Does the conversion from string to rune slice make a copy?

Question

I'm teaching myself Go from a C background. The code below works as I expect (the first two Printf() will access bytes, the last two Printf() will access codepoints).

What I am not clear is if this involves any copying of data.

package main

import "fmt"

var a string

func main() {
    a = "èe"
    fmt.Printf("%d\n", a[0])
    fmt.Printf("%d\n", a[1])
    fmt.Println("")
    fmt.Printf("%d\n", []rune(a)[0])
    fmt.Printf("%d\n", []rune(a)[1])
}

In other words:

does []rune("string") create an array of runes and fill it with the runes corresponding to "string", or it's just the compiler that figures out how to get runes from the string bytes?

Answer 1

It involves a copy because:

• strings are immutable; if the conversion []rune(s) didn't make a copy, you would be able to index the rune slice and change the string contents
• a string value is a "(possibly empty) sequence of bytes", where byte is an alias of uint8, whereas a rune is a "an integer value identifying a Unicode code point" and an alias of int32. The types are not identical and even the lengths may not be the same:

    a = "èe"
    r := []rune(a)
    fmt.Println(len(a)) // 3 (3 bytes)
    fmt.Println(len(r)) // 2 (2 Unicode code points)

Answer 2

It is not possible to turn []uint8 (i.e. a string) into []int32 (an alias for []rune) without allocating an array.

Also, strings are immutable in Go but slices are not, so the conversion to both []byte and []rune must copy the string's bytes in some way or another.