Structure

String.UTF8View

A view of a string’s contents as a collection of UTF-8 code units.

Overview

You can access a string’s view of UTF-8 code units by using its utf8 property. A string’s UTF-8 view encodes the string’s Unicode scalar values as 8-bit integers.

let flowers = "Flowers 💐"
for v in flowers.utf8 {
    print(v)
}
// 70
// 108
// 111
// 119
// 101
// 114
// 115
// 32
// 240
// 159
// 146
// 144

A string’s Unicode scalar values can be up to 21 bits in length. To represent those scalar values using 8-bit integers, more than one UTF-8 code unit is often required.

let flowermoji = "💐"
for v in flowermoji.unicodeScalars {
    print(v, v.value)
}
// 💐 128144

for v in flowermoji.utf8 {
    print(v)
}
// 240
// 159
// 146
// 144

In the encoded representation of a Unicode scalar value, each UTF-8 code unit after the first is called a continuation byte.

UTF8View Elements Match Encoded C Strings

Swift streamlines interoperation with C string APIs by letting you pass a String instance to a function as an Int8 or UInt8 pointer. When you call a C function using a String, Swift automatically creates a buffer of UTF-8 code units and passes a pointer to that buffer. The code units of that buffer match the code units in the string’s utf8 view.

The following example uses the C strncmp function to compare the beginning of two Swift strings. The strncmp function takes two const char* pointers and an integer specifying the number of characters to compare. Because the strings are identical up to the 14th character, comparing only those characters results in a return value of 0.

let s1 = "They call me 'Bell'"
let s2 = "They call me 'Stacey'"

print(strncmp(s1, s2, 14))
// Prints "0"
print(String(s1.utf8.prefix(14)))
// Prints "They call me '"

Extending the compared character count to 15 includes the differing characters, so a nonzero result is returned.

print(strncmp(s1, s2, 15))
// Prints "-17"
print(String(s1.utf8.prefix(15)))
// Prints "They call me 'B"

Topics

Instance Properties

var count: Int

The number of elements in the collection.

var customMirror: Mirror

Returns a mirror that reflects the UTF-8 view of a string.

var endIndex: String.UTF8View.Index

The “past the end” position—that is, the position one greater than the last valid subscript argument.

var first: Unicode.UTF8.CodeUnit?

The first element of the collection.

var indices: DefaultIndices<String.UTF8View>

The indices that are valid for subscripting the collection, in ascending order.

var isEmpty: Bool

A Boolean value indicating whether the collection is empty.

var lazy: LazySequence<String.UTF8View>

A sequence containing the same elements as this sequence, but on which some operations, such as map and filter, are implemented lazily.

var lazy: LazyCollection<String.UTF8View>

A view onto this collection that provides lazy implementations of normally eager operations, such as map and filter.

var startIndex: String.UTF8View.Index

The position of the first code unit if the UTF-8 view is nonempty.

var underestimatedCount: Int

A value less than or equal to the number of elements in the collection.

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var underestimatedCount: Int

Returns a value less than or equal to the number of elements in the sequence, nondestructively.

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Instance Methods

func contains(Unicode.UTF8.CodeUnit)

Returns a Boolean value indicating whether the sequence contains the given element.

func contains(where: (Unicode.UTF8.CodeUnit) -> Bool)

Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate.

func drop(while: (UInt8) -> Bool)

Returns a subsequence by skipping elements while predicate returns true and returning the remaining elements.

func dropFirst()

Returns a subsequence containing all but the first element of the sequence.

func dropFirst(Int)

Returns a subsequence containing all but the given number of initial elements.

func dropLast()

Returns a subsequence containing all but the last element of the sequence.

func dropLast(Int)

Returns a subsequence containing all but the specified number of final elements.

func elementsEqual<OtherSequence>(OtherSequence)

Returns a Boolean value indicating whether this sequence and another sequence contain the same elements in the same order.

func elementsEqual<OtherSequence>(OtherSequence, by: (Unicode.UTF8.CodeUnit, Unicode.UTF8.CodeUnit) -> Bool)

Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements, using the given predicate as the equivalence test.

func enumerated()

Returns a sequence of pairs (n, x), where n represents a consecutive integer starting at zero, and x represents an element of the sequence.

