Objective-C: Overriding Getter & Setter with Instance Variable (using _)

Question

I'm learning the Swift programing language and during this I sometimes get in touch with the old Objective-C programming language and its code.

I'm an absolutely beginner and therefore I have some question for better understanding the Setter and Getter.

So, I know that I can create an instance variable through curly braces in the .h file but normally I use properties. These properties are backed by an instance variable and offer automatically a Getter and Setter Method.

Example:

Vehicle .h file:

@interface Vehicle : NSObject 
@property int myProperty;
@end

Because I created this property I don't have to declare a Getter and Setter method in the vehicle.m file because they are automatically created by the compiler. So I can create a vehicle-object, set and get the value.

Example

main.m

Vehicle *myvehicle = [[vehicle alloc] init];
[myvehicle myProperty] // myvehicle.myProperty
[myvehicle setMyProperty : 10] // myvehicle.myProperty = 10;

Now I read that it is possible to override the automatically created Getter and Setter method of my created property "myProperty". When declaring my own version of the Getter and Setter I have to declare two methods in the vehicle.h and vehicle.m file. In the vehicle.m file I don't call the object by using the self keyword but by using it's automatically created instance variable (_myProperty). Is it right?

I tried it but alway get an error and I don't know why and what is the point.

Example

Vehicle .h file:

@interface Vehicle : NSObject 
@property int myProperty;
-(int) myProperty; //my new Getter method
-(void) setMyProperty: (int)updatedMyProperty; //My new Setter method
@end

vehicle .m file:

    @implementation Vehicle

    -(int) myProperty {
      if (! _myProperty) {
        _myProperty = NO;
    }
      return _myProperty;
    }

   -(void) setMyProperty: (int)updatedMyProperty {
    if (_myProperty == updatedMyProperty) return;
    _myProperty = updatedMyProperty;
    }
    @end

I always get the error "Use of undeclared identifier" and I don't know why. If I understand right I don't have to declare the ivar or its name using @synthesize because the compiler automatically creates the ivar called _myProperty for me. I just have to use @synthesize when I want to change the ivar's name.

I'm not sure why I get stuck and what the point is. Could you explain it? Thanks in advance!

Answer 1

If you implement all of the accessor methods, the compiler will no longer automatically synthesize the ivar for you. In this case, you have to explicitly do so yourself. E.g.

@synthesize myProperty = _myProperty;

This is only necessary when you have manually implemented all of the accessor methods. The reason is that the compiler is smart enough to know that if you're taking over the accessor methods, you may well not need the ivar, namely that you might be doing something radically different, e.g. computing values from some other property, setting/getting values from some different store, etc. You may want the compiler to synthesize the ivar (in which case you add the above @synthesize statement), but it's equally likely that you've implemented the accessor methods because no backing ivar is needed (in which case you'd omit the above @synthesize statement).

Anyway, staying with your simple example, you get something like:

@interface Vehicle : NSObject 
@property (nonatomic) int myProperty;  // if you don't write atomic accessor methods, you really should be explicit that this is nonatomic

// as an aside, even if you implement accessor methods, you don't have to declare them here
//
// -(int) myProperty; //my new Getter method
// -(void) setMyProperty: (int)updatedMyProperty; //My new Setter method

@end

And

@implementation Vehicle

// since you implemented all of the accessor properties, you have to manually synthesize the ivar

@synthesize myProperty = _myProperty;

- (int) myProperty {
    // do whatever you want here; note, the following doesn't make sense
    //
    // if (! _myProperty) {
    //     _myProperty = NO;
    // }

    return _myProperty;
}

- (void)setMyProperty:(int)updatedMyProperty {
    if (_myProperty == updatedMyProperty) return;
    _myProperty = updatedMyProperty;
}
@end

Clearly, there's no point in writing these particular accessor methods in the above example, because you're not offering any new functionality, so you wouldn't. You'd just avail yourself of the auto-synthesized accessor methods.

But in those cases that you really need to write your own accessor methods, then you have to explicitly tell the compiler whether you need it to synthesize the ivar for you, too, or not.