With the example of GRDB, a lightweight sqlite wrapper:

class DbManager {
    static let sharedInstance = DbManager()

    var dbPath: String!  <--- !!!
    var dbQueue: DatabaseQueue!

    private init() {
        do {
            dbQueue = try connectToDatabase()
            try createTable()
        } catch {
            print(error)
        }
    }
}

Then use it like this:

try DbManager.sharedInstance.dbQueue.write {...}

The catch is to use the exclamation mark while creating class variables. Otherwise you’ll get the error:

Variable 'self.dbQueue' used before being initialized

The use of ? is widely used in various languages to describe an optional value, as in:

obj?.attr

Indicating that we’ll only try to access attr on obj if it exists. The use of ! however is a fun one. It declares that we’re 100% certain this variable is not nil. If it is nil we’ll get into runtime errors, so make sure it isn’t.