How do I send strings to another viewcontroller

You can pass strings through viewcontrollers. Make therefore a segue between the two viewcontroller and named it for example "secondVC"

the when you want to switch to other view make this call

[self performSegueWithIdentifier:@"secondVC"];

and implement this method.

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
if ([[segue identifier] isEqualToString:@"secondVC"]) {
    SecondViewController *second = (SecondViewController *)[segue destinationViewController];
    second.userString = self.user;
}
}

You can use NSUserDefaults to store the username and use it in other ViewControllers.

For example, save it in your current ViewController.

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults setObject:username forKey:@"UserName"];
[defaults synchronize];

get username it in another ViewController.

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSString *username = [defaults objectForKey:@"UserName"];

pass the userName to second view controller.

You can achieve it using following approaches.

try to pass the username when you are initialising your secondViewController like

SecondViewController * sVC = [SecondViewController alloc] initWithUserName: username];

for this in your SecondViewController class you have to modify the init method and add a property of userName {typeOF string}

@property (nonatomic, strong) NSString * userName

- (id) initWithUserName: (NSString *) name
{
    self = [super initWithNibName: @"SecondViewController"
                           bundle: nil];
    if (self)
    {
        self.userName = name;
    }
    return self;
}

Enjoy!!

Create object of first class in second class like this..

//First class

//Pass the values to seconClass

secondClass *appdelegate = [NSApp delegate];
[appdelegate initwithDetails:userName withPassword:password];

//Second class

//In second class declare the function and get the values

//When u create the object, init function will call first

-(id)initwithDetails:(NSString *)user withPassword:(NSString *)password 
{
    userName = [NSString stringWithFormat:@"%@", user];
    newPass=[NSString stringWithFormat:@"%@", password];
    return self;
}

If the second viewcontroller is spawned from the first you could just create a delegate protocol and corresponding delegate so that either the first is delegate of the second or the second is delegate of the first:

Here is an example of a protocol which includes one method, notice the instance variable delegate is of type id, as it will be unknown at compile time the type of class that will adopt this protocol.

#import <Foundation/Foundation.h>

@protocol ProcessDataDelegate <NSObject>
@required
- (void) processSuccessful: (BOOL)success;
@end

@interface ClassWithProtocol : NSObject 
{
 id <ProcessDataDelegate> delegate;
}

@property (retain) id delegate;

-(void)startSomeProcess;

@end

Inside the implementation section for the interface defined above we need to do two things at a minimum – first synthesize the delegate instance variable and second, call the method defined in the protocol as needed (more on that in a moment).

Let’s look at a bare bones implementation of the ClassWithProtocol.m:

#import "ClassWithProtocol.h"

@implementation ClassWithProtocol

@synthesize delegate;

- (void)processComplete
{
  [[self delegate] processSuccessful:YES];
}

-(void)startSomeProcess
{
  [NSTimer scheduledTimerWithTimeInterval:5.0 target:self 
    selector:@selector(processComplete) userInfo:nil repeats:YES];
}

@end

Read more at this tutorial