Related
In Programming in Objective C, 4e, Chapter 9, Program 9.3:
#import "Square.h"
int main (int argc, char * argv[])
{
#autoreleasepool {
Square *mySquare = [[Square alloc] init];
...
// respondsTo:
if ( [mySquare respondsToSelector: #selector (setSide:)] == YES )
NSLog (#"mySquare responds to setSide: method");
...
if ( [Square respondsToSelector: #selector (alloc)] == YES )
NSLog (#"Square class responds to alloc method");
...
}
return 0;
}
Q1:
Since -respondsToSelector: is an instance method, not a class method, why would it be possible to use it on Square class directly?
Q2:
The book says you can use Square here instead of [Square class]. Is it only a exceptional shortcut, or is there any mechanism behind this?
Any help would be really appreciated! Thanks in advance!
From The Objective-C Programming Language, Objects, class, and Messaging,
All objects, classes and instances alike, need an interface to the
runtime system. Both class objects and instances should be able to
introspect about their abilities and to report their place in the
inheritance hierarchy. It’s the province of the NSObject class to
provide this interface.
So that NSObject methods don’t have to be implemented twice—once to
provide a runtime interface for instances and again to duplicate that
interface for class objects—class objects are given special dispensation to perform instance methods defined in the root class.
When a class object receives a message that it can’t respond to with a
class method, the runtime system determines whether there’s a root
instance method that can respond. The only instance methods that a
class object can perform are those defined in the root class, and only if there’s no class method that can do the job.
In this case, NSObject is the root class. As NSObject instances all comply with NSObject protocol, where -respondsToSelector: is defined, most class objects should be able to perform -respondsToSelector:.
Q1:
The simple answer is that, in addition to class methods, you can call any instances method of the root class (whatever the root class of your class is; in this case, NSObject) on a class object.
The more complicated answer is that class objects are instances of metaclasses. Whereas instance methods are methods on an instance, which are defined in the class; class methods are methods on the class object, which are defined in the metaclass. Each class has its own metaclass. The inheritance of metaclasses follows that of their classes; i.e. NSString's metaclass inherits from NSObject's metaclass. Ultimately, the root class's metaclass inherits from the root class; i.e. NSObject's metaclass inherits from NSObject. That is why all of NSObject's instance methods are available to class objects.
Q2:
[Square class] calls the class method +class (this is unrelated to -class). +class is essentially an identity method that simply returns the thing called on it (just like -self); i.e. if foo is a pointer to a class object, then [foo class] is the same as foo.
So +class seems pretty useless; why do we use it? That is because in the grammar of the Objective-C language, a class name is not a valid expression (unlike Smalltalk). So you cannot say id bar = Square;; that would not compile. As a special case in the grammar, a class name is allowed in place of the receiver in a message call expression, and the message is sent to the class object; i.e. [Square something]. So if you want to use the class object in any other expression context, we do this in a roundabout way by calling an identity method like +class; i.e. [Square class] is an expression that can be used in any expression context ([Square self] would also work, but we use [Square class] by convention, which is unfortunate, since it is confused with -class); we would have liked to just use Square, but can't due to the language.
In your case, it is already the receiver in a message call expression, so it is unnecessary to do [Square class]; Square already works there.
//Q1:
Since -respondsToSelector: is an instance method, not a class method, why would it be possible to use it on Square class directly?//
You seem to have this notion that class methods cannot be called from instance methods (and vice versa). On the contrary, it would seem to be the intent of the method -respondsToSelector to do so, most likely by getting the class of the sender with the -class method, then querying if the class responds to the selector and returning YES or NO. In a more localized example, consider the following:
-(void)someInstanceMethod{
[MyCurrentClass doClassMethod]; //basic equivalent of [self doClassMethid];
}
Is perfectly valid in Objective-C, provided MyCurrentClass is all alloc'd and init'ed.
//Q2:
The book says you can use Square here instead of [Square class]. Is it only a exceptional shortcut, or is there any mechanism behind this?//
It is completely redundant to send -class to a Class! It makes little sense, and is just extra unnecessary code. -class just queries for the reciever's class, no matter if it is an instance or Class object.
The Objective C run-time currently implements a class as an instance object of some other class. Thus a class will response to certain instance methods.
The real implementation, straight out of NSObject.m is as such:
- (BOOL)respondsToSelector:(SEL)aSelector {
PF_HELLO("")
return class_respondsToSelector( isa, aSelector );
}
Now, I have no idea why that PF_HELLO("") is there, but as you can see, it's literally ASKING the CLASS in the RUNTIME "Hey, do you have a method for this isa [instance] called aSelector?"
