Forcing a subclass to call its superclass method in Objective-C

Question

I have two classes, Vehicle and Car; Car is a subclass of Vehicle.

There is a method logVehicleDetail in the Vehicle class.

Is there any way I can force Car to call its superclass method logVehicleDetail when overriding this method? If the subclass does not do that, then the complier should generate a warning or error.

For example, when compiling without ARC, the compiler warns if you don't call [super dealloc] in your own dealloc.

Answer 1

There are various ways of doing this. Perhaps the easiest and most straightforward way would be to add __attribute__((objc_requires_super)) to your method declaration:

@interface Vehicle : NSObject
  -(void)logDetails __attribute__((objc_requires_super));
@end

@implementation Car
  -(void) logDetails {
} // WARNING: Method possibly missing a [super logDetails] call

A different way to achieve a similar outcome would be to create a protocol for vehicles that log details e.g.

@protocol VehicleLogger <NSObject>

-(void)logDetails;

@end

in your Vehicle header file. Then, in every subclass of Vehicle that you want to log details, make it conform to that protocol.

@implementation Car <VehicleLogger>

That way, you will receive a warning in the compiler to implement that method. And in the implementation you can call super's implementation of logDetails:

-(void)logDetails {
   [super logDetails];
   //Do more stuff here, if I want
}

If on the other hand all you want is for every subclass to call Vehicle's implementation, then simply don't implement that method in the subclass.