接口值的基础指针类型

How do I get the underlying pointer type from an interface?

package main

import (
    "fmt"
)

type Car interface  {
    Drive() string
}
type MyCar struct   {
    name string
}
func (MyCar) Drive (  ) string  {
    return "rum rum"
}


func main() {
    var car Car
    mycar := &MyCar{name:"mycar"}
    car = mycar
    mycarptr, err :=  car.(*MyCar)
    mycarvalue, err2 :=  car.(MyCar)
    fmt.Printf( "as ptr failed: %t, as value failed: %t
", err, err2 )
    fmt.Printf( "as ptr: %+v, as value: %+v", mycarptr, mycarvalue)
}

Your first assertion to *MyCar works fine

Here is a playground example to illustrate

Your second assertion to MyCar will fail since it's not a pointer.

To be able to modify the car you need to use a pointer to it (like you already did), however to make it more clear to others (and yourself) you should define the interface method on the pointer:

type Drivable interface {
    Drive() string
}

type Car struct {
    name string
}

func (*Car) Drive() string {
    return "rum rum"
}

type SpaceShip struct {
    name string
}

func (*SpaceShip) Drive() string {
    return "sound spaceships makes when you drive / fly them"
}

func Drive(d Drivable) {
    switch d := d.(type) { // d now is the actual type
    case *Car:
        fmt.Println("Got a car named", d.name)
    case *SpaceShip:
        fmt.Println("Got a spaceship named", d.name)
    }
}

playground

I recommend going through Effective Go, and pay extra attention to the Interfaces And Methods section.