Go的sql.NullString的值是否可以预测为空？

Question

After using db.Query and rows.Scan to populate a sql.NullString with a NULL value such that its .Valid field is false, is it guaranteed that its .String will hold the empty string ""?

I ask because it would be snazzy if I could use this:

res := ns.String

Instead of this:

var res string
if ns.Valid {
    res = ns.String
}

https://golang.org/pkg/database/sql/#NullString

Answer 1

In the current implementation yes, but the docs explicitly say to check Valid before using String. Ignore the docs at your own risk.

If you want to be safe, simply copy the type into your own package; then you know it's not going to change with the next Go release.

If you don't do that, I strongly suggest to write a test:

func TestEmptyNullString(t *testing.T) {
    var x sql.NullString
    x.String = "foo"
    x.Scan(nil)
    if x.String != "" {
        t.Error("string not empty after Scan")
    }
}