Singleton 单例

单例的实现依赖 sync.Once 中的 Do 函数，因为每个 sync.Once 类型变量中的 Do 函数永远只会执行一次。

package main
import (
    "fmt"
    "sync"
    "time"
    "unsafe"
)
func main() {
    for i := 0; i < 10; i++ {
        go func() {
            user := getUser()
            user2 := getUser2()
            fmt.Printf("user address: %x\n", unsafe.Pointer(user))
            fmt.Printf("user2 address: %x\n", unsafe.Pointer(user2))
        }()
    }
    time.Sleep(time.Millisecond)
}
type User struct {
}
type User2 struct {
}
var (
    userInstance  *User
    user2Instance *User2
    userOnce      sync.Once
)
func getUser() *User {
    userOnce.Do(func() {
        userInstance = new(User)
    })
    return userInstance
}
func getUser2() *User2 {
    // 变量userOnce的Do函数只会执行一次，所以user2Instance永远不会被初始化
    userOnce.Do(func() {
        user2Instance = new(User2)
    })
    // 永远返回nil
    return user2Instance
}