interface_type_assertion.go

package main
import (
"fmt"
"math"
)

type Square struct {
  side float32
}

type Circle struct {
  radius float32
}

type Shaper interface {
  Area() float32
  Circum() float32
}

func main() {
  var areaIntf Shaper
  sq1 := Square{5} // Try using val
  // sq1.side = 5
  areaIntf = sq1
  // Is Square the type of areaIntf ?
  if t, ok := areaIntf.(*Square); ok {
    fmt.Printf("The type of areaIntf is: %T\n", t)
  }
  if u, ok := areaIntf.(*Circle); ok {
    fmt.Printf("The type of areaIntf is: %T\n", u)
  } else {
    fmt.Println("areaIntf does not contain a variable of type Circle")
  }
}

func (sq *Square) Area() float32 {
  return sq.side * sq.side
}

func (sq *Square) Circum() float32 {
  return 4 *sq.side
}

func (ci Circle) Area() float32 {
  return ci.radius * ci.radius * math.Pi
}

func (ci Circle) Circum() float32 {
  return 2 * ci.radius * math.Pi
}

When you call a method on an interface, it must either have an identical receiver type or it must be directly discernible from the concrete type:

Pointer methods can be called with pointers.
Value methods can be called with values.
Value-receiver methods can be called with pointer values because they can be dereferenced first.
Pointer-receiver methods cannot be called with values; however, because the value stored inside an interface has no address.