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
package main
import (
"fmt"
"gopkg.in/mgo.v2"
)
func ConnectToMongo(session **mgo.Session) {
if session == nil {
var err error
*session, err = mgo.Dial("localhost:27028")
if err != nil {
panic(err)
}
}
}
func main() {
var session *mgo.Session
ConnectToMongo(&session)
if session == nil {
fmt.Println("nil. Why?")
} else {
fmt.Println("not nil. Ok.")
}
}
相同的输出：
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
package main
import (
"fmt"
"gopkg.in/mgo.v2"
)
func ConnectToMongo(session *mgo.Session) {
if session == nil {
var err error
session, err = mgo.Dial("localhost:27028")
if err != nil {
panic(err)
}
}
}
func main() {
var session *mgo.Session
ConnectToMongo(session)
if session == nil {
fmt.Println("nil. Why?")
}
}
It outputs:
nil. Why?
Update
package main
import (
"fmt"
"gopkg.in/mgo.v2"
)
func ConnectToMongo(session **mgo.Session) {
if session == nil {
var err error
*session, err = mgo.Dial("localhost:27028")
if err != nil {
panic(err)
}
}
}
func main() {
var session *mgo.Session
ConnectToMongo(&session)
if session == nil {
fmt.Println("nil. Why?")
} else {
fmt.Println("not nil. Ok.")
}
}
The same output:
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。
package main
import (
"fmt"
"gopkg.in/mgo.v2"
)
func ConnectToMongo(session **mgo.Session) {
if *session == nil {
var err error
*session, err = mgo.Dial("localhost:27028")
if err != nil {
panic(err)
}
}
}
func main() {
var session *mgo.Session
ConnectToMongo(&session)
if session == nil {
fmt.Println("nil. Why?")
}
}
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.
package main
import (
"fmt"
"gopkg.in/mgo.v2"
)
func ConnectToMongo(session **mgo.Session) {
if *session == nil {
var err error
*session, err = mgo.Dial("localhost:27028")
if err != nil {
panic(err)
}
}
}
func main() {
var session *mgo.Session
ConnectToMongo(&session)
if session == nil {
fmt.Println("nil. Why?")
}
}
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