为什么该切片会更改其容量？

Question

I'm new to golang. A Tour of Go has this snippet:

package main

import "fmt"

func main() {
    s := []int{2, 3, 5, 7, 11, 13}
    printSlice(s)

    // Slice the slice to give it zero length.
    s = s[:0]
    printSlice(s)

    // Extend its length.
    s = s[:4]
    printSlice(s)

    // Drop its first two values.
    s = s[2:]
    printSlice(s)
}

func printSlice(s []int) {
    fmt.Printf("len=%d cap=%d %v
", len(s), cap(s), s)
}

and the result:

len=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=4 cap=6 [2 3 5 7]
len=2 cap=4 [5 7]

My confusion is the cap=4 in the last line - I thought it should remain 6. What is the cause of the capacity changing from 6 to 4 in the last line? Also, why only the last line changes its capacity but the others don't ?

Answer 1

Remember that slice holds data in an array. By dropping first two elements we moved the beginning of the slice to the right and now there are fewer slots between the start of the slice inside the array and the end of the array.

Droping elements at the end of the slice has no effect on capacity as the distance between start of the slice inside the array and the end of the backing array does not change.

Neither of the operations modify the backing array, they just modify the slice data.

See https://blog.golang.org/go-slices-usage-and-internals the observed behaviour is explained in section Slice internals

By printing the slice header you can see the changes happening

func printSlice(s []int) {
    sh := (*reflect.SliceHeader)(unsafe.Pointer(&s))
    fmt.Printf("header=%+v len=%d cap=%d %v
", sh, len(s), cap(s), s)
}

In the last call, the data pointer is moved ahead.

header=&{Data:272990208 Len:6 Cap:6} len=6 cap=6 [2 3 5 7 11 13]
header=&{Data:272990208 Len:0 Cap:6} len=0 cap=6 []
header=&{Data:272990208 Len:4 Cap:6} len=4 cap=6 [2 3 5 7]
header=&{Data:272990216 Len:2 Cap:4} len=2 cap=4 [5 7]