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

NSString *str = @"hello";
NSData *data = [NSData dataWithBytes:str.UTF8String length:str.length];

以防Swift开发者来这里，

将NSString / String转换为NSData

var _nsdata = _nsstring.dataUsingEncoding(NSUTF8StringEncoding)

objective - c

NSString *str = @"Hello World";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:NO];

斯威夫特

let str = "Hello World"
let data = string.data(using: String.Encoding.utf8, allowLossyConversion: false)

NSString *str = @"Banana";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:true];

在Swift中，有一个返回非可选参数的API

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