Example 2

package main

import (
    "fmt"
    "sync"
)

var once sync.Once

type single struct {
}

var singleInstance *single

func getInstance() *single {
    if singleInstance == nil {
        once.Do(
            func() {
                fmt.Println("Creating single instance now.")
                singleInstance = &single{}
            })
    } else {
        fmt.Println("Single instance already created.")
    }

    return singleInstance
}

package main

import (
    "fmt"
)

func main() {

    for i := 0; i < 30; i++ {
        go getInstance()
    }

    // Scanln is similar to Scan, but stops scanning at a newline and
    // after the final item there must be a newline or EOF.
    fmt.Scanln()
}

Creating single instance now.
Single instance already created.
Single instance already created.