Convert UTF-8 encoded NSData to NSString - ios

Question

I have UTF-8 encoded NSData from windows server and I want to convert it to NSString for iPhone. Since data contains characters (like a degree symbol) which have different values on both platforms, how do I convert data to string?

Answer 1

If the data is not null-terminated, you should use -initWithData:encoding:
NSString* newStr = [[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding];
If the data is null-terminated, you should instead use -stringWithUTF8String: to avoid the extra \0 at the end.
NSString* newStr = [NSString stringWithUTF8String:[theData bytes]];
(Note that if the input is not properly UTF-8-encoded, you will get nil.)
Swift variant:
let newStr = String(data: data, encoding: .utf8)
// note that `newStr` is a `String?`, not a `String`.
If the data is null-terminated, you could go though the safe way which is remove the that null character, or the unsafe way similar to the Objective-C version above.
// safe way, provided data is \0-terminated
let newStr1 = String(data: data.subdata(in: 0 ..< data.count - 1), encoding: .utf8)
// unsafe way, provided data is \0-terminated
let newStr2 = data.withUnsafeBytes(String.init(utf8String:))

Answer 2

You could call this method
+(id)stringWithUTF8String:(const char *)bytes.

Answer 3

I humbly submit a category to make this less annoying:
#interface NSData (EasyUTF8)
// Safely decode the bytes into a UTF8 string
- (NSString *)asUTF8String;
#end
and
#implementation NSData (EasyUTF8)
- (NSString *)asUTF8String {
return [[NSString alloc] initWithData:self encoding:NSUTF8StringEncoding];
}
#end
(Note that if you're not using ARC you'll need an autorelease there.)
Now instead of the appallingly verbose:
NSData *data = ...
[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
You can do:
NSData *data = ...
[data asUTF8String];

Answer 4

The Swift version from String to Data and back to String:
Xcode 10.1 • Swift 4.2.1
extension Data {
var string: String? {
return String(data: self, encoding: .utf8)
}
}
extension StringProtocol {
var data: Data {
return Data(utf8)
}
}
extension String {
var base64Decoded: Data? {
return Data(base64Encoded: self)
}
}
Playground
let string = "Hello World" // "Hello World"
let stringData = string.data // 11 bytes
let base64EncodedString = stringData.base64EncodedString() // "SGVsbG8gV29ybGQ="
let stringFromData = stringData.string // "Hello World"
let base64String = "SGVsbG8gV29ybGQ="
if let data = base64String.base64Decoded {
print(data) // 11 bytes
print(data.base64EncodedString()) // "SGVsbG8gV29ybGQ="
print(data.string ?? "nil") // "Hello World"
}
let stringWithAccent = "Olá Mundo" // "Olá Mundo"
print(stringWithAccent.count) // "9"
let stringWithAccentData = stringWithAccent.data // "10 bytes" note: an extra byte for the acute accent
let stringWithAccentFromData = stringWithAccentData.string // "Olá Mundo\n"

Answer 5

Sometimes, the methods in the other answers don't work. In my case, I'm generating a signature with my RSA private key and the result is NSData. I found that this seems to work:
Objective-C
NSData *signature;
NSString *signatureString = [signature base64EncodedStringWithOptions:0];
Swift
let signatureString = signature.base64EncodedStringWithOptions(nil)

Answer 6

Just to summarize, here's a complete answer, that worked for me.
My problem was that when I used
[NSString stringWithUTF8String:(char *)data.bytes];
The string I got was unpredictable: Around 70% it did contain the expected value, but too often it resulted with Null or even worse: garbaged at the end of the string.
After some digging I switched to
[[NSString alloc] initWithBytes:(char *)data.bytes length:data.length encoding:NSUTF8StringEncoding];
And got the expected result every time.

Answer 7

With Swift 5, you can use String's init(data:encoding:) initializer in order to convert a Data instance into a String instance using UTF-8. init(data:encoding:) has the following declaration:
init?(data: Data, encoding: String.Encoding)
Returns a String initialized by converting given data into Unicode characters using a given encoding.
The following Playground code shows how to use it:
import Foundation
let json = """
{
"firstName" : "John",
"lastName" : "Doe"
}
"""
let data = json.data(using: String.Encoding.utf8)!
let optionalString = String(data: data, encoding: String.Encoding.utf8)
print(String(describing: optionalString))
/*
prints:
Optional("{\n\"firstName\" : \"John\",\n\"lastName\" : \"Doe\"\n}")
*/