Not creating a new instance of NSMutableDictionary when passing to another class

Question

i have an NSMutableDictionary declared in a class but i want to print get access to the contents of it in another class for example

@interface MyClass0 : NSObject
{

}

@property (nonatomic, strong) NSMutableDictionary *valuee;
@end

and in implementation i do

@implementation MyClass0

- (void)viewDidLoad{
  [super viewDidLoad];

[valuee setObject:@"name" forKey:@"Aryan"];

}

@end

Now i create a new class called MyClass1 where i want to access these

  @interface MyClass1 : NSObject
    {
    }

    @property (nonatomic, strong) NSMutableDictionary *dict;

    @end

and the implementation

@implementation MyClass1
@synthesize dict;

- (void)viewDidLoad{
  [super viewDidLoad];

 self.dict = [[NSMutableDictionary alloc] init];
 MyClass0 *c = [[MyClass0 alloc] init];

 self.dict = c.valuee;

  // dict is not nil but the contents inside is nil so it clearly creates a new instance


}

@end

You are allocing and initing you MyClass0 but it's view is not being loaded so nothing is getting set in its dictionary. On the other hand you have specified copy for your dict property so, yes, it is creating a copy. — Remover, Jul 02 '16 at 14:16

John Farkerson · Answer 1 · 2016-07-02T17:29:50.560

If it's just a simple NSMutableDictionary that has the same contents every time you can create a class method in MyClass0 like so:

+ (NSMutableDictionary *) getDict {
    NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
    [dict setObject:@"name" forKey:@"Aryan"];//did you mean [dict setObject:@"Aryan" forKey:@"name"]?
    return dict;
}

To access this, declare the method in the MyClass0.h file like so: + (NSMutableDictionary *) getDict; and simply call [MyClass0 getDict]; in your MyClass1.m file.

If it doesn't have the same contents every time, you'll have to pass the dictionary forward to each view controller in prepareForSegue:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Make sure your segue name in storyboard is the same as this next line
    if ([[segue identifier] isEqualToString:@"MySegue"]) {

        MyClass1 *mc = [segue destinationViewController];
        mc.dict = self.valuee;
    }
}

score 1 · Accepted Answer · answered Jul 02 '16 at 17:36

You are creating the instance of MyClass0 and valuee is declared but not initialized.

The closest solution to your code is

MyClass0 *c = [[MyClass0 alloc] init];
c.valuee = [[NSMutableDictionary alloc] init];

self.dict = c.valuee;

If a value is assigned to a declared property then an explicit initialization is not necessary.

Not creating a new instance of NSMutableDictionary when passing to another class

2 Answers2