Please explain Getter and Setters in Objective C

Question

Possible Duplicate:
Setters and Getters (Noobie) - iPhone SDK

I am a beginner here. I have just started learning iOS for the last two months and I do not have any programming background. (Little bit of Java though). Can anyone please explain what is getter and setter in Objective C? What is the use of them? Why do we use @property and @synthesize?

Answer 1

Getter is a method which gets called every time you access (read value from) a property (declared with @property). Whatever that method returns is considered that property's value:

@property int someNumber;

...

- (int)someNumber {
    return 42;
}

...

NSLog("value = %d", anObject.someNumber); // prints "value = 42"

Setter is a method which gets called every time property value is changed.

- (void)setSomeNumber: (int)newValue { // By naming convention, setter for `someValue` should 
                                    // be called `setSomeValue`. This is important!
    NSLog("someValue has been assigned a new value: %d", newValue);
}

...

anObject.someNumber = 19; // prints "someValue has been assigned a new value: 19"

Usually it doesn't make much sense to just return the same value from getter and print new value in setter. To actually store something you have to declare an instance variable (ivar) in your class:

@interface SomeClass : NSObject {
    int _someNumber;
}

and make accessors (the collective name for getters and setters) to store/retrieve it's value:

- (int)someNumber {
    return _someNumber;
}

- (void)setSomeNumber:(int)newValue {
    _someNumber = newValue;
}

...

SomeClass *anObject = [[SomeClass alloc]init];
anObject.someNumber = 15;
NSLog(@"It's %d", anObject.someNumber); // prints "It's 15"

Okay, now that property behaves just like the usual variable. What's the point in writing all that code?

First, from now on you can add some extra code to the accessors, which will get executed each time the property is accessed or changed. There are multiple reasons for doing that, for example I may want to do some kind of hidden calculations, or updating my object's state, caching stuff etc.

Second, there are cool mechanisms called Key-Value Coding (KVC) and Key-Value Observing (KVO) in Cocoa. They depend on properties. You can read about them in the Developer Library: KVC Programming Guide and KVO Programming Guide. Those are advanced topics though.

Last, in Objective C there is no static allocation for objects. All the objects are dynamically allocated (reason). If you want to keep your object pointers in instance variables (as opposed to properties) you will have to do all the memory management manually every time you assign new value to your ivar (not true when Automatic Reference Counting is on). Using properties you could put some memory management code in the accessors and make your life easier.

I don't believe this explanation will make much sense to someone who is not familiar with Objective C memory management, so, either read some real docs/tutorials on it, or just use properties (instead of instance variables) until you learn all the details one way or another. Personally, I don't like the second option, but it's up to you.

You can use @synthesize to make the compiler generate basic accessors and underlying instance variables for you automatically. Instead of the code above (-(int)someNumber and -(void)setSomeNumber:) you could just write

@synthesize someNumber = _someNumber; // = _someNumbers tells compiler 
                                      // to name the instance variable `_someNumber`. 
                                      // You could replace it with = `_somethingElse`, of
                                      // course, but that's an ill idea.

This single line generates int _someNumber variable, someNumber getter and setSomeNumber setter for you. If you want the accessors to do anything more complex than just store/retrieve the value from some instance variable, you will have to write them yourself.

Hope all this makes any sense.