当使用*

Question

我正在按照本书（https://github.com/PacktPublishing/Go-Design-Patterns/blob/master/Chapter02/main.go）中所述的Singleton设计模式进行操作，并将此代码保存在文件“ singleton2.go”中：

package singleton2

type Singleton interface {
  AddOne() int
}

type singleton struct {
  count int
}

//var instance = &singleton{}
var instance *singleton

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

func (s *singleton) AddOne() int {
  s.count++
  return s.count
}

然后我有此测试文件（singleton2_test.go）：

package singleton2

import "testing"

func TestGetInstance(t *testing.T) {
  counter1 := GetInstance()
  if counter1 == nil {
    t.Fatal("expected pointer to Singleton after calling GetInstance(), not nil")
  }

  expectedCounter := counter1

  currentCount := counter1.AddOne()
  if currentCount != 1 {
    t.Errorf("After calling for the first time to count, the counter must be 1 but it is %d", currentCount)
  }

  counter2 := GetInstance()
  if counter2 != expectedCounter {
    t.Errorf("Expected same instance in counter2 but it got a different instance.")
    t.Logf("Got %v, want %v", counter2, expectedCounter)
  }

  currentCount = counter2.AddOne()
  if currentCount != 2 {
    t.Errorf("After calling AddOne() using second counter, the count must be 2, but it was %d", currentCount)
  }

}

问题是测试总是失败：

--- FAIL: TestGetInstance (0.00s)
    singleton2_test.go:20: Expected same instance in counter2 but it got a different instance.
    singleton2_test.go:21: Got &{0}, want &{1}
    singleton2_test.go:26: After calling AddOne() using second counter, the count must be 2, but it was 1
FAIL
exit status 1
FAIL    github.com/d3c3/design_patterns/singleton/singleton2    0.029s

有趣的是，如果我将var instance *singleton行更改为var instance = &singleton{}行，则测试通过！这是为什么？ IMO，它也应与“ var instance * singleton”一起使用

有人能解释这种行为差异吗？

Answer 1

遵循代码的逻辑，很明显问题出在哪里。

声明但不初始化单例（var instance *singleton）时，instance为nil。调用GetInstance会将instance == nil评估为true，并在每次调用时返回一个新的*singleton。这就是counter2永远不会等于expectedCounter的原因-对GetInstance的每次调用都返回一个新的计数器实例。

初始化单例实例（var instance = &singleton{}）时，对GetInstance的调用将返回该单例实例，因为它不是nil。

我想您想将GetInstance修改为以下内容：

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

编辑

尽管您没有提到需要该单例是线程安全的，但用户colminator正确地指出，以这种方式实例化一个单例会导致多个go例程实例化其自己的单例实例，挫败了拥有一个人的目的。鉴于Golang中的并发性很高，了解更多信息可能会有所帮助！ Check out the link he posted here。