为什么变量是nil，尽管我将对象的引用放在那里？

huangapple
go评论117阅读模式
英文:

why variable is nil although I put there reference to object

问题

我无法弄清楚为什么在调用ConnectToMongo之后，变量session仍然为nil。如果ConnectToMongo接受的是非引用类型，比如ConnectToMongo(session mgo.Session)，我可以理解这种情况，但是返回函数ConnectToMongo后，引用变量类型*mgo.Session必须被保存。

它的输出是：

nil. Why?

更新

  1. package main
  3. import (
  4. 	"fmt"
  5. 	"gopkg.in/mgo.v2"
  6. )
  8. func ConnectToMongo(session **mgo.Session) {
  9. 	if session == nil {
  10. 		var err error
  11. 		*session, err = mgo.Dial("localhost:27028")
  12. 		if err != nil {
  13. 			panic(err)
  14. 		}
  15. 	}
  16. }
  18. func main() {
  19. 	var session *mgo.Session
  20. 	ConnectToMongo(&session)
  21. 	if session == nil {
  22. 		fmt.Println("nil. Why?")
  23. 	} else {
  24. 		fmt.Println("not nil. Ok.")
  25. 	}
  26. }

相同的输出：

  1. nil. Why?
英文:

I cannot figure out why after calling ConnectToMongo variable session is still nil. I would understand it if ConnectToMongo accepts not reference type like ConnectToMongo(session mgo.Session) but reference variable type *mgo.Session must be saved after returning function ConnectToMongo

  1. package main
  3. import (
  4. 	"fmt"
  5. 	"gopkg.in/mgo.v2"
  6. )
  8. func ConnectToMongo(session *mgo.Session) {
  9. 	if session == nil {
  10. 		var err error
  11. 		session, err = mgo.Dial("localhost:27028")
  12. 		if err != nil {
  13. 			panic(err)
  14. 		}
  15. 	}
  16. }
  18. func main() {
  19. 	var session *mgo.Session
  20. 	ConnectToMongo(session)
  21. 	if session == nil {
  22. 		fmt.Println("nil. Why?")
  23. 	}
  24. }

It outputs:

nil. Why?

Update

  1. package main
  3. import (
  4. 	"fmt"
  5. 	"gopkg.in/mgo.v2"
  6. )
  8. func ConnectToMongo(session **mgo.Session) {
  9. 	if session == nil {
  10. 		var err error
  11. 		*session, err = mgo.Dial("localhost:27028")
  12. 		if err != nil {
  13. 			panic(err)
  14. 		}
  15. 	}
  16. }
  18. func main() {
  19. 	var session *mgo.Session
  20. 	ConnectToMongo(&session)
  21. 	if session == nil {
  22. 		fmt.Println("nil. Why?")
  23. 	} else {
  24. 		fmt.Println("not nil. Ok.")
  25. 	}
  26. }

The same output:

  1. nil. Why?

答案1

得分: 2

你需要传递一个指向指针的指针来存储指针的值。否则，你只是将指针的值复制给ConnectToMongo函数。

  1. package main
  3. import (
  4.     "fmt"
  5.     "gopkg.in/mgo.v2"
  6. )
  8. func ConnectToMongo(session **mgo.Session) {
  9.     if *session == nil {
  10.         var err error
  11.         *session, err = mgo.Dial("localhost:27028")
  12.         if err != nil {
  13.             panic(err)
  14.         }
  15.     }
  16. }
  18. func main() {
  19.     var session *mgo.Session
  20.     ConnectToMongo(&session)
  21.     if session == nil {
  22.         fmt.Println("nil. Why?")
  23.     }
  24. }
英文:

You need to pass a pointer to pointer to store the value of the pointer. Otherwise your are copying the value of the pointer to the ConnectToMongo function.

  1. package main
  3. import (
  4.     "fmt"
  5.     "gopkg.in/mgo.v2"
  6. )
  8. func ConnectToMongo(session **mgo.Session) {
  9.     if *session == nil {
  10.         var err error
  11.         *session, err = mgo.Dial("localhost:27028")
  12.         if err != nil {
  13.             panic(err)
  14.         }
  15.     }
  16. }
  18. func main() {
  19.     var session *mgo.Session
  20.     ConnectToMongo(&session)
  21.     if session == nil {
  22.         fmt.Println("nil. Why?")
  23.     }
  24. }

huangapple
  • 本文由 发表于 2015年1月16日 10:10:06
  • 转载请务必保留本文链接：https://go.coder-hub.com/27976275.html
  • go
  • mgo
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