UTF8View

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"