How do I create a private global variable in an NSObject class?

Question

I want to create a private global variable in an NSObject class. Here is what I have so far:

+ (MyClass *)sharedInstance
{
    static MyClass *sharedInstance;

    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];
    });

    return sharedInstance;
}

I want to create and assign an NSDictionary that can be accessed throughout the .m file.

For example: In a regular .m UIViewController class, there is:

@interface MyViewControllerClassName ()

// Here I would declare private global variables.
// Example:
@property (strong, nonatomic) NSArray *myArray;

@end

Then in viewDidLoad I would assign it a value, and I will be able to access it throughout the .m file.

My question is: How can I create the same thing (a global private variable) in an NSObject class, which doesn't have @interface and viewDidLoad?

Update

I'm trying to add objects in an array, and use it throughout my NSObject class. Instead of creating it in every method that I'm using it in.

Example:

NSArray *myArray = [[NSArray alloc] initWithObjects: @"One", @"Two", nil];

- (void)firstMethod {
    NSLog(@"%@", [self.myArray firstObject]);
}

- (void)secondMethod {
    NSLog(@"%@", [self.myArray lastObject]);
}

Update 2

When you create a new file with a subclass of NSObject, the .m file doesn't come with @interface myObjectClass () ... @end. Therefore, I don't know where to create my variables that I want to access throughout the myObjectClass.m file.

Answer 1

You say:

When you create a new file with a subclass of NSObject, the .m file doesn't come with @interface myObjectClass () ... @end. Therefore, I don't know where to create my variables that I want to access throughout the myObjectClass.m file.

That's true, but there's nothing to stop you from adding this (called a "private class extension") yourself. In fact, that precisely what you should do. Then, all of your private class properties should go inside this private class extension.

See Class Extensions Extend the Internal Implementation.

---

Going back to your example, you might end up with a public interface defined in MyClass.h that looks like:

@interface MyClass : NSObject

+ (MyClass *)sharedInstance;

- (void)firstMethod;
- (void)secondMethod;

@end

The .m file might include a private class extension which defines the private properties:

@interface MyClass ()

@property (nonatomic, strong) NSArray *myArray;

@end

@implementation MyClass

+ (MyClass *)sharedInstance {
    static MyClass *sharedInstance;

    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];
    });

    return sharedInstance;
}

- (MyClass *)init {
    self = [super init];
    if (self) {
        self.myArray = @[@"one", @"two"];
    }
    return self;
}

- (void)firstMethod {
    NSLog(@"%@", [self.myArray firstObject]);
}

- (void)secondMethod {
    NSLog(@"%@", [self.myArray lastObject]);
}

@end

Bottom line, the class extension may not be added automatically for you, but you can add it yourself.

Answer 2

for that you define MyClass object out side of sharedInstance method.

static MyClass *sharedInstance = nil;  
+ (MyClass *)sharedInstance
{
             static dispatch_once_t once;
             dispatch_once(&once, ^{
                    sharedInstance = [[self alloc] init];
              });

              return sharedInstance;
}

----------Edited---------

Option 1 - MyViewControllerClassName.m

    @interface MyViewControllerClassName ()
    {
    @private // << note: protected is the default when declared in this scope.
       NSArray *myArray;
    }
    @end

Option 2 - MyViewControllerClassName.h

@interface MyViewControllerClassName : NSObject
{
@private // << note: protected is the default when declared in this scope.
   NSArray * myArray;
}
@end

Option 3 - MyViewControllerClassName.m

@implementation MyViewControllerClassName
{
@private // << note: private is the default when declared in this scope.
   NSArray * myArray;
}

@end

For more reference:Class variables explained comparing Objective-C and C++ approaches

Answer 3

I think this is what you want to implement with your NSObject class.

Any concern warm welcome