Go function parameter passing: Why does pointer variable remain unchanged after being modified inside the function?

Pointer variable trap in Go function parameter passing: Why is it invalid to modify external variables in the function?

This article analyzes common problems about pointer variable assignment in a Go language program. The program tries to modify the pointer variable to the database connection object inside the function, but the variable in the main function has not been modified.

The following code snippet shows this problem:

 var db *sql.DB

func main() {
    initDB(db)
    fmt.Println(db) // Print<nil>
}

func initDB(db *sql.DB) {
    var err error
    db, err = sql.Open("mysql", "root:123456@tcp(127.0.0.1:3306)/data")
    checkErr(err)
    db.SetMaxOpenConns(100)
    db.SetMaxIdleConns(100)
    db.SetConnMaxLifetime(time.Minute * 3)
    if err := db.Ping(); err != nil {
        checkErr(err)
    }
    fmt.Println(db) // What is printed here is the correct value}

func checkErr(err error) {
    if err != nil {
        panic(err)
    }
}</nil>

db printed by main function is still nil , and the initDB function has successfully connected to the database. This is because the function parameter passing mechanism of Go language is value passing. Even if the parameter is a pointer type, the passed copy of the pointer value.

In the initDB function, db, err := sql.Open(...) creates a new *sql.DB object and assigns the object to the local variable db inside initDB function. This does not modify the pointing of the db variable in the main function. After the initDB function ends, its local variable db is destroyed, and db in the main function is still nil .

To correctly modify the db variable in the main function, you need to use a pointer to a pointer, or use a pointer receiver in the initDB function. Here is how to use pointers to solve this problem:

 var db *sql.DB

func main() {
    initDB(&db)
    fmt.Println(db) // The correct value will be printed now}

func initDB(db **sql.DB) {
    var err error
    *db, err = sql.Open("mysql", "root:123456@tcp(127.0.0.1:3306)/data")
    checkErr(err)
    (*db).SetMaxOpenConns(100)
    (*db).SetMaxIdleConns(100)
    (*db).SetConnMaxLifetime(time.Minute * 3)
    if err := (*db).Ping(); err != nil {
        checkErr(err)
    }
    fmt.Println(*db) // What is printed here is the correct value}

func checkErr(err error) {
    if err != nil {
        panic(err)
    }
}

By passing the address of db ( &db ), the initDB function can directly modify the pointing of the db variable in the main function.

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