Understanding self in Objective-C

Question

The code below is from an iTunes U course on iPhone dev in Objective-C. I've read the Apple documentation and it's all very very clear with the exception of self. I sort of understand self to be a pointer to myself, but what exactly does that mean? In the code below what exactly does self mean? What is the difference between self.topSpeed and self.nearestWormhole in the implementation file or does self refer to the same thing on both occasions? Does self.topSpeed refer to Planet * and self.nearestWormhole refer to Wormhole * ? Thanks to anyone who answers, I've learned C and now trying to learn OOP so any input is appreciated.

(Header file)
#import "Vehicle.h"
#import "Planet.h"
@interface Spaceship : Vehicle
@property (nonatomic) double topSpeed;
- (void)orbitPlanet:(Planet *)aPlanet
         atAltitude:(double)km;
@end





(Implementation file)
#import "Spaceship.h"
@interface Spaceship()
@property (nonatomic, strong) Wormhole *nearestWormhole;
@end

@implementation Spaceship
@synthesize topSpeed = _topSpeed;
@synthesize nearestWormhole = _nearestWormhole;

- (void)setTopSpeed:(double)speed
{
    if ((speed < 1) && (speed > 0)) _topSpeed = speed;
}

- (void)orbitPlanet:(Planet *)aPlanet atAltitude:(double)km
{
    double speed = self.topSpeed;
    if (speed > MAX_RELATIVE) speed = MAX_RELATIVE;
    [self.nearestWormhole travelToPlanet:aPlanet
                                 atSpeed:speed];
}
@end

Answer 1

self (or this in C++) refers to the object which is executing the method (or "on which the method is being invoked").

Suppose I have a room with three people, Arthur, Betty, and Ziggy, and a box of hats. We also define that

Arthur's teacher is Betty.

Betty's teacher is Ziggy.

Ziggy does not have a teacher.

I want to give the following set of instructions to all three people:

1. Put a hat on Ziggy's head.

This is pretty easy. "Ziggy" means the same person to Arthur, Betty, and even Ziggy. No matter who follows this instruction the same person receives the hat.

2. Put a hat on the head of your teacher, if you have one.

This instruction will have a different effect depending on who's following it, because teacher refers to someone different for each of the three. But each can ask him/herself "who is my teacher, if I have one?" and find that person.

But the next thing I want is for Arthur to put a hat on Arthur's head, Betty to put a hat on Betty's head, and Ziggy to put a hat on Ziggy's head. We can't refer to that person by name (like Ziggy) because it depends on who is doing it. Suppose we treat it like "teacher" and establish a variable "foo" such that Arthur's foo is Arthur, and Betty's foo is Betty… but it should be obvious that the idea we are really expressing is that Ziggy's foo is Ziggy, and Jack's foo would be Jack, and Skip's foo would be Skip… do we really need to establish a "foo"? No! Everyone has a foo: it's your self. So let's define an implicit variable "self" that is not declared anywhere but always refers to the person carrying out the action.

3. Put a hat on the head of your self.

This works for Arthur, Betty, Ziggy, and even Jack. It works for anyone.

In your code self refers to the Spaceship whose topSpeed needs to be accessed. You create many Spaceships and each needs to know the topSpeed of that one Spaceship which exists (we know it does because it's calling the method) but has no name (like myWingman.topSpeed) - one's self.