How can I create a String from UTF8 in Swift? - swift

Question

We know we can print each character in UTF8 code units?
Then, if we have code units of these characters, how can we create a String with them?

Answer 1

With Swift 5, you can choose one of the following ways in order to convert a collection of UTF-8 code units into a string.
#1. Using String's init(_:) initializer
If you have a String.UTF8View instance (i.e. a collection of UTF-8 code units) and want to convert it to a string, you can use init(_:) initializer. init(_:) has the following declaration:
init(_ utf8: String.UTF8View)
Creates a string corresponding to the given sequence of UTF-8 code units.
The Playground sample code below shows how to use init(_:):
let string = "Café 🇫🇷"
let utf8View: String.UTF8View = string.utf8
let newString = String(utf8View)
print(newString) // prints: Café 🇫🇷
#2. Using Swift's init(decoding:as:) initializer
init(decoding:as:) creates a string from the given Unicode code units collection in the specified encoding:
let string = "Café 🇫🇷"
let codeUnits: [Unicode.UTF8.CodeUnit] = Array(string.utf8)
let newString = String(decoding: codeUnits, as: UTF8.self)
print(newString) // prints: Café 🇫🇷
Note that init(decoding:as:) also works with String.UTF8View parameter:
let string = "Café 🇫🇷"
let utf8View: String.UTF8View = string.utf8
let newString = String(decoding: utf8View, as: UTF8.self)
print(newString) // prints: Café 🇫🇷
#3. Using transcode(_:from:to:stoppingOnError:into:) function
The following example transcodes the UTF-8 representation of an initial string into Unicode scalar values (UTF-32 code units) that can be used to build a new string:
let string = "Café 🇫🇷"
let bytes = Array(string.utf8)
var newString = ""
_ = transcode(bytes.makeIterator(), from: UTF8.self, to: UTF32.self, stoppingOnError: true, into: {
newString.append(String(Unicode.Scalar($0)!))
})
print(newString) // prints: Café 🇫🇷
#4. Using Array's withUnsafeBufferPointer(_:) method and String's init(cString:) initializer
init(cString:) has the following declaration:
init(cString: UnsafePointer<CChar>)
Creates a new string by copying the null-terminated UTF-8 data referenced by the given pointer.
The following example shows how to use init(cString:) with a pointer to the content of a CChar array (i.e. a well-formed UTF-8 code unit sequence) in order to create a string from it:
let bytes: [CChar] = [67, 97, 102, -61, -87, 32, -16, -97, -121, -85, -16, -97, -121, -73, 0]
let newString = bytes.withUnsafeBufferPointer({ (bufferPointer: UnsafeBufferPointer<CChar>)in
return String(cString: bufferPointer.baseAddress!)
})
print(newString) // prints: Café 🇫🇷
#5. Using Unicode.UTF8's decode(_:) method
To decode a code unit sequence, call decode(_:) repeatedly until it returns UnicodeDecodingResult.emptyInput:
let string = "Café 🇫🇷"
let codeUnits = Array(string.utf8)
var codeUnitIterator = codeUnits.makeIterator()
var utf8Decoder = Unicode.UTF8()
var newString = ""
Decode: while true {
switch utf8Decoder.decode(&codeUnitIterator) {
case .scalarValue(let value):
newString.append(Character(Unicode.Scalar(value)))
case .emptyInput:
break Decode
case .error:
print("Decoding error")
break Decode
}
}
print(newString) // prints: Café 🇫🇷
#6. Using String's init(bytes:encoding:) initializer
Foundation gives String a init(bytes:encoding:) initializer that you can use as indicated in the Playground sample code below:
import Foundation
let string = "Café 🇫🇷"
let bytes: [Unicode.UTF8.CodeUnit] = Array(string.utf8)
let newString = String(bytes: bytes, encoding: String.Encoding.utf8)
print(String(describing: newString)) // prints: Optional("Café 🇫🇷")

Answer 2

It's possible to convert UTF8 code points to a Swift String idiomatically using the UTF8 Swift class. Although it's much easier to convert from String to UTF8!
import Foundation
public class UTF8Encoding {
public static func encode(bytes: Array<UInt8>) -> String {
var encodedString = ""
var decoder = UTF8()
var generator = bytes.generate()
var finished: Bool = false
do {
let decodingResult = decoder.decode(&generator)
switch decodingResult {
case .Result(let char):
encodedString.append(char)
case .EmptyInput:
finished = true
/* ignore errors and unexpected values */
case .Error:
finished = true
default:
finished = true
}
} while (!finished)
return encodedString
}
public static func decode(str: String) -> Array<UInt8> {
var decodedBytes = Array<UInt8>()
for b in str.utf8 {
decodedBytes.append(b)
}
return decodedBytes
}
}
func testUTF8Encoding() {
let testString = "A UTF8 String With Special Characters: 😀🍎"
let decodedArray = UTF8Encoding.decode(testString)
let encodedString = UTF8Encoding.encode(decodedArray)
XCTAssert(encodedString == testString, "UTF8Encoding is lossless: \(encodedString) != \(testString)")
}
Of the other alternatives suggested:
Using NSString invokes the Objective-C bridge;
Using UnicodeScalar is error-prone because it converts UnicodeScalars directly to Characters, ignoring complex grapheme clusters; and
Using String.fromCString is potentially unsafe as it uses pointers.

