Defer playground

defer.go

package main

import "sync"

// deferはreturnの後だよ
func A() string {
	str := "A"
	defer func() {
		str = "B"
	}()
	defer func() {
		str = "C"
	}()
	defer func() {
		str = "D"
	}()
	return str
}

// named returnだと戻り値を上書きできるよ
func B() (str string) {
	str = "A"
	defer func() {
		str = "B"
	}()
	defer func() {
		str = "C"
	}()
	defer func() {
		str = "D"
	}()
	return str
}

func C() {
	str := "1"
	print(str)

	// その時の変数はキャプチャされるよ
	defer func() {
		print(str)
	}()

	str = "2"
	print(str)
}

func D() {
	str := "1"
	print(str)

	// 関数スコープで定義するとその時点の値が取れるよ
	defer func(s string) {
		print(s)
	}(str)

	str = "2"
	print(str)
}

// Goroutine
func E() {
	var wg sync.WaitGroup
	for _, s := range []string{"A", "B", "C"} {
		wg.Add(1)
		go func(s string) {
			defer print(s)
			wg.Done()
		}(s)
	}

	println("Start")
	wg.Wait()
	println()
	println("End")
}

func main() {
	print(A())
	println()
	print(B())
	println()
	C()
	println()
	D()
	println()
	E()
}