range-buggy.go

package main

import (
	"fmt"
	"sync"
)

// more to see https://github.com/golang/go/wiki/CommonMistakes
func main() {
	m := []string{"a", "b", "c", "d"}
	var wg sync.WaitGroup

	// Buggy way, output closure v:dddd
	fmt.Print("closure v:")
	wg.Add(len(m))
	for _, v := range m {
		go func() {
			defer wg.Done()
			fmt.Print(v)
		}()
	}
	wg.Wait()
	fmt.Println()

	// Buggy way, output go func v:dddd
	fmt.Print("go func v:")
	wg.Add(len(m))
	for _, v := range m {
		go trap1(&wg, &v)
	}
	wg.Wait()
	fmt.Println()

	// Right way, output closure v:dcba
	fmt.Print("closure v:")
	wg.Add(len(m))
	for _, v := range m {
		go func(k string) {
			defer wg.Done()
			fmt.Print(k)
		}(v)
	}
	wg.Wait()
	fmt.Println()

	// Right way, output: go func v:dcba
	fmt.Print("go func v:")
	wg.Add(len(m))
	for _, v := range m {
		v1 := v
		go trap1(&wg, &v1)
	}
	wg.Wait()
	fmt.Println()

}

func trap1(wg *sync.WaitGroup, name *string) {
	defer wg.Done()
	fmt.Print(*name)
}