golang slice，用 slice[a :b:c] slice

Question

我读了go slice usage and internals和 Slice和 Effective go#slice但是没有像这样用 3 个数字 slice 的方法:slice[a:b:c]

例如这段代码:

package main

import "fmt"

func main() {
    var s = []string{"a", "b", "c", "d", "e", "f", "g"}
    fmt.Println(s[1:2:6], len(s[1:2:6]), cap(s[1:2:6]))
    fmt.Println(s[1:2:5], len(s[1:2:5]), cap(s[1:2:5]))
    fmt.Println(s[1:2], len(s[1:2]), cap(s[1:2]))

}

go playground结果是这样的:

[b] 1 5
[b] 1 4
[b] 1 6

我能理解第三个是关于容量的东西，但是这个具体是什么意思呢？
我是否遗漏了文档中的某些内容？

Answer 1

语法已经在 Go 1.2 中引入，正如我在“Re-slicing slices in Golang”中提到的。
它记录在 Full slice expressions 中:

a[low : high : max]

constructs a slice of the same type, and with the same length and elements as the simple slice expression a[low : high].
Additionally, it controls the resulting slice's capacity by setting it to max - low.
Only the first index may be omitted; it defaults to 0.

After slicing the array a:

a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]

the slice t has type []int, length 2, capacity 4, and elements

t[0] == 2
t[1] == 3

design document该功能具有以下理由:

It would occasionally be useful, for example in custom []byte allocation managers, to be able to hand a slice to a caller and know that the caller cannot edit values beyond a given subrange of the true array.

The addition of append to the language made this somewhat more important, because append lets programmers overwrite entries between len and cap without realizing it or even mentioning cap.