单例模式中singleton结构体字段为空，多次赋值instance值是一样的

[joengtou]

如下代码验证了空结构体多次赋值结果一样，在单例模式中golang-design-pattern/03_singleton/singleton.go line23，即使不用sync.Once 也能通过单元测试，应该给singleton添加一个字段

`package main

import (
"fmt"
)

var a1 *singleton1
var a2 *singleton2

type singleton1 struct {
}
type singleton2 struct {
d int
}

func getInstance1() *singleton1 {
a1 = &singleton1{}
return a1
}
func getInstance2() *singleton2 {
a2 = &singleton2{}
return a2
}
func main() {
b1, c1 := getInstance1(), getInstance1()
fmt.Println(b1 == c1)
//true
b2, c2 := getInstance2(), getInstance2()
fmt.Println(b2 == c2)
//false
}
`