A special type of class where only one instance per process is created and used is Singleton class. Singleton classes are commonly used the time when general services were offered by some classes. In Singleton class we can store values and wherever we want to access those values we can just use its sharedInstance which prevents from allocating memory every time.
It provides a global point for accessing the resources of its class. Thus, Singleton class are useful in situations where single point of control is required for different classes.

Here is an example which shows the UserName and UserPassword is being saved in the Singleton class and we can access it by using its sharedInstance :-

Create a Singleton class which is a subclass of NSObject. In your singleton class .h file include the following code ..

@interface DemoSingleton : NSObject

+(DemoSingleton *)objectOfSingleton;     // class method to return the singleton object

/* declare the property for the values which you want to save in singleton class */

@property(nonatomic,retain)NSString *name;            
@property(nonatomic,retain)NSString *passwordOfUser;

@end

Include this code in your singleton class .m file :-

#import "DemoSingleton.h"

@implementation DemoSingleton

+(DemoSingleton* )objectOfSingleton{    
    
    static DemoSingleton *objectOfSIngleton=nil;
    static dispatch_once_t  oncePredecate;  
    dispatch_once(&oncePredecate,^{
       objectOfSIngleton = [[DemoSingleton alloc] init];  // memory allocated to the object of singleton
        
    });
    return objectOfSIngleton;
    
}
@end

Now we have to access the properties of Singleton class so #include the header of Singleton in your class.. We can set the value or save the value in singleton class as :

   DemoSingleton *object=[DemoSingleton objectOfSingleton];

   object.name = self.txtName.text;    // self.txtName.text is the outlet of textField in which we enter the name to save

   object.passwordOfUser = self.txtPassword.text;     //self.txtPassword.text is the outlet of textField in which we enter the password to save

The values are saved and wherever we want to use it we can just access the properties of singleton class using its object as :-

DemoSingleton *obj=[DemoSingleton objectOfSingleton];

self.lblNameOfUser.text = obj.name;              //self.lblNameOfUser.text is the label to display name

self.lblPasswordlOfUser.text = obj.eMailOfUser;     //self.lblPasswordlOfUser.text is the label to display password

It's done !