取消引用结构指针是否会复制结构？

Question

I have this code:

type countHolder struct {
    count int
}

func main() {
    a := &countHolder{1}
    b := *a
    a.count = 2
    println(b.count)
}

I expected the output to be 2, but the output was 1.

My understanding was that:

1. a := &countHolder{1} // a is pointer to struct with data starting at address x
2. b := *a // b now equals address x
3. a.count = 2 // the struct stored at address x has its count value changed to 2

Where am I wrong? is b := *a creating a copy of the struct?

Answer 1

From the fine specification:

For an operand x of type T, the address operation &x generates a pointer of type *T to x. [...]

For an operand x of pointer type *T, the pointer indirection *x denotes the variable of type T pointed to by x. [...]

That means that the unary & operator gives you the address of something so a in:

a := &countHolder{1}

is a pointer. The unary * operator in:

b := *a

dereferences the pointer a and leaves you with a countHolder struct on the right side so b is a copy of the struct that a points to. Since b is a copy of the struct, modifying a.count:

a.count = 2

(which could also be written as (*a).count = 2) won't have any affect on b.

You could also have a look at (https://play.golang.org/p/Zubs8qYBA_K):

func main() {
    a := &countHolder{1}
    b := *a
    fmt.Printf("%T
%T
", a, b)
}

to have a quick look at what types a and b are (*counterHolder and counterHolder, respectively, in this case).