Question

I am just trying to make sure I am getting things right as I move forward with Objective-C, two quick questions if I may:

(1) Am I accessing the Position object correctly from within Rectangle? am I right to access the Position object contained within by the pointer I set in the init, or is there a better way?

(2) In [setPosX: andPosY:] Which of the two ways of setting the Position instance variables is best, or does it really not matter?

// INTERFACE
@interface Position: NSObject {
    int posX;
    int posY;
}
@property(assign) int posX;
@property(assign) int posY;
@end

@interface Rectangle : NSObject {
    Position *coord;
}
-(void) setPosX:(int) inPosX andPosY:(int) inPosY;

// IMPLEMENTATION
@implementation Rectangle
-(id) init {
    self = [super init];
    if (self) {
        NSLog(@"_init: %@", self);
        coord = [[Position alloc] init];
        // Released in dealloc (not shown)
    }
    return(self);
}
-(void) setPosX:(int) inPosX andPosY:(int) inPosY {
    //[coord setPosX:inPosX];
    //[coord setPosY:inPosY];
    coord.posX = inPosX;
    coord.posY = inPosY;
}

EDIT_01

Do I then call -(id)initWithX:andY: from the Rectangle object when I init it? and if so how do I go about setting posX and posY from within main()? or do I replace the init for rectangle with a further -(id)initWithX:andY: and pass the values through?

@implementation Rectangle
-(id) init {
    self = [super init];
    if (self) {
        NSLog(@"_init: %@", self);
        coord = [[Position alloc] initWithX:1234 andY:5678];
    }
    return(self);
}
...

cheers gary

Answer 1

(1) You’re accessing it correctly.

(2) In objective-c 2.0, the assignments have identical affect.

Design wise, you would want to make:

-(void) setPosX:(int) inPosX andPosY:(int) inPosY;

…into a method of Position. This encapsulates both the data and the methods related to into one object. So you could have calls like:

coord = [[Position alloc] initWithX:inPosX andY:inPosY];

or
[coord setPosX:inPosX andPosY:inPosY];

all much cleaner and easier to maintain.

---

Edit O1

Do I then call -(id)initWithX:andY:
from the Rectangle object when I init
it?

That depends on your design. If the coord property is absolutely vital to the Rectangle instance, then you should call it when you initialize a Rectangle instance. You might even write an initializers for Rectangle that takes a position or x and y as input. eg:

-(id) initWithPosition:(Position *) aPos {
    self = [super init];
    if (self) {
        NSLog(@"_init: %@", self);
        coord = aPos;
        // Released in dealloc (not shown)
    }
    return self;
}

You should also write a connivence initializer for the Position class:

-(id) initWithX:(NSInteger) x andY:(NSInteger) y{
     self=[super init];
     self.posX=x;
     self.posY=y;
     return self;
}

You would then call like:

Position *aPos=[[Position alloc] initWithX:100 andY:50];
Rectangle *aRec=[[Rectangle alloc] initWithPosition:aPos];

Or you could write another combination initializer for Rectangle:

-(id) initWithXCoordinate:(NSInteger) x andYCoordinate:(NSInteger) y{
     self=[super init];
     Position *aPos=[[Position alloc] initWithX:x andY:y];
     self.coord=aPos;
     return self;
}

and call it like:

Rectangle *aRec=[[Rectangle alloc] initWithXCoordinate:100 
                                        andYCoordinate:50];   

These are rough examples but you get the idea. Objective-c gives you a lot of flexibility in setting up initializer so you can create any initializers you find convenient.

You do generally want to avoid using actual functions instead of methods inside classes.