Difference between @class directive and defining class through .h file

[veliborsantic74]

Hi Steve,
First sorry for my english.
I am little stumped on this chapter, because use of @class directive in real life is not crystal clear. As I could understand you, no need for it at all, because we can do it all through defining new class XYPoint (including all variables in that class). This is my version (sorry, it is on croatian), and please be patient to look at it, and please help me to understand what is difference by including @class directive in another .h file, and defining separate class.
This is the code:

XYPoint.h

Code: (Objective-C)

#import <Foundation/Foundation.h>

@interface XYPoint : NSObject 
{
	int x;
	int y;
}

@property int x, y;

-(void) setX: (int) xKoordinata setY: (int) yKoordinata;

@end

XYPoint.m

Code: (Objective-C)

#import "XYPoint.h"


@implementation XYPoint

@synthesize x, y;

-(void) setX: (int) xKoordinata setY: (int) yKoordinata
{
	x = xKoordinata;
	y = yKoordinata;
}

@end

Pravokutnik.h

Code: (Objective-C)

#import <Foundation/Foundation.h>

@interface Pravokutnik : NSObject 
{
	int sirina;
	int visina;
}

@property int sirina, visina;

-(int) povrsina;
-(int) opseg;
-(void) setSirina:(int) s setVisina: (int) v;

@end

Pravokutnik.m

Code: (Objective-C)

#import "Pravokutnik.h"


@implementation Pravokutnik

@synthesize sirina, visina;

-(int) povrsina
{
	return (sirina + visina) * 2;
}

-(int) opseg
{
	return sirina * visina;
}

-(void) setSirina:(int) s setVisina: (int) v
{
	sirina = s;
	visina = v;
}

@end

mojPravokutnik.m

Code: (Objective-C)

#import <Foundation/Foundation.h>
#import "Pravokutnik.h"
#import "XYPoint.h"

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
	Pravokutnik *mojPravokutnik = [[Pravokutnik alloc] init];
	XYPoint *mojaLokacija = [[XYPoint alloc] init];
	
	[mojaLokacija setX:100 setY:200];
	[mojPravokutnik setSirina:5 setVisina:8];
		
	NSLog (@"Sirina = %i, Visina = %i", mojPravokutnik.sirina, mojPravokutnik.visina);
	NSLog (@"Povrsina = %i, Opseg = %i", mojPravokutnik.povrsina, mojPravokutnik.opseg);
	NSLog (@"Lokacija na %i, %i", mojaLokacija.x, mojaLokacija.y);
	
	[mojaLokacija release];
	[mojPravokutnik release];
    [pool drain];
    return 0;
}

[skochan]

The @class directive doesn't replace the need to define the class.  It only obviates the need to import the corresponding .h file when the compiler doesn't need to know any more information about a class.   For example, in this interface section:

Code: (Objective-C)

@class XYPoint;

@interface Rectangle : NSObject
{
    int  width;
    int  height;
    XYPoint  *origin;
}

The compiler is just happy to know that XYPoint is the name of a class.  However, in this example:

Code: (Objective-C)

origin.x = 100;

The @class directive wouldn't give the compiler enough information.  The compiler doesn't know that the XYPoint object has a setter method called setX:.   You have to import the XYPoint.h header file for it to know that.

So again, the bottom line is that if you use a method from a class, you need to import the header file.  If you just  reference the class name, then the @class suffices to tell the compiler all it needs to know.

Cheers,

Steve