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Accessing the underlying struct of an interface

Converting between an interface and a struct can be done leveraging type assertions in Go. Let’s take the following example code and see how it would work:

package main

import (

type shape interface {
	area() float64

type rectangle struct {
	length float64
	width  float64

type circle struct {
	radius float64

func (r rectangle) area() float64 {
	return r.length * r.width

func (c circle) area() float64 {
	return math.Pi * math.Pow(c.radius, 2)

func main() {
	// Create a slice of shapes to iterate over
	shapes := []shape{
		rectangle{length: 2, width: 2},
		circle{radius: 3},

	for _, item := range shapes {
		if rect, ok := item.(rectangle); ok {
			// Check if the item is a rectangle, if it is, then enter this if block
			fmt.Println("I'm a rectangle")
			fmt.Printf("length: %f, width %f, area: %f\n", rect.length, rect.width, item.area())
		} else if circ, ok := item.(circle); ok {
			// Check if the item is a cirlce, if it is, then enter this if block
			fmt.Println("I'm a circle")
			fmt.Printf("radius: %f, area: %f\n", circ.radius, item.area())
		} else {
			// If it's anything else, we aren't sure what it is
			fmt.Println("I don't know what I am")

When running this code you should see the following output:

I'm a rectangle
length: 2.000000, width 2.000000, area: 4.000000
I'm a circle
radius: 3.000000, area: 28.274334

As you can see in the above example you can leverage type assertions to get access to the underlying data of an object. This can be extremely useful for things like custom errors, where you might have additional information, like what HTTP status code to return, or more detailed information on the error.