And, in Objective-C, class methods ALSO belong to instances, but, however, take lower precedence (the instance method of the same name as the class method is called before the class method).
Another aspect of Objective-C's Dynamic Typing is that the id type is in fact declared as follows:
typedef struct objc_class *Class;
typedef struct objc_object {
Class isa;
} *id;
So your instance object is in fact, a Class pointer. This means your -respondsToSelector messages go to the Class of the instance type as well. In your case, it means that -respondsToSelector is going to the objc_class FIRST.
Now in a test case, (straight out of libFoundation), my answer would be summed up like this:
Test *tst = [Test new];
fail_unless([tst respondsToSelector:#selector(testInstanceMethod)], "-[Test respondsToSelector:] returned NO for a valid instance method (testInstanceMethod).");
fail_if([tst respondsToSelector:#selector(testClassMethod)], "-[Test respondsToSelector:] returned YES for a class method (testInstanceMethod).");
fail_unless([Test respondsToSelector:#selector(testClassMethod)], "+[Test respondsToSelector:] returned NO for a valid class method (testClassMethod).");
fail_if([Test respondsToSelector:#selector(testInstanceMethod)], "+[Test respondsToSelector:] returned YES for an instance method (testInstanceMethod).");
fail_unless([tst respondsToSelector:#selector(init)], "-[Test respondsToSelector:] returned NO for an inherited instance method (-[NSObject init].");
fail_unless([Test respondsToSelector:#selector(alloc)], "+[Test respondsToSelector:] returned NO for an inherited class method (+[NSObject alloc]).");
[tst release];
I have a category on NSObject which supposed to so some stuff. When I call it on an object, I would like to override its dealloc method to do some cleanups.
I wanted to do it using method swizzling, but could not figure out how. The only examples I've found are on how to replace the method implementation for the entire class (in my case, it would override dealloc for ALL NSObjects - which I don't want to).
I want to override the dealloc method of specific instances of NSObject.
#interface NSObject(MyCategory)
-(void)test;
#end
#implementation NSObject(MyCategory)
-(void)newDealloc
{
// do some cleanup here
[self dealloc]; // call actual dealloc method
}
-(void)test
{
IMP orig=[self methodForSelector:#selector(dealloc)];
IMP repl=[self methodForSelector:#selector(newDealloc)];
if (...) // 'test' might be called several times, this replacement should happen only on the first call
{
method_exchangeImplementations(..., ...);
}
}
#end
You can't really do this since objects don't have their own method tables. Only classes have method tables and if you change those it will affect every object of that class. There is a straightforward way around this though: Changing the class of your object at runtime to a dynamically created subclass. This technique, also called isa-swizzling, is used by Apple to implement automatic KVO.
This is a powerful method and it has its uses. But for your case there is an easier method using associated objects. Basically you use objc_setAssociatedObject to associate another object to your first object which does the cleanup in its dealloc. You can find more details in this blog post on Cocoa is my Girlfriend.
Method selection is based on the class of an object instance, so method swizzling affects all instances of the same class - as you discovered.
But you can change the class of an instance, but you must be careful! Here is the outline, assume you have a class:
#instance MyPlainObject : NSObject
- (void) doSomething;
#end
Now if for just some of the instances of MyPlainObject you'd like to alter the behaviour of doSomething you first define a subclass:
#instance MyFancyObject: MyPlainObject
- (void) doSomething;
#end
Now you can clearly make instances of MyFancyObject, but what we need to do is take a pre-existing instance of MyPlainObject and make it into a MyFancyObject so we get the new behaviour. For that we can swizzle the class, add the following to MyFancyObject:
static Class myPlainObjectClass;
static Class myFancyObjectClass;
+ (void)initialize
{
myPlainObjectClass = objc_getClass("MyPlainObject");
myFancyObjectClass = objc_getClass("MyFancyObject");
}
+ (void)changeKind:(MyPlainObject *)control fancy:(BOOL)fancy
{
object_setClass(control, fancy ? myFancyObjectClass : myPlainObjectClass);
}
Now for any original instance of MyPlainClass you can switch to behave as a MyFancyClass, and vice-versa:
MyPlainClass *mpc = [MyPlainClass new];
...
// masquerade as MyFancyClass
[MyFancyClass changeKind:mpc fancy:YES]
... // mpc behaves as a MyFancyClass
// revert to true nature
[MyFancyClass changeKind:mpc: fancy:NO];
(Some) of the caveats:
You can only do this if the subclass overrides or adds methods, and adds static (class) variables.
