A view of a string’s contents as a collection of UTF-8 code units.
SDK
- Xcode 6.0.1+
Framework
- Swift Standard Library
Declaration
@frozen struct UTF8View
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"