How to access the content of a custom cell in swift using button tag?

@IBAction func buttonTap(sender: UIButton) {
        let button = sender as UIButton
        let indexPath = self.tableView.indexPathForRowAtPoint(sender.center)!
}

OPTION 1. Handling it with delegation

The right way of handling events fired from your cell's subviews is to use delegation.

So you can follow the steps:

1. Above your class definition write a protocol with a single instance method inside your custom cell:

protocol CustomCellDelegate {
    func cellButtonTapped(cell: CustomCell)
} 

2. Inside your class definition declare a delegate variable and call the protocol method on the delegate:

var delegate: CustomCellDelegate?

@IBAction func buttonTapped(sender: AnyObject) {
    delegate?.cellButtonTapped(self)
}

3. Conform to the CustomCellDelegate in the class where your table view is:

 class ViewController: CustomCellDelegate

4. Set your cell's delegate

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as CustomCell
    cell.delegate = self

    return cell
}

5. Implement the required method in your view controller class.

EDIT: First define an empty array and then modify it like this:

private var selectedItems = [String]()

func cellButtonTapped(cell: CustomCell) {
    let indexPath = self.tableView.indexPathForRowAtPoint(cell.center)!
    let selectedItem = items[indexPath.row]

    if let selectedItemIndex = find(selectedItems, selectedItem) {
        selectedItems.removeAtIndex(selectedItemIndex)
    } