go - 为什么当你删除前 n 项而不是最后 n 项时， slice 的容量会改变？

Question

我正在浏览 Go 并想了解以下内容:

package main

import "fmt"

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

    // Drop its last two values
    s = s[:len(s)-2]
    printSlice(s)

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

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

结果:

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

为什么当您删除最后 2 项时 slice 的容量保持不变，但当您删除前 2 项时 slice 的容量会发生变化？

https://play.golang.org/p/ZNKwOYKDqOi

Answer 1

Go slice 被实现为一个结构:

src/runtime/slice.go:

type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}

---

修改您的 printSlice 函数以显示指向底层数组的指针:

package main

import "fmt"

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

    // Drop its last two values
    s = s[:len(s)-2]
    printSlice(s)

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

func printSlice(s []int) {
    var ptr *int
    if cap(s) >= 1 {
        ptr = &s[:cap(s)][0]
    }
    fmt.Printf("ptr=%p len=%d cap=%d %v\n", ptr, len(s), cap(s), s)
}

Playground :https://play.golang.org/p/pk3cpE_LsUV

输出:

ptr=0x450000 len=6 cap=6 [2 3 5 7 11 13]
ptr=0x450000 len=4 cap=6 [2 3 5 7]
ptr=0x450008 len=2 cap=4 [5 7]

查看 slice 操作如何调整指针、长度和容量。 slice 只是底层数组的 View 或窗口。

---

引用资料:

The Go Blog: Go Slices: usage and internals

The Go Programming Language Specification:

• Slice types

• Slice expressions