golang 切片的一个奇怪的现象

有如下求全排列代码：

package main

import (
	"fmt"
)

func main() {
	res := permute([]int{5,4,6,2})
	fmt.Println(res)
}

var result [][]int
func permute(nums []int) [][]int {
	result = make([][]int,0)
	track := make([]int,0)
	trackBack(nums,track)
	return result
}

func trackBack(nums []int, track []int) {
	if len(track) == len(nums) {
		//tmp := make([]int,len(nums))
		//copy(tmp,track)
		//result = append(result,tmp)
		
		result = append(result,track)
		return
	}

	for _,i := range nums {
		// 已经在路径中的剪枝剪掉
		if isInSlice(track,i) {
			continue
		}
		track = append(track,i)
		trackBack(nums,track)
		track = track[:len(track)-1]
	}

}

func isInSlice(nums []int,item int) bool {
	for _,i := range nums {
		if i == item {
			return true
		}
	}
	return false
}

运行结果如下：

[[5 4 2 6] [5 4 2 6] [5 6 2 4] [5 6 2 4] [5 2 6 4] [5 2 6 4] [4 5 2 6] [4 5 2 6] [4 6 2 5] [4 6 2 5] [4 2 6 5] [4 2 6 5] [6 5 2 4] [6 5 2 4] [6 4 2 5] [6 4 2 5] [6 2 4 5] [6 2 4 5] [2 5 6 4] [2 5 6 4] [2 4 6 5] [2 4 6 5] [2 6 4 5] [2 6 4 5]]

由于切片底层都是公用一份内存,修改子切片的值也会影响到父切片,所以正常的使用方式应该是在每次 append 的时候要使用一个 tmp 切片来 copy 一个复制的值,然后 append tmp 切片，不然结果里的值都将会是最后 append 的元素的值（因为它们都是公用的一份内存数据），但是这个问题比较诡异的地方是，他只有相邻的两两元素是一样的，不符合预期，有大佬遇到过类似的问题么？

