在golang中，如何在函数中重新分配外部引用？

Question

I'm probably not expressing this correctly in the question, but perhaps this code will make it clearer:

package main
import "fmt"

type Blob struct {
    Message string
}

func assign1(bb **Blob) {
    *bb = &Blob{"Internally created by assign1"}
}

func (b *Blob) assign2() {
    *b = Blob{"Internally created by assign2"}
}

func main() {
    x1 := &Blob{"Hello World"}
    assign1(&x1)
    fmt.Printf("x1 = %+v
", *x1)

    x2 := Blob{"Hello World"}
    x2.assign2()
    fmt.Printf("x2 = %+v
", x2)
}

Produces, as desired:

x1 = {Message:Internally created by assign1}
x2 = {Message:Internally created by assign2}

I want to pass a reference (pointer to a pointer) into a function and have the function assign a new value to the pointer such that the calling scope will see that new value.

I've figured out the above two ways of doing this, but I'd like to know if they are actually correct or if there is some hidden flaw. Also, are either of them more idiomatic than the other?

Coming from Java, assign2 just seems wrong but I'm sure I've seen something similar in the encoding/json package. What is that actually doing?

Thanks!

Answer 1

James answers the mechanics of assign2. I'll touch a bit on when to use it.

Let's take a simpler example, first.

type Counter uint

func (c *Counter) Increment() {
    *c += 1
}

In the counter example the entire state of the receiver is changing. Similarly for the encoding/json package the entire state of the receiver is changing. That's really the only time I would use that style.

One major advantage of the style: you can define an interface for the change, just like the GobDecoder does.

When I first saw the assign2 style it was a little grating. But then I remembered that (c *Counter) Increment gets translated to Increment(c *Counter) in the machine code and it didn't bother me anymore. I personally prefer assign1-style. (Though, there is no need for the double pointers as orignally posted.)

package main
import "fmt"

type Blob struct {
    Message string
}

func assign1(b *Blob) {
    *b = Blob{"Internally created by assign1"}
}

func main() {
    x1 := Blob{"Hello World"}
    assign1(&x1)
    fmt.Printf("x1 = %+v
", *x1)
}