How do I convert an NSString value to NSData?

NSString* str = @"teststring";
NSData* data = [str dataUsingEncoding:NSUTF8StringEncoding];

Objective-C:

NSString to NSData:

NSString* str= @"string";
NSData* data=[str dataUsingEncoding:NSUTF8StringEncoding];

NSData to NSString:

NSString* newStr = [[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding];

Swift:

String to Data:

var testString = "string"
var somedata = testString.data(using: String.Encoding.utf8)

Data to String:

var backToString = String(data: somedata!, encoding: String.Encoding.utf8) as String!

Update Swift 5.x

let str = "teststring"
let data = str.data(using: .utf8)

Swift:

Swift 5.x

let myStringToConvert = "My String to Convert in Data"
let myData = str.data(using: .utf8)

String to Data:

var myStringToConvert = "My String to Convert in Data"
var myData = testString.data(using: String.Encoding.utf8)

Data to String:

var backToMyString = String(data: myData!, encoding: String.Encoding.utf8) as String!

OBJECTIVE C:

NSString to NSData :

NSString* myStringToConvert= @"My String to Convert in Data";
NSData* myData=[str dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:NO];

NSData to NSString :

NSString* backToMyString = [[NSString alloc] initWithData: myData encoding:NSUTF8StringEncoding];

In Swift there is an API which returns a non-optional

let str = "teststring"
let data = Data(str.utf8)