Question

I’m trying to work my way through an Objective-C tutorial. In the book there is this example:

@interface
{
 int width;
 int height;
 XYPoint *origin;
}
@property int width, height;

I thought, “hey there’s no getter/setter for the XYPoint object. The code does work though.” Now i’m going maybe to answer my own question :).

I thinks its because “origin” is a pointer already, and whats happening under the hood with “width” and “height”, is that there is going te be created a pointer to them..

Am i right, or am i talking BS 🙂 ??

I just dont get it. here’s main:

#import "Rectangle.h"
#import "XYPoint.h"
int main (int argc, char *argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    Rectangle *myRect = [[Rectangle alloc] init];
    XYPoint *myPoint = [[XYPoint alloc] init];

    [myPoint setX: 100 andY: 200];
    [myRect setWidth: 5 andHeight: 8];

    myRect.origin = myPoint; 

    NSLog (@"Rectangle w = %i, h = %i",
           myRect.width, myRect.height); 

    NSLog (@"Origin at (%i, %i)",
           myRect.origin.x, myRect.origin.y);

    NSLog (@"Area = %i, Perimeter = %i",
           [myRect area], [myRect perimeter]);

    [myRect release];
    [myPoint release];
    [pool drain];

    return 0;
}

And here’s the Rectangle object:

#import "Rectangle.h"
#import "XYPoint.h"

@implementation Rectangle
@synthesize width, height;

-(void) setWidth: (int) w andHeight: (int) h
{
    width = w;
    height = h;
}

- (void) setOrigin: (XYPoint *) pt
{
    origin = pt;
}
-(int) area
{
    return width * height;
}
-(int) perimeter
{
    return (width + height) * 2;
}
-(XYPoint *) origin
{
    return origin;
}
@end

What i dont understand is this line in main: myRect.origin = myPoint; I did not make a setter for it..

BTW thanks for your fast reply’s

Answer 1

What i dont understand is this line in main: myRect.origin = myPoint; I did not make a setter for it..

There is both a getter and a setter (collectively referred to as accessors) created for origin in the Rectangle class. If you have a look in the implementation for Rectangle, this is the getter:

-(XYPoint *) origin
{
    return origin;
}

and this is the setter:

- (void) setOrigin: (XYPoint *) pt
{
    origin = pt;
}

And as of Objective-C 2.0 calling:

myRect.origin = myPoint;

is equivalent to:

[myRect setOrigin:myPoint];

Declaring getters and setters using @property (and then implementing them using @synthesize) is only one way of declaring and creating accessors, and is there for a convenience if you have lots of properties to declare in the class interface. As Schildmeijer said, @property int width is equivalent to declaring two methods:

- (int)width;
- (void)setWidth:(int)newWidth;

Due to the dynamically-bound nature of Objective-C method calls, you don’t even have to declare the getter and setter methods in the interface, although it is generally best practice to do so if you are advertising them as publicly available to other classes.