struct UTF8

A codec for translating between Unicode scalar values and UTF-8 code units.

Inheritance UnicodeCodec View Protocol Hierarchy →
Associated Types
CodeUnit = UInt8

A type that can hold code unit values for this encoding.

Import import Swift



Creates an instance of the UTF-8 codec.



Static Methods

static func encode(_:into:)

Encodes a Unicode scalar as a series of code units by calling the given closure on each code unit.

For example, the musical fermata symbol ("𝄐") is a single Unicode scalar value (\u{1D110}) but requires four code units for its UTF-8 representation. The following code encodes a fermata in UTF-8:

var bytes: [UTF8.CodeUnit] = []
UTF8.encode("𝄐", into: { bytes.append($0) })
// Prints "[240, 157, 132, 144]"

Parameters: input: The Unicode scalar value to encode. processCodeUnit: A closure that processes one code unit argument at a time.


static func encode(_ input: UnicodeScalar, into processCodeUnit: (UTF8.CodeUnit) -> Swift.Void)
static func isContinuation(_:)

Returns a Boolean value indicating whether the specified code unit is a UTF-8 continuation byte.

Continuation bytes take the form 0b10xxxxxx. For example, a lowercase "e" with an acute accent above it ("é") uses 2 bytes for its UTF-8 representation: 0b11000011 (195) and 0b10101001 (169). The second byte is a continuation byte.

let eAcute = "é"
for codePoint in eAcute.utf8 {
    print(codePoint, UTF8.isContinuation(codePoint))
// Prints "195 false"
// Prints "169 true"

byte: A UTF-8 code unit. Returns: true if byte is a continuation byte; otherwise, false.


static func isContinuation(_ byte: UTF8.CodeUnit) -> Bool

Instance Methods

mutating func decode(_:)

Starts or continues decoding a UTF-8 sequence.

To decode a code unit sequence completely, call this method repeatedly until it returns UnicodeDecodingResult.emptyInput. Checking that the iterator was exhausted is not sufficient, because the decoder can store buffered data from the input iterator.

Because of buffering, it is impossible to find the corresponding position in the iterator for a given returned UnicodeScalar or an error.

The following example decodes the UTF-8 encoded bytes of a string into an array of UnicodeScalar instances. This is a demonstration only---if you need the Unicode scalar representation of a string, use its unicodeScalars view.

let str = "✨Unicode✨"
// Prints "[226, 156, 168, 85, 110, 105, 99, 111, 100, 101, 226, 156, 168]"

var bytesIterator = str.utf8.makeIterator()
var scalars: [UnicodeScalar] = []
var utf8Decoder = UTF8()
Decode: while true {
    switch utf8Decoder.decode(&bytesIterator) {
    case .scalarValue(let v): scalars.append(v)
    case .emptyInput: break Decode
    case .error:
        print("Decoding error")
        break Decode
// Prints "["\u{2728}", "U", "n", "i", "c", "o", "d", "e", "\u{2728}"]"

input: An iterator of code units to be decoded. input must be the same iterator instance in repeated calls to this method. Do not advance the iterator or any copies of the iterator outside this method. Returns: A UnicodeDecodingResult instance, representing the next Unicode scalar, an indication of an error, or an indication that the UTF sequence has been fully decoded.


mutating func decode<I where I : IteratorProtocol, I.Element == UTF8.CodeUnit>(_ input: inout I) -> UnicodeDecodingResult