You also need a sub-class for ever class you wish to change the behaviour of, you can't have a single class which can change the behaviour of many different classes.
I made a swizzling API that also features instance specific swizzling. I think this is exactly what you're looking for: https://github.com/JonasGessner/JGMethodSwizzler
It works by creating a dynamic subclass for the specific instance that you're swizzling at runtime.
I've worked with a few different languages such as Java, C#, and Objective-C.
In most languages, methods that don't require an instance of an object are called static methods. However, when it comes to Objective-C, some people get defensive when you call them static methods, and they expect you to call them class methods.
Why are they called class methods instead of static methods? What is the difference between a static method and a class method?
So my question is why are they called class methods instead of a
static method? What is the difference between a static method and a
class method?
From Wikipedia: Static methods neither require an instance of the class nor can they implicitly access the data (or this, self, Me, etc.) of such an instance.
This describes exactly what Objective-C's class methods are not.
An Objective-C class method very much requires an instance that is the target of the method invocation. That is, it requires an instance of the metaclass that describes the class object being invoked.
Unlike static methods, Objective-C's class methods can be inherited (which, in combination with having the aforementioned self, is exactly why many classes can share a single, simple, implementation of +alloc on NSObject without needing their own custom implementations) and invoking a class method goes through the exact same objc_msgSend* based dispatch mechanism as any other method call site.
Objective-C's class methods can be overridden across the class hierarchy and they can be swizzled. None of which is supported in languages that typically offer static methods in lieu of class methods.
The bottom line is that static methods and class methods are very different. While that difference is mostly transparent for day to day coding purposes, there are still situations where knowing how class methods work can save you a ton of unnecessary lines of code.
For example, you can't do this with static methods:
#interface AbstractClass:NSObject
+ factory;
#end
#implementation AbstractClass
+ factory
{
return [[[self alloc] init] autorelease];
}
#end
#interface Concrete1:AbstractClass
#end
#implementation Concrete1
#end
#interface Concrete2:AbstractClass
#end
#implementation Concrete2
#end
void foo() {
Concrete1 *c = [Concrete1 factory];
Concrete2 *d = [Concrete2 factory];
... etc ...
}
Because it's dynamically bound, not static.
Because it's really a class object's instance method.
Objective-C class method is actually an object's class object's instance method.
It's hard to describe with text. See nice illustration here.
http://www.sealiesoftware.com/blog/archive/2009/04/14/objc_explain_Classes_and_metaclasses.html
Though class methods and static methods are in practice the same most of the time, they are different. With static methods the class is acting as a namespace qualifier. With class methods the class itself is an object and so class methods are to the class object exactly the same thing instance methods are to an instance; as a consequence you can do the following
#interface TestClass : NSObject
+ (void)classOrInstanceMethod;
- (void)classOrInstanceMethod;
#end
...
NSArray * arr = [NSArray arrayWithObjects:
[[[TestClass alloc] init] autorelease],
[TestClass class],
nil];
for( id obj in arr )
[obj classOrInstanceMethod];
which version of classOrInstanceMethod is called depends on whether obj is a class object or and instance. If you are familiar with the factory class pattern, this pattern is part of the Objective-C language.
This is purely a historical difference, mostly stemming from the fact that Objective-C was developed contemporaneously with C++, and before C++ or later languages like Java and C# had much influence. Objective-C was essentially a port of the Smalltalk object model to C, so its syntax and terminology don't necessarily seem as "C-like" as that used by C++. However, Objective-C was in no way bucking a trend by not using the term "static method", because that trend wasn't well-established back in 1983.
I'd like to specify an Objective-C protocol with an optional routine. When the routine is not implemented by a class conforming to the protocol I'd like to use a default implementation in its place. Is there a place in the protocol itself where I can define this default implementation? If not, what is the best practice to reduce copying and pasting this default implementation all over the place?
Objective-C protocols have no affordance for default implementations. They are purely collections of method declarations that can be implemented by other classes. The standard practice in Objective-C is to test an object at runtime to see if it responds to the given selector before calling that method on it, using -[NSObject respondsToSelector:]. If e object does not respond to the given selector, the method isn't called.
One way you could achieve the result you're looking for would be to define a method encapsulating the default behavior you're looking for in the calling class, and call that method if the object doesn't pass the test.
Another approach would be to make the method be required in the protocol, and provide default implementations in the superclasses of any classes wherein you may not want to provide a specific implementation.
There are probably other options as well, but generally speaking there isn't a particular standard practice in Objective-C, except perhaps to just not call the given method if it hasn't been implement by the object, per my first paragraph, above.
