Question

I’m a beginner and I wrote this code:

#import <Foundation/Foundation.h>

@interface XYPoint : NSObject {

int pointX;
int pointY;

}    

- (void) print;
- (void) setX: (int) x; 
- (void) setY: (int) y; 

@end

@implementation XYPoint

-(void) print {

NSLog(@"X is %i and Y is %i", pointX, pointY);

}

-(void) setX: (int) x {
pointX = x;
}

-(void) setY: (int) y {
pointY = y;
}

@end

int main (int argc, const char * argv[])
{

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

XYPoint *myCord = [[XYPoint alloc] init];

[myCord setX: 4];
[myCord setY: 6];

[myCord print];

[myCord release];


[pool drain];
return 0;
}

What I need help with is I cannot for the life of me understand the relationship between “pointX” and setX and “x”

Answer 1

PointX is called an instance variable – this is what would be called in most other languages a private class-level variable. It represents a piece of information that your class needs to store internally, and by default is not exposed to other objects in the system.

setX is a method that you explicitly create, that allows other objects to assign a value to the private PointX instance variable. x is the parameter that the external calling object passes to the setX method.

Note that the most common way of exposing access to a private instance variable is through the use of defined properties. In your case, you would add something like this in your interface:

@property (nonatomic) int PointX;

and then this in your implementation:

@synthesize PointX;

This syntax allows you to (effectively) directly access the PointX instance variable, by automagically creating a wrapper property (with matching -get and -set methods) with the same name.