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package singleton

var instance *singleton

func init() {
    instance = &singleton{}
}

type singleton struct{}

func GetInstance() *singleton {
    return instance
}

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package singleton

var instance *singleton

type singleton struct{}

func GetInstance() *singleton {
    if instance == nil {
        instance = &singleton{}
    }
    return instance
}

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package singleton

import "sync"

var (
    instance *singleton
    mutex    sync.Mutex
)

type singleton struct{}

func GetInstance() *singleton {
    mutex.Lock() // 如果实例存在没有必要加锁
    defer mutex.Unlock()

    if instance == nil {
        instance = &singleton{}
    }
    return instance
}

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package singleton

import "sync"

var (
    instance *singleton
    mutex    sync.Mutex
)

type singleton struct{}

func GetInstance() *singleton {
    if instance == nil { // // 不太完美 因为这里不是完全原子的
        mutex.Lock()
        defer mutex.Unlock()

        if instance == nil {
            instance = &singleton{}
        }
    }
    return instance
}

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package singleton

import "sync"
import "sync/atomic"

var (
    initialized uint32
    instance *singleton
    mutex    sync.Mutex
)

type singleton struct{}

func GetInstance() *singleton {
    if atomic.LoadUInt32(&initialized) == 1 {  // 原子操作 
        return instance
    }

    mu.Lock()
    defer mu.Unlock()

    if initialized == 0 {
         instance = &singleton{}
         atomic.StoreUint32(&initialized, 1)
    }

    return instance
}

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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

import (
    "sync/atomic"
)
// Once is an object that will perform exactly one action.
//
// A Once must not be copied after first use.
type Once struct {
    // done indicates whether the action has been performed.
    // It is first in the struct because it is used in the hot path.
    // The hot path is inlined at every call site.
    // Placing done first allows more compact instructions on some architectures (amd64/386),
    // and fewer instructions (to calculate offset) on other architectures.
    done uint32
    m    Mutex
}

// Do calls the function f if and only if Do is being called for the
// first time for this instance of Once. In other words, given
//  var once Once
// if once.Do(f) is called multiple times, only the first call will invoke f,
// even if f has a different value in each invocation. A new instance of
// Once is required for each function to execute.
//
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
//  config.once.Do(func() { config.init(filename) })
//
// Because no call to Do returns until the one call to f returns, if f causes
// Do to be called, it will deadlock.
//
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
//
func (o *Once) Do(f func()) {
    // Note: Here is an incorrect implementation of Do:
    //
    // if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    //      f()
    //  }
    //
    // Do guarantees that when it returns, f has finished.
    // This implementation would not implement that guarantee:
    // given two simultaneous calls, the winner of the cas would
    // call f, and the second would return immediately, without
    // waiting for the first's call to f to complete.
    // This is why the slow path falls back to a mutex, and why
    // the atomic.StoreUint32 must be delayed until after f returns.

    if atomic.LoadUint32(&o.done) == 0 {
        // Outlined slow-path to allow inlining of the fast-path.
        o.doSlow(f)
    }
}

func (o *Once) doSlow(f func()) {
    o.m.Lock()
    defer o.m.Unlock()
    if o.done == 0 {
        defer atomic.StoreUint32(&o.done, 1)
        f()
    }
}

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var once sync.Once

once.Do(func() {
    // 在这里执行安全的初始化
})

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package singleton

import (
    "sync"
)

var (
    instance *singleton
    once     sync.Once
)

type singleton struct{}

func GetInstance() *singleton {
    once.Do(func() {
        instance = &singleton{}
    })
    return instance
}