How to make a class and use it in objective c?

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Hi,

im a beginner with objective c programming, and i have problem with my simple class.
i have coded a simple class with three properties, and then i tried to use it but i am facing a problem which it seems that the object of that class does not reference at all and as a result does not hold any value. can anyone help me please?

The class code:
1-city.h

#import <Foundation/Foundation.h>

@interface City : NSObject{
    NSString *cityname;
    NSString *description;
    UIImage *cityPic;
}

@property (nonatomic,retain) NSString *cityName;
@property (nonatomic,retain) NSString *description;
@property (nonatomic,retain) UIImage *cityPic;

@end

2- city.m

#import "City.h"

@implementation City:NSObject
@synthesize cityName;
@synthesize description;
@synthesize cityPic;

@end

3- the object definition and use

- (void)viewDidLoad
{
       
   City *thiscity; 
    
    thiscity.cityName=@"London";
    thiscity.description=@"Capital of Britain";
    UIAlertView *alert=[[UIAlertView alloc] initWithTitle:thiscity.cityName message:thiscity.description delegate:self cancelButtonTitle:nil otherButtonTitles:@"ok", nil];
    [alert show];
    [super viewDidLoad];
	
}

The output is an empty alert.

Posted 6-Jun-12 7:40am

Brainy Girl

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TRK3 · Accepted Answer · 2012-06-06T11:31:00

You have not initialized thiscity.

Add:

City *thiscity = [[City alloc] init];

In Objective C, uninitialized variables are initialized to zero or Null.

So your pointer starts out as null.

You don't get an error when you do:

thiscity.cityName = @"London";
thiscity.description = @"Capital of Britain";

Because for properties the . operator is just syntactic sugar for:

[thiscity setCityName: @"London"];
[thiscity setDescription: @"Capital of Britain"];

And in Objective C, you can send any message you want to a null object -- it just quietly returns null.

Similarly when you are accessing the properties, you are sending getCityName to Null and getting back null.

So you end up with a blank alert.