How to correctly cast to subclass in Swift?

Question

I have a UITableView with a lot of different cells, based on whats in the content array of the datasource they should show custom content.

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        var cell : UITableViewCell? = nil
        let objectAtIndexPath: AnyObject = contentArray![indexPath.row]

        if let questionText = objectAtIndexPath as? String {
            cell = tableView.dequeueReusableCellWithIdentifier("questionCell", forIndexPath: indexPath) as QuestionTableViewCell
            cell.customLabel.text = "test"
        }

        return cell!
    }

Here I get the error that

UITableViewCell does not have the attribute customLabel

which QuestionTableViewCell does have. Whats wrong with my cast to QuestionTableViewCell?

Answer 1

The problem is not your cast but your declaration of cell. You declared it as an optional UITableViewCell and that declaration remains forever - and is all that the compiler knows.

Thus you must cast at the point of the call to customLabel. Instead of this:

cell.customLabel.text = "test"

You need this:

(cell as QuestionTableViewCell).customLabel.text = "test"

You could make this easier on yourself by declaring a different variable (since you know that in this particular case your cell will be a QuestionTableViewCell), but as long as you are going to have just one variable, cell, you will have to constantly cast it to whatever class you believe it really will be. Personally, I would have written something more like this, exactly to avoid that repeated casting:

    if let questionText = objectAtIndexPath as? String {
        let qtv = tableView.dequeueReusableCellWithIdentifier("questionCell", forIndexPath: indexPath) as QuestionTableViewCell
        qtv.customLabel.text = "test"
        cell = qtv
    }