GO type cast and assignment using interfaces-CodePudding

I can't get my head around type casting using interfaces.

There is an example for setting value using pointers:

func main() {
    a := &A{}
    
    cast(a, "BBB")

    fmt.Println(a.s)
}

type A struct {
    s string
}

func cast(a *A, b interface{}) {
    a.s = b.(string)
}

The output of this program will print BBB.

Now my problem is that what if I want to set more than string? I imagine that I want to do something like this:

func main() {
    a := &A{}
    
    cast(&(a.s), "BBB")

    fmt.Println(a.s)

}

type A struct {
    s string
}

func cast(a interface{}, b interface{}) {
    // Here could be type switch to determine what kind of type I want to cast to, but for know string is enough...
    a = b.(string)
}

And this code's output is an empty string... Could anyone help me to understand what I've been doing wrong?

CodePudding user response：

The second program assigns to local variable a, not to the caller's variable a.

You must dereference the pointer to assign to the caller's value. To do, that you need a pointer type. Use a type assertion to get the pointer type:

func cast(a interface{}, b interface{}) {
    *a.(*string) = b.(string)
}