I was just going through properties documentation in the link: http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html
The section' You Can Implement Custom Accessor Methods' describes that Properties don’t always have to be backed by their own instance variables. The example of fullName also does not use its own corresponding member.
Won't the compiler implicitly create a member when a property is defined (_propertyName)?
If you create a property using #property, then an ivar is created automatically and its setter and getters are created by LLVM compiler.
Whereas when you create an ivar, you have to create a setter/getter for it. Typically you do with #property and leave rest on the compiler.
However in some cases you create ivar and property with different name and put a reference to it as :
#synthesize boxDescription=boxName;
Now boxName is accessor for boxDescription, now you cant use boxDescription in your code.
if you override the default setter and getter, and you dont provide the #synthesize (and as well you dont access the _ivar), no variable is created for you by the compiler. in that case you have a property that is not backed by an ivar.
as a very random example:
#property (nonatomic) int x;
- (int)x
{
return 1;
}
- (void)setX:(int)x
{
NSLog(#"%d", x);
}
a more userfull usage can be if you have a facade for other classes, and you use this setter/getter to set/get those.
in case you don't override the set/getter and now that #synthesize is not mandatory anymore by default the compiler creates an ivar named _propertyname, you can override this behavious by #synthesize prop_name=ivar_name
I was just wondering where I should set the values of variables for use in all my methods.
For example, let's say in my .h I say:
#property NSString *name; and then synthesize it in the .m. Where do I assign it a value so in my functions, say -(NSString *)changeUsername:(NSString *) changes and -(void)deleteUsername, I can access that data?
main() is the first thing that gets called in a command line program. Wouldn't you do it there (or somewhere called from main())?
Since you're talking about properties, you must have a class that you're instantiating. That class's designated initializer (-init or similar) is the right place to set up your properties and/or instance variables.
The only reason that Cocoa Touch apps defer some initialization tags to -viewDidLoad is that view controllers don't load their views when they're initialized and some properties or ivars are related to the view(s) that will be loaded. Those things clearly can't be set up until the view is loaded (or created), so -viewDidLoad becomes the best place for setting up those sorts of things.
Well you COULD do so in an initializer for your class and indeed, this would be the approach in C++ or maybe Java. In objective-c, you usually use lazy instantiation, and the best place to do that is in the getter for that property.
If someone assigns a value to the property the setter is called and everything is fine.
If someone asks FOR the value and it has not been set yet (is nil) you can create the object and/or assign a default value in the getter.
// Override accessor for name
- (NSString*)name
{
if (!_name) {
_name = #"unknown";
}
return _name;
}
The accessor methods are the only place you should be accessing instance variables directly.
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.
The top voted answer to this SA question ( Objective C Static Class Level variables ) outlines my question quite well but to it, I'd like to add one more criteria:
Issue Description
You want your ClassA to have a ClassB class variable.
You are using Objective-C as programming language.
Objective-C does not support class variables as C++ does.
I want to access ClassA's class variable from subclass ClassASub
or even better
4a. I want ClassA's method to access the class variable as it is, overridden in ClassASub
Any ideas? Or is this just bending Objective-C one step too far?
Just make a normal getter method for your class variable, and you can override it in the subclass. Just remember to access it through the method.
static SomeClass *gClassVar;
#implementation ClassA
+ (SomeClass *)classVar {
if (!gClassVar)
gClassVar = ...;
return gClassVar;
}
+ (...)someMethod {
[[self classVar] doSomething];
}
#end
Then,
static SomeClass *gClassVar;
#implementation ClassASubclass
+ (SomeClass *)classVar {
if (!gClassVar)
gClassVar = ...;
return gClassVar;
}
#end
So, when you call [ClassA someMethod], it will operate on the ClassA instance of classVar. When you call [ClassASubclass someMethod], it will operate on the ClassASubclass instance.
The idea of having variables of any sort attached to an object (class or instance) is a feature that is kind of "stapled on" to Objective C. Any time you want to do anything object-oriented using Objective C, start by working with methods. (Almost) everything else is just syntactic sugar for things you can do with methods.
The concept of private / protected / public is somewhat alien to Objective C, even though access control is supported for member variables. The best you can do for methods is to define them in a separate header (and this applies to class variables and properties, if we implement both using methods).
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];