go-写入接口的功能{}

Question

I want to pass a pointer to something into a function, without knowing its type at compile time, have the function write to it. Here's what I thought would work:

func foo(dest interface{}) {
    switch (dest).(type) {
    case *int:
        fmt.Println("got int")
        *dest = 1
    // handle other cases...
    }
}

However, calling this with an *int input

func main() {
    bar := 2
    foo(&bar)
    fmt.Println(bar) // expect 1
}

yields the compiler error

invalid indirect of dest (type interface {}).

What am I doing wrong here?

Answer 1

In this piece of code (btw, you don't need the parens around dest), you are basically forgetting the type once you enter a case:

func foo(dest interface{}) {
    switch dest.(type) {
    case *int:
        fmt.Println("got int")
        *dest = 1
    // handle other cases...
    }
}

That is, dest is still of type interface{} according to the compiler, and that makes *dest = 1 wrong.

You could use more type assertions like this...

func foo(dest interface{}) {
    switch dest.(type) {
    case *int:
        fmt.Println("got int")
        *dest.(*int) = 1
        // handle other cases...
    }
}

...but a switch that actually 'remembers' the type would be much better (from Effective Go)

func foo(dest interface{}) {
    switch dest := dest.(type) {
    case *int:
        fmt.Println("got int")
        *dest = 1
    // handle other cases...
    }
}