实例上替换的字符串数组不反映

Question

I have an instance of a struct that has an array field. When a goroutine replaces the array on the instance, all the other handles to that instance still have the original array. Why is this? What should I change so that the instance of Container below will reflect the change made on the goroutine?

A simple example is also reflected here: https://play.golang.org/p/dXVKN6o8aP

func Create() Container {
    instance := *&Container{
        values: []string{"initial value"},
    }
    go func() {
        instance.values = []string{"modified value"}
        fmt.Print("values updated")
    }()
    return instance
}

func main() {
    instance := Create()
    time.Sleep(100 * time.Millisecond)
    fmt.Printf("%[1]v", instance.values)
    //prints "initial value", but I expected "modified value"
}

Answer 1

It is because you are using the slice's value. You return the value of the slice which is returned to main by copy. Goroutine then executes and updates the original value. Make the following changes:

func Create() *Container { // return a pointer to container

    // instance is now initialized and points to a Container value
    // (i.e. instance is of type *Container)
    instance := &Container{
        values: []string{"initial value"},
    }
    go func() {
        instance.values = []string{"modified value"}
        fmt.Print("values updated")
    }()
    return instance
}

Now that main has a pointer to the above instance, modifications done by goroutine will be visible.

Example: https://play.golang.org/p/AKwUZfTu0s

Also, to create a new value, you can directly use the type's name:

instance := Container{ // instance is of type Container
    // ...
}

And to create and initialize a pointer of a type, use:

instance := &Container{ // instance is of type *Container
    // ...
}

Doing something like below is redundant:

instance := *&Container{ // instance is of type Container. Same as first declaration
    // ...
}