20210110190726-interface.org

Interface

tags

Golang

source

Embedding in Go: Part 3 - interfaces in structs

Understand Interface

Interfaces are implemented as a pair of pointers, one to the underlying type and one to the underlying value. Only when both pointers are nil, the interface is nil.

var i interface{} // underlying type is nil, value is nil
fmt.Println(i == nil) // true

var x []int // value is nil
var j interface{} = x // underlying type is []int, value is nil

fmt.Println(j == nil, reflect.TypeOf(j)) // true, []int

• an interface is a collection of method declarations
• a type is an interface if it implements all the methods declared in that interface
• interface can be used as data type

package main

import (  
	"fmt"
)

type ptr interface {
	p()
}

type myint int

type mystring string

func (i myint) p() {
	fmt.Printf("my int is %v\n", i)
}

func (s mystring) p() {
	fmt.Printf("holy shit %s!\n", s)
}

func myprint(a ptr) {
	a.p()
}

func main() {
	var a myint = 1
	var s mystring = "Jeck"
	myprint(a)
	myprint(s)
}

Type assertion

type man struct {
	name string
}

type speaker interface {
	say()
}

func (m man) say() {
	fmt.Printf("hi, my name is %s", m.name)
	fmt.Println()
}

type fucker interface {
	fuck()
}

func (m man) fuck() {
	fmt.Printf("hi, my name is %s, fuck you", m.name)
	fmt.Println()
}

func greet(m speaker) {
        // if the underlying type also implemented fucker interface,
        // call fuck() then.
	if m, ok := m.(fucker); ok {
		m.fuck()
		return
	}
	m.say()
}

func main() {
	x := man{name: "Bob"}
	x.say()
	x.fuck()
	greet(x)
}

Type Switches

see Type Switches