Convert UTF-8 encoded NSData to NSString

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?

Seton

UTF-8 is UTF-8 everywhere. Once it's UTF-8, there's no different values for different platforms. That's the whole point of it. – Polyandrous

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:))

Rhnegative

watch out!! if using stringWithUTF8String, don't pass it a NULL argument or it will throw an exception – Busey

MIND THIS: when using "stringWithUTF8String:" on a string that is not null-terminated, the result is unpredictable! – Slumgullion

Both of the solutions returning nil for me. – Lemuroid

You can also drop the NS if using swift 2 making it: let newStr = String(data: data, encoding: NSUTF8StringEncoding) – Hundredpercenter

I meet a crash: NSString jsonStr = [NSString stringWithUTF8String:[jsonData bytes]]; And NSString newStr = [[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding]; – Cepeda

Dumb question probably (just learning Swift, don't know Objective-C at all): You show two variants for Objective-C depending on whether the input is null-terminated or not, but only one variant for Swift? – Shapiro

How do you know whether your NSData is null-terminated or not? See Tom Harrington's answer at: #27935554. In my experience, one should not ever assume NSData is either null-terminated or not: it can differ from one transmission to the next, even from a known server. – Paludal

@ElisevanLooij Thanks for the link. I'd argue that if the transmitted data could be randomly null-terminated or not the protocol is ill-defined. – Rhnegative

@Rhnegative No doubt you're right, but sometimes one has to row with the oars that one has. (Apologies for translating a Dutch saying) – Paludal

Answer 2

You could call this method

+(id)stringWithUTF8String:(const char *)bytes.

Irrecusable

Only if the data is null-terminated. Which it may not be (and, in fact, probably is not). – Mudskipper

i don't know why on earth this would break on non-null-terminated strings seeing how the NSData knows how many bytes it has... – Saad

@Claudiu, you're not passing in an NSData object, you're passing it a (const char *) obtained with [data bytes], which is just a pointer, no size information. Hence the data block it points to must be null terminated. Check out the documentation, it says so explicitly. – Kreisler

@jbat100: Of course. I wasn't clear. I meant, given that it's possible to go from a non-null-terminated NSData to an NSString (see KennyTM's answer), I'm surprised there isn't a +(id)stringWithUTF8Data:(NSData *)data which just works. – Saad

stringWithUTF8Data, hence most of us create a NSString+Foo category and create the method. – Gazehound

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];

Saad

Answer 4

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)

Yila

how to get that string to nsdata ? – Thought

@DarshanKunjadiya: Objective-C: [[NSData alloc] initWithBase64EncodedString:signatureString options:0]; Swift: NSData(base64EncodedString: str options: nil) – Yila

Answer 5

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"

Alwyn

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.

Billie

Its important that you understand <i>why</i> you got 'garbage' results. – Lamination

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}")
*/

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