How to use global var across files in a package?

Edit: The problem is that you used Short variable declaration := and you just stored the created *DB value in a local variable and not in the global one.

This line:

db, err := NewDB(dbinfo)

Creates 2 local variables: db and err, and this local db has nothing to do with your global db variable. Your global variable will remain nil. You have to assign the created *DB to the global variable. Do not use short variable declaration but simple assignment, e.g:

var err error
db, err = NewDB(dbinfo)
if err != nil {
    log.Fatal(err)
}

Original answer follows.

It's a pointer type, you have to initialize it before you use it. The zero value for pointer types is nil.

You don't have to export it (that's what starting it with a capital letter does). Note that it doesn't matter that you have multiple files as long as they are part of the same package, they can access identifiers defined in one another.

A good solution would be to do it in the package init() function which is called automatically.

Note that sql.Open() may just validate its arguments without creating a connection to the database. To verify that the data source name is valid, call DB.Ping().

For example:

var db *sql.DB

func init() {
    var err error
    db, err = sql.Open("yourdrivername", "somesource")
    if err != nil {
        log.Fatal(err)
    }
    if err = db.Ping(); err != nil {
        log.Fatal(err)
    }
}

icza has already correctly answered your specific problem but it's worth adding some additional explanation on what you're doing wrong so you understand how not to make the mistake in the future. In Go, the syntax := for assignment creates new variables with the names to the left of the :=, possibly shadowing package, or even parent scope function/method variables. As an example:

package main

import "fmt"

var foo string = "global"

func main() {
    fmt.Println(foo) // prints "global"

    // using := creates a new function scope variable 
    // named foo that shadows the package scope foo
    foo := "function scope" 
    fmt.Println(foo) // prints "function scope"
    printGlobalFoo() // prints "global"

    if true {
        foo := "nested scope"
        fmt.Println(foo) // prints "nested scope"
        printGlobalFoo() // prints "global" 
    } 
    // the foo created inside the if goes out of scope when 
    // the code block is exited

    fmt.Println(foo) // prints "function scope"
    printGlobalFoo() // prints "global"

    if true {
        foo = "nested scope" // note just = not :=
    }

    fmt.Println(foo) // prints "nested scope"
    printGlobalFoo() // prints "global"

    setGlobalFoo()
    printGlobalFoo() // prints "new value"
}

func printGlobalFoo() {
    fmt.Println(foo)
}

func setGlobalFoo() {
    foo = "new value" // note just = not :=
}

Note Go has no way to delete or unset a variable, so once you have shadowed a higher scope variables (such as by creating a function scope variable of the same name as a package scope variable), there is no way to access the higher scope variable within that code block.

Also be aware that := is a shorthand for var foo =. Both act in exactly the same way, however := is only valid syntax within a function or method, while the var syntax is valid everywhere.

For who came here and wants a fast answer.

in db.go file:

package db

var db *DB

type DB struct {
    *gorm.DB // or what database you want like *mongo.Client
}

func GetDB() *DB {
    if db == nil{
        db = ConnectToYourDbFunc("connection_string")
    }
    return db
}

then in your other packages you can get it just with this:

db := db.GetDB()

thats all.

How to use global var across files in a package?

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