structs-interfaces.go

package main

import (
	"fmt"
	"encoding/json"
	"reflect"
)

type circle struct {
	Radius	float64	`json:"radius"`
}

type square struct {
	Edge	float64	`json:"edge"`
}

func (c square) area() float64 {
	return c.Edge * c.Edge
}

type shape interface {
	area() float64
}

func (c circle) area() float64 {
	return 3.14 * c.Radius * c.Radius
}

func info(s shape) {
	fmt.Println("area", s.area())
}

func getType(myvar interface{}) string {
	if t := reflect.TypeOf(myvar); t.Kind() == reflect.Ptr {
		return "*" + t.Elem().Name()
	} else {
		return t.Name()
	}
}

func main() {
	js1 := []byte(`{"radius": 2.32}`)
	js2 := []byte(`{"edge": 1.2}`)

	c := circle{}
	s := square{}

	json.Unmarshal(js1, &c)
	json.Unmarshal(js2, &s)

	// c - 16.900736
	// s - 1.44
	fmt.Println(c.area(), s.area())

	// What about:

	// Works fine
	var i shape
	i = circle{1.2}
	info(i)

	// Not working..
	var i2 shape // <-- nil
	i2 = circle{}

	// It should be no more empty interface, Circle struct instead!
	fmt.Println(getType(i2))  // <-- Yep, It's circle!

	// QUESTION 1:  This will not assign values, but why, because it's interface?
	// Fields are not accessible only methods circle.area() !

	// QUESTION 2: How to assign values from JSON string to such struct`s?
	json.Unmarshal(js1, &i2)

	info(i2) // out: area 0
}

Answer:

	// Interface
	var i2 shape // <-- nil

	// Actual literal
	qc := circle{}

	// QUESTION 1:  This will not assign values, but why, because it's interface?
	// Fields are not accessible only methods circle.area() !
	// i2 - interface
	// qc - actual literal

	// QUESTION 2: How to assign values from JSON string to such struct`s?

	// Unmarshal to actual literal
	json.Unmarshal(js1, &qc)

	// Assign literal to interface
	i2 = qc

	info(i2) // out: area 16.900736