Answer 3

improve on Martin R's answer
import AppKit
let utf8 : CChar[] = [65, 66, 67, 0]
let str = NSString(bytes: utf8, length: utf8.count, encoding: NSUTF8StringEncoding)
println(str) // Output: ABC
import AppKit
let utf8 : UInt8[] = [0xE2, 0x82, 0xAC, 0]
let str = NSString(bytes: utf8, length: utf8.count, encoding: NSUTF8StringEncoding)
println(str) // Output: €
What happened is Array can be automatic convert to CConstVoidPointer which can be used to create string with NSSString(bytes: CConstVoidPointer, length len: Int, encoding: Uint)

Answer 4

Swift 3
let s = String(bytes: arr, encoding: .utf8)

Answer 5

I've been looking for a comprehensive answer regarding string manipulation in Swift myself. Relying on cast to and from NSString and other unsafe pointer magic just wasn't doing it for me. Here's a safe alternative:
First, we'll want to extend UInt8. This is the primitive type behind CodeUnit.
extension UInt8 {
var character: Character {
return Character(UnicodeScalar(self))
}
}
This will allow us to do something like this:
let codeUnits: [UInt8] = [
72, 69, 76, 76, 79
]
let characters = codeUnits.map { $0.character }
let string = String(characters)
// string prints "HELLO"
Equipped with this extension, we can now being modifying strings.
let string = "ABCDEFGHIJKLMONP"
var modifiedCharacters = [Character]()
for (index, utf8unit) in string.utf8.enumerate() {
// Insert a "-" every 4 characters
if index > 0 && index % 4 == 0 {
let separator: UInt8 = 45 // "-" in ASCII
modifiedCharacters.append(separator.character)
}
modifiedCharacters.append(utf8unit.character)
}
let modifiedString = String(modifiedCharacters)
// modified string == "ABCD-EFGH-IJKL-MONP"

Answer 6

// Swift4
var units = [UTF8.CodeUnit]()
//
// update units
//
let str = String(decoding: units, as: UTF8.self)

Answer 7

I would do something like this, it may be not such elegant than working with 'pointers' but it does the job well, those are pretty much about a bunch of new += operators for String like:
#infix func += (inout lhs: String, rhs: (unit1: UInt8)) {
lhs += Character(UnicodeScalar(UInt32(rhs.unit1)))
}
#infix func += (inout lhs: String, rhs: (unit1: UInt8, unit2: UInt8)) {
lhs += Character(UnicodeScalar(UInt32(rhs.unit1) << 8 | UInt32(rhs.unit2)))
}
#infix func += (inout lhs: String, rhs: (unit1: UInt8, unit2: UInt8, unit3: UInt8, unit4: UInt8)) {
lhs += Character(UnicodeScalar(UInt32(rhs.unit1) << 24 | UInt32(rhs.unit2) << 16 | UInt32(rhs.unit3) << 8 | UInt32(rhs.unit4)))
}
NOTE: you can extend the list of the supported operators with overriding + operator as well, defining a list of the fully commutative operators for String.
and now you are able to append a String with a unicode (UTF-8, UTF-16 or UTF-32) character like e.g.:
var string: String = "signs of the Zodiac: "
string += (0x0, 0x0, 0x26, 0x4b)
string += (38)
string += (0x26, 76)

Answer 8

This is a possible solution (now updated for Swift 2):
let utf8 : [CChar] = [65, 66, 67, 0]
if let str = utf8.withUnsafeBufferPointer( { String.fromCString($0.baseAddress) }) {
print(str) // Output: ABC
} else {
print("Not a valid UTF-8 string")
}
Within the closure, $0 is a UnsafeBufferPointer<CChar> pointing to the array's contiguous storage. From that a Swift String can be created.
Alternatively, if you prefer the input as unsigned bytes:
let utf8 : [UInt8] = [0xE2, 0x82, 0xAC, 0]
if let str = utf8.withUnsafeBufferPointer( { String.fromCString(UnsafePointer($0.baseAddress)) }) {
print(str) // Output: €
} else {
print("Not a valid UTF-8 string")
}

Answer 9

If you're starting with a raw buffer, such as from the Data object returned from a file handle (in this case, taken from a Pipe object):
let data = pipe.fileHandleForReading.readDataToEndOfFile()
var unsafePointer = UnsafeMutablePointer<UInt8>.allocate(capacity: data.count)
data.copyBytes(to: unsafePointer, count: data.count)
let output = String(cString: unsafePointer)

Answer 10

There is Swift 3.0 version of Martin R answer
public class UTF8Encoding {
public static func encode(bytes: Array<UInt8>) -> String {
var encodedString = ""
var decoder = UTF8()
var generator = bytes.makeIterator()
var finished: Bool = false
repeat {
let decodingResult = decoder.decode(&generator)
switch decodingResult {
case .scalarValue(let char):
encodedString += "\(char)"
case .emptyInput:
finished = true
case .error:
finished = true
}
} while (!finished)
return encodedString
}
public static func decode(str: String) -> Array<UInt8> {
var decodedBytes = Array<UInt8>()
for b in str.utf8 {
decodedBytes.append(b)
}
return decodedBytes
}
}
If you want show emoji from UTF-8 string, just user convertEmojiCodesToString method below. It is working properly for strings like "U+1F52B" (emoji) or "U+1F1E6 U+1F1F1" (country flag emoji)
class EmojiConverter {
static func convertEmojiCodesToString(_ emojiCodesString: String) -> String {
let emojies = emojiCodesString.components(separatedBy: " ")
var resultString = ""
for emoji in emojies {
var formattedCode = emoji
formattedCode.slice(from: 2, to: emoji.length)
formattedCode = formattedCode.lowercased()
if let charCode = UInt32(formattedCode, radix: 16),
let unicode = UnicodeScalar(charCode) {
let str = String(unicode)
resultString += "\(str)"
}
}
return resultString
}
}