Cannot convert nil (untyped nil value) to struct

user_10583765 0
Viewed 14858

A Go noob's learning to use sqlc here. I'm trying to create a method that creates a user.

How do I return nil for a struct?

type user struct{}

func (u *user) Create(ctx context.Context, db *sql.DB) (pg.User, error) {
    q := pg.New(db)
    user, err := q.CreateUser(ctx, "Someone")
    if err != nil {
        return nil, err
        //      ^
        // [compiler] [E] cannot convert nil (untyped nil value) to pg.User
    }
    return user, nil
}

This is the pg.User struct:

type User struct {
    ID int64 `json:"id"`
    // can not be empty
    Name      string    `json:"name"`
    CreatedAt time.Time `json:"created_at"`
    UpdatedAt time.Time `json:"updated_at"`
}

Currently, I'm returning pointer instead, which gives no error:

func (u *user) Create(ctx context.Context, db *sql.DB) (*pg.User, error) {
    q := pg.New(db)
    var usr *pg.User
    user, err := q.CreateUser(ctx, "Someone")
    if err != nil {
        return nil, err
    }

    fmt.Println(">> user:", user)
    // >> user: {1 Someone 2020-09-14 18:36:05.94079 +0000 UTC 2020-09-14 18:36:05.94079 +0000 UTC}

    usr = &user
    fmt.Println(">> usr:", usr)
    // >> usr: &{1 Someone 2020-09-14 18:36:05.94079 +0000 UTC 2020-09-14 18:36:05.94079 +0000 UTC}

    return usr, nil
}

One more question though, which one is better, returning pointer or not?

go
1 Answers

Using a pointer return type is perfectly fine.

If you would rather return a non-pointer struct, then simply return a zero value of the struct:

return pg.User{}, err

or change your function signature to include return named variables (note: the use of = not := as the variables are already considered declared):

func (u *user) Create(ctx context.Context, db *sql.DB) (u pg.User, err error) {
    q := pg.New(db)
    u, err = q.CreateUser(ctx, "Someone")
    
    return // will implicitly return u & err for you
}

and since the if err logic has been removed, the above can be reduced to a one-liner:

func (u *user) Create(ctx context.Context, db *sql.DB) (pg.User, error) {
    return pg.New(db).CreateUser(ctx, "Someone")
}
user_12185120
Related