If you adopt NSCopying, your object knows how to make copies of itself:
@interface Vehicle: NSObject<NSCopying>
{
NSColor* color;
}
// methods…
@end

-(id) copyWithZone: (NSZone *)zone {
     Vehicle * vehicleCopy = [[[self class] allocWithZone: zone] initWithColor: color];
     return vehicleCopy;
}

@interface Car: Vehicle // Car includes the <NSCopying> protocol
{
    float drivingForce;
}
// methods…
@end

// Need to implement copyWithZone, though.
// Make sure Car’s drivingForce instance variable is copied.
-(id) copyWithZone: (NSZone *)zone {
     Car * vehicleCopy = [super copyWithZone: zone];
     [vehicleCopy setDrivingForce: drivingForce];
     return vehicleCopy;
}

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/
Protocols/NSCopying_Protocol/Reference/Reference.html