Type Assertion

• A type assertion is an operation applied to an interface value.
• Extract dynamic value from interface value.
• v := x.(T)
  • Checks interface values's dynamic type is identical to concrete type typename, return dynamic value.
  • If check fails, then the operation panics.
• v, ok := x.(T)
  • The second return value is boolean.
  • If type assertion is successful, v == dynamic value, ok == true.
  • If type assertion is fails, v == zero value of type, ok == false.
  • No run-time panic occurs in this case.

Exercises

https://go.dev/play/p/oeKqBMBMiVT

package main

import (
    "fmt"
    "math"
)

type circle struct {
    radius float64
}
type triangle struct {
    // lengths of the sides of a triangle.
    a, b, c float64
}
type rectangle struct {
    a, b float64
}

func (c circle) area() float64 {
    return math.Pi * c.radius * c.radius
}

// Heron's Formula for the area of a triangle
func (t triangle) area() float64 {
    p := (t.a + t.b + t.c) / 2.0 // perimeter half
    return math.Sqrt(p * (p - t.a) * (p - t.b) * (p - t.c))
}

func (t rectangle) area() float64 {
    return t.a * t.b
}

func (t triangle) angles() []float64 {
    return []float64{angle(t.b, t.c, t.a), angle(t.a, t.c, t.b), angle(t.a, t.b, t.c)}
}
func angle(a, b, c float64) float64 {
    return math.Acos((a*a+b*b-c*c)/(2*a*b)) * 180.0 / math.Pi
}

type shape interface {
    area() float64
}

func main() {
    shapes := []shape{
        circle{1.0},
        rectangle{5, 10},
        triangle{10, 4, 7},
    }
    for _, v := range shapes {
        fmt.Println(v, "\tArea:", v.area())
        if t, ok := v.(triangle); ok {
            fmt.Println("Angles:", t.angles())
        }
    }
}