What does #property in a header file mean in Obj C? - objective-c

Question

I'm trying to learn objective C and I'm looking through some simple code to figure it out. Here's an example of a header file:
#import <Foundation/Foundation.h>
#class XYPoint;
#interface Rectangle: NSObject
{
int width;
int height;
XYPoint *origin;
}
#property int width, height;
-(XYPoint *) origin;
-(void) setOrigin: (XYPoint *) pt;
-(void) setWidth: (int) w andHeight: (int) h;
-(int) area;
-(int) perimeter;
#end
Can anyone explain the meaning of the line "#property int width, height;"? Thanks!

Answer 1

The property syntax lets you declare a combination of a getter and a setter in a short syntax. In your code, the declaration creates four methods:
-(int) width;
-(void)setWidth:(int)val;
-(int) height;
-(void)setHeight:(int)val;
There is probably an implementation, too, which ties these methods to their "backing variables" with names width and height defined above; there may also be a #synthesize directive in the .m file.
In the current version of Objective-C the declaration of backing variables is unnecessary: all properties are synthesized by default, but you can provide your own implementations if you wish.
Properties support an alternative way of invoking getter and setter methods in Objective-C: in addition to the standard
int h = [point height];
[point setWidth:123];
you can write
int h = point.height;
point.width = 123;
It is not necessary to declare properties to use the dot syntax: you can call any value-returning method with no arguments or a setter method with a single argument using dot syntax.

Answer 2

It is just a way for other classes to refer to the private iVars declared between { and }.
Basically, it will create a getter and a setter, that, for primitive types like int will look like this:
- (int)width
{
return width;
}
- (void)setWidth:(int)width
{
_width = width;
}
And you can override these 2 methods if you want.
According to the conventions, the underlying instance variables should be prefixed with "_":
#interface Rectangle: NSObject
{
int _width;
int _height;
XYPoint *_origin;
}
If you do not declare them, the compiler does that for you, but be careful, in case you override both the getter and the setter, you need to add this kind of statements in the ".m" file:
#synthesize width = _width;
because in that case the compiler will not generate the "_width" ivar on your behalf.