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

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 } `