func filter((Unicode.UTF8.CodeUnit) -> Bool)

Returns an array containing, in order, the elements of the sequence that satisfy the given predicate.

func first(where: (Unicode.UTF8.CodeUnit) -> Bool)

Returns the first element of the sequence that satisfies the given predicate.

func forEach((Unicode.UTF8.CodeUnit) -> Void)

Calls the given closure on each element in the sequence in the same order as a for-in loop.

func index(after: String.UTF8View.Index)

Returns the next consecutive position after i.

func index(of: UInt8)

Returns the first index where the specified value appears in the collection.

func index(where: (UInt8) -> Bool)

Returns the first index in which an element of the collection satisfies the given predicate.

func lexicographicallyPrecedes<OtherSequence>(OtherSequence)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the less-than operator (<) to compare elements.

func lexicographicallyPrecedes<OtherSequence>(OtherSequence, by: (Unicode.UTF8.CodeUnit, Unicode.UTF8.CodeUnit) -> Bool)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements.

func map<T>((Unicode.UTF8.CodeUnit) -> T)

Returns an array containing the results of mapping the given closure over the sequence’s elements.

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func map<T>((UInt8) -> T)

Returns an array containing the results of mapping the given closure over the sequence’s elements.

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func max()

Returns the maximum element in the sequence.

func max(by: (Unicode.UTF8.CodeUnit, Unicode.UTF8.CodeUnit) -> Bool)

Returns the maximum element in the sequence, using the given predicate as the comparison between elements.

func min()

Returns the minimum element in the sequence.

func min(by: (Unicode.UTF8.CodeUnit, Unicode.UTF8.CodeUnit) -> Bool)

Returns the minimum element in the sequence, using the given predicate as the comparison between elements.

func prefix(Int)

Returns a subsequence, up to the specified maximum length, containing the initial elements of the collection.

func prefix(through: String.Index)

Returns a subsequence from the start of the collection through the specified position.

func prefix(upTo: String.Index)

Returns a subsequence from the start of the collection up to, but not including, the specified position.

func prefix(while: (UInt8) -> Bool)

Returns a subsequence containing the initial elements until predicate returns false and skipping the remaining elements.

func reduce<Result>(Result, (Result, Unicode.UTF8.CodeUnit) -> Result)

Returns the result of combining the elements of the sequence using the given closure.

func reversed()

Returns an array containing the elements of this sequence in reverse order.

func sorted()

Returns the elements of the sequence, sorted.

func sorted(by: (Unicode.UTF8.CodeUnit, Unicode.UTF8.CodeUnit) -> Bool)

Returns the elements of the sequence, sorted using the given predicate as the comparison between elements.

func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator: (UInt8) -> Bool)

Returns the longest possible subsequences of the collection, in order, that don’t contain elements satisfying the given predicate.

func split(separator: UInt8, maxSplits: Int, omittingEmptySubsequences: Bool)

Returns the longest possible subsequences of the collection, in order, around elements equal to the given element.

func starts<PossiblePrefix>(with: PossiblePrefix)

Returns a Boolean value indicating whether the initial elements of the sequence are the same as the elements in another sequence.

func starts<PossiblePrefix>(with: PossiblePrefix, by: (Unicode.UTF8.CodeUnit, Unicode.UTF8.CodeUnit) -> Bool)

Returns a Boolean value indicating whether the initial elements of the sequence are equivalent to the elements in another sequence, using the given predicate as the equivalence test.

func suffix(Int)

Returns a subsequence, up to the given maximum length, containing the final elements of the collection.

func suffix(from: String.Index)

Returns a subsequence from the specified position to the end of the collection.

Subscripts

subscript(String.UTF8View.Index)

Accesses the code unit at the given position.

subscript(Range<String.Index>)

Accesses a contiguous subrange of the collection’s elements.

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See Also

Related String Types

struct Substring

A slice of a string.

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protocol StringProtocol

A type that can represent a string as a collection of characters.

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struct String.CharacterView

A view of a string’s contents as a collection of characters.

struct String.UnicodeScalarView

A view of a string’s contents as a collection of Unicode scalar values.

struct String.UTF16View

A view of a string’s contents as a collection of UTF-16 code units.

Beta Software

This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.

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