重新分配指针方法接收器

Question

What I understand about pointer method receiver and non-pointer method receiver is first one can be modified in the method and next one isn't.

So, following worked exactly as I expected.

type student struct {
    name string
    age  int
}

func (s *student) update() {
    s.name = "unknown"
    s.age = 0
}

func main() {
    s := student{"hongseok", 13}
    fmt.Println(s)

    s.update()
    fmt.Println(s)
}

It prints hongseok/13 and unknown/0.

But, I want to replace whole s in update method at once with reassigning. So, I've just altered update method as bellow.

func (s *student) update() {
    s = &student{"unknown", 0}
}

And it doesn't change s in main method and prints double hongseok/13.

func (s *student) update() {
    *s = student{"unknown", 0}
}

Above change fix the problem.

I think there's no semantic difference. What am I missing?

Answer 1

In the first example:

func (s *student) update() {
    s = &student{"unknown", 0}
}

You are assigning an entirely new "pointer value" to s, and the new *s points at a new student value. The variable s is scoped only to the method body, so there are no side effects after this returns.

In the second example

func (s *student) update() {
    *s = student{"unknown", 0}
}

You are dereferencing s, and changing the value of *s to point to a new student value, or to put it differently, you are putting a new student value at the address where s points.