Golang中的数组分配：内容副本或内容指针副本

Question

While learning nuances of array data structure in Golang I came across an interesting confusion. I have learnt from blog.golang.org that -

when you assign or pass around an array value you will make a copy of its contents.

To check this myself I wrote the following piece of code:

package main

import "fmt"

func main() {
    x := []int{2, 4, 5}
    y := x

    y[0] = -10

    // expecting 2 here but getting -10, since  y := x is supposed to be a content copy
    fmt.Println(x[0])

    // not same
    println("&x: ", &x)
    println("&y: ", &y)

    // same
    println("&x[0]: ", &x[0])
    println("&y[0]: ", &y[0])
}

Doing exactly same thing is java I got same internal object address for x and y as expected.

Go Code: https://play.golang.org/p/04l0l84eT4J

Java code: https://pastebin.ubuntu.com/p/S3fHMTj5NC/

Answer 1

Your x:

x := []int{2, 4, 5}

is a slice, not an array.

If you make x an array:

x := [3]int{2, 4, 5}
// ---^ Now it is an array

then you'll see the results you're expecting:

package main

import "fmt"

func main() {
    x := [3]int{2, 4, 5}
    y := x

    y[0] = -10

    // expecting 2 here, since it is supposed to be a content copy
    fmt.Println(x[0])

    // not same
    fmt.Println("&x: ", &x)
    fmt.Println("&y: ", &y)

    // same (not anymore...)
    fmt.Println("&x[0]: ", &x[0])
    fmt.Println("&y[0]: ", &y[0])
}

produces output like this:

2
&x:  &[2 4 5]
&y:  &[-10 4 5]
&x[0]:  0x10414020
&y[0]:  0x10414030

https://play.golang.org/p/-3ZNPwlD1WT