将引用类型为“ slice”的变量分配给另一个变量，为什么它们不同时更改？

Question

anotherSlice := theSlice
anotherSlice = append(anotherSlice, newEle)
fmt.Println(len(anotherSlice) == len(theSlice))

This snippet will output false. Why?

And here are some other experiments:

package main

import "fmt"

func main() {
    theSlice := []int{3,3,2,5,12,43}
    anotherSlice := theSlice
    fmt.Println(anotherSlice[3], theSlice[3])
    anotherSlice[3] = anotherSlice[3]+2
    fmt.Println(anotherSlice[3], theSlice[3])
    anotherSlice = append(anotherSlice[:3], anotherSlice[4:]...)
    fmt.Println(len(anotherSlice),len(theSlice))
}

The output is like below:

5 5
7 7
5 6

Program exited.

Answer 1

Whenever appended slice anotherSlice has no capacity for the new element, append function creates new slice and returns it. Since then the slices anotherSlice and theSlice are different - they are backed by separate arrays.

Reslicing slice with a shorter length anotherSlice[:3] has no impact on the original capacity of the slice.

The following line:

anotherSlice = append(anotherSlice[:3], anotherSlice[4:]...)

cuts out fourth (index 3) element. Because anotherSlice[:3] has capacity to hold all elements of anotherSlice[4:] no new allocation happens and therefore both slices are modified.

package main

import (
        "fmt"
)   

func main() {
        x := []int{1, 2, 3, 4, 5, 6}
        fmt.Println(cap(x[:3]) >= len(x[:3])+len(x[4:]))
        y := append(x[:3], x[4:]...)
        fmt.Println(x, y)
}

<kbd>Playground</kbd>

• Go Slices
• Capacity function
• Slice tricks