=chrome&ie=UTF-8&q=static+keyword+c%2B%2B

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Class methods are used with classes but instance methods are used with objects of that class i.e instance

//Class method example
className *objectName = [[className alloc]init]; 
[objectName methodName];

//Instance method example
[className methodName];

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instance methods use an instance of a class, whereas a class method can be used with just the class name. + sign is used before the Class Method where as single desh (-) is used before the instance variable.

@interface MyClass : NSObject

+ (void)aClassMethod;
- (void)anInstanceMethod;

@end

They could also be used like so,

[MyClass aClassMethod];

MyClass *object = [[MyClass alloc] init];
[object anInstanceMethod];

or another example is:

[

NSString string]; //class method

NSString *mystring = [NSString alloc]init];
[mystring changeText]; //instance Method

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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:

// class methods must be called on the class itself
[MyClass aClassMethod];

// instance method require an instance of the class
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.