track int nums result

1 条回复 • 2020-10-31 21:51:32 +08:00

vzard

2020 年 10 月 31 日

找到原因了，是切片内存自动扩容和缩容的坑：
```
// The append built-in function appends elements to the end of a slice. If
// it has sufficient capacity, the destination is resliced to accommodate the
// new elements. If it does not, a new underlying array will be allocated.
// Append returns the updated slice. It is therefore necessary to store the
// result of append, often in the variable holding the slice itself:
// slice = append(slice, elem1, elem2)
// slice = append(slice, anotherSlice...)
// As a special case, it is legal to append a string to a byte slice, like this:
// slice = append([]byte("hello "), "world"...)
func append(slice []Type, elems ...Type) []Type
```
当 cap 不够的时候会自动扩容，当多次执行`track = track[:len(track)-1]`这样的操作之切片又会自动缩容，每次扩容和缩容都会导致底层数组的内存重新申请，每两次内存重新申请之前的 append 的值就会一样，增加日志可以验证：
```
Cap Before: 1, track: [5]
Cap Before: 2, track: [5 4]
Cap Before: 4, track: [5 4 6]
Cap Before: 4, track: [5 4 6 2]
Cap After: 4, track: [5 4 6]
Cap After: 4, track: [5 4]
Cap Before: 4, track: [5 4 2]
Cap Before: 4, track: [5 4 2 6]
Cap After: 4, track: [5 4 2]
Cap After: 4, track: [5 4]
Cap After: 2, track: [5]
Cap Before: 2, track: [5 6]
Cap Before: 4, track: [5 6 4]
Cap Before: 4, track: [5 6 4 2]
Cap After: 4, track: [5 6 4]
Cap After: 4, track: [5 6]
Cap Before: 4, track: [5 6 2]
Cap Before: 4, track: [5 6 2 4]
Cap After: 4, track: [5 6 2]
Cap After: 4, track: [5 6]
Cap After: 2, track: [5]
Cap Before: 2, track: [5 2]
Cap Before: 4, track: [5 2 4]
Cap Before: 4, track: [5 2 4 6]
Cap After: 4, track: [5 2 4]
Cap After: 4, track: [5 2]
Cap Before: 4, track: [5 2 6]
Cap Before: 4, track: [5 2 6 4]
Cap After: 4, track: [5 2 6]
Cap After: 4, track: [5 2]
Cap After: 2, track: [5]
Cap After: 1, track: []
Cap Before: 1, track: [4]
Cap Before: 2, track: [4 5]
Cap Before: 4, track: [4 5 6]
Cap Before: 4, track: [4 5 6 2]
Cap After: 4, track: [4 5 6]
Cap After: 4, track: [4 5]
Cap Before: 4, track: [4 5 2]
Cap Before: 4, track: [4 5 2 6]
Cap After: 4, track: [4 5 2]
Cap After: 4, track: [4 5]
Cap After: 2, track: [4]
Cap Before: 2, track: [4 6]
Cap Before: 4, track: [4 6 5]
Cap Before: 4, track: [4 6 5 2]
Cap After: 4, track: [4 6 5]
Cap After: 4, track: [4 6]
Cap Before: 4, track: [4 6 2]
Cap Before: 4, track: [4 6 2 5]
Cap After: 4, track: [4 6 2]
Cap After: 4, track: [4 6]
Cap After: 2, track: [4]
Cap Before: 2, track: [4 2]
Cap Before: 4, track: [4 2 5]
Cap Before: 4, track: [4 2 5 6]
Cap After: 4, track: [4 2 5]
Cap After: 4, track: [4 2]
Cap Before: 4, track: [4 2 6]
Cap Before: 4, track: [4 2 6 5]
Cap After: 4, track: [4 2 6]
Cap After: 4, track: [4 2]
Cap After: 2, track: [4]
Cap After: 1, track: []
Cap Before: 1, track: [6]
Cap Before: 2, track: [6 5]
Cap Before: 4, track: [6 5 4]
Cap Before: 4, track: [6 5 4 2]
Cap After: 4, track: [6 5 4]
Cap After: 4, track: [6 5]
Cap Before: 4, track: [6 5 2]
Cap Before: 4, track: [6 5 2 4]
Cap After: 4, track: [6 5 2]
Cap After: 4, track: [6 5]
Cap After: 2, track: [6]
Cap Before: 2, track: [6 4]
Cap Before: 4, track: [6 4 5]
Cap Before: 4, track: [6 4 5 2]
Cap After: 4, track: [6 4 5]
Cap After: 4, track: [6 4]
Cap Before: 4, track: [6 4 2]
Cap Before: 4, track: [6 4 2 5]
Cap After: 4, track: [6 4 2]
Cap After: 4, track: [6 4]
Cap After: 2, track: [6]
Cap Before: 2, track: [6 2]
Cap Before: 4, track: [6 2 5]
Cap Before: 4, track: [6 2 5 4]
Cap After: 4, track: [6 2 5]
Cap After: 4, track: [6 2]
Cap Before: 4, track: [6 2 4]
Cap Before: 4, track: [6 2 4 5]
Cap After: 4, track: [6 2 4]
Cap After: 4, track: [6 2]
Cap After: 2, track: [6]
Cap After: 1, track: []
Cap Before: 1, track: [2]
Cap Before: 2, track: [2 5]
Cap Before: 4, track: [2 5 4]
Cap Before: 4, track: [2 5 4 6]
Cap After: 4, track: [2 5 4]
Cap After: 4, track: [2 5]
Cap Before: 4, track: [2 5 6]
Cap Before: 4, track: [2 5 6 4]
Cap After: 4, track: [2 5 6]
Cap After: 4, track: [2 5]
Cap After: 2, track: [2]
Cap Before: 2, track: [2 4]
Cap Before: 4, track: [2 4 5]
Cap Before: 4, track: [2 4 5 6]
Cap After: 4, track: [2 4 5]
Cap After: 4, track: [2 4]
Cap Before: 4, track: [2 4 6]
Cap Before: 4, track: [2 4 6 5]
Cap After: 4, track: [2 4 6]
Cap After: 4, track: [2 4]
Cap After: 2, track: [2]
Cap Before: 2, track: [2 6]
Cap Before: 4, track: [2 6 5]
Cap Before: 4, track: [2 6 5 4]
Cap After: 4, track: [2 6 5]
Cap After: 4, track: [2 6]
Cap Before: 4, track: [2 6 4]
Cap Before: 4, track: [2 6 4 5]
Cap After: 4, track: [2 6 4]
Cap After: 4, track: [2 6]
Cap After: 2, track: [2]
Cap After: 1, track: []

```