There is no standard way for doing that as protocols should not define any implementations.
Since Objective-C comes with a neat runtime, you can of course add such a behavior if you really think you need to do it that way (and there's no possibility by achieving the same with inheritance).
Say you declared MyProtocol, then just add an interface with the same name in the .h file under your protocol declaration:
#interface MyProtocol : NSObject <MyProtocol>
+ (void)addDefaultImplementationForClass:(Class)conformingClass;
#end
And create a corresponding implementation file (using MAObjCRuntime for readability here, but the standard runtime functions wouldn't be much more code):
#implementation MyProtocol
+ (void)addDefaultImplementationForClass:(Class)conformingClass {
RTProtocol *protocol = [RTProtocol protocolWithName:#"MyProtocol"];
// get all optional instance methods
NSArray *optionalMethods = [protocol methodsRequired:NO instance:YES];
for (RTMethod *method in optionalMethods) {
if (![conformingClass rt_methodForSelector:[method selector]]) {
RTMethod *myMethod = [self rt_methodForSelector:[method selector]];
// add the default implementation from this class
[conformingClass rt_addMethod:myMethod];
}
}
}
- (void)someOptionalProtocolMethod {
// default implementation
// will be added to any class that calls addDefault...: on itself
}
Then you just have to call
[MyProtocol addDefaultImplementationForClass:[self class]];
in the initializer of your class conforming to the protocol and all default methods will be added.
A truly fascinating way is to use the runtime. At the start-up, very early in the program execution, do the following:
Enumerate all the classes, find classes which implement the protocol
Check if the class implements a method
If not, add to the class the default implementation
It can be achieved without that much trouble.
I agree with "w.m." A very nice solution is to put all the default implementations into an interface (with the same name as the protocol). In the "+initialize" method of any subclass it can simply copy any unimplemented methods from the default interface into itself.
The following helper functions worked for me
#import <objc/runtime.h>
// Get the type string of a method, such as "v#:".
// Caller must allocate sufficent space. Result is null terminated.
void getMethodTypes(Method method, char*result, int maxResultLen)
{
method_getReturnType(method, result, maxResultLen - 1);
int na = method_getNumberOfArguments(method);
for (int i = 0; i < na; ++i)
{
unsigned long x = strlen(result);
method_getArgumentType(method, i, result + x, maxResultLen - 1 - x);
}
}
// This copies all the instance methods from one class to another
// that are not already defined in the destination class.
void copyMissingMethods(Class fromClass, Class toClass)
{
// This gets the INSTANCE methods only
unsigned int numMethods;
Method* methodList = class_copyMethodList(fromClass, &numMethods);
for (int i = 0; i < numMethods; ++i)
{
Method method = methodList[i];
SEL selector = method_getName(method);
char methodTypes[50];
getMethodTypes(method, methodTypes, sizeof methodTypes);
if (![toClass respondsToSelector:selector])
{
IMP methodImplementation = class_getMethodImplementation(fromClass, selector);
class_addMethod(toClass, selector, methodImplementation, methodTypes);
}
}
free(methodList);
}
Then you call it in your class initializer such as...
#interface Foobar : NSObject<MyProtocol>
#end
#implementation Foobar
+(void)initialize
{
// Copy methods from the default
copyMissingMethods([MyProtocol class], self);
}
#end
Xcode will give you warnings about Foobar missing methods, but you can ignore them.
This technique only copies methods, not ivars. If the methods are accessing data members that do not exist, you could get strange bugs. You must ensure that the data is compatible with the code. It is as if you did a reinterpret_cast from Foobar to MyProtocol.
As Ryan mention there are no default implementations for protocols, another option to implementing in the superclass would be is to implement a "Handler" kind of class that can be contained in any class that want to provide the default implementation, the appropriate method then calls the default handlers implementation.
I ended up creating a macro that has a default implementation of the method.
I've defined it in the protocol's header file, and then it's just a one-liner in each implementation.
This way, I do not have to change the implementation several places, and it's done on compile time, so no run-time magic is necessary.
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?
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.
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.
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!!
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() { ... }
}
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
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).
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.
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.
Instances methods operate on instances of classes (ie, "objects"). Class methods are associated with classes (most languages use the keyword static for these guys).
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.
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.
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];
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
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.
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.
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 & class method after automatic reference counting came to existence in Objective-C.
For Example[NS StringWithformat:..] a class method & [[NSString alloc] initwihtformat:..] an instance method, both are same after ARC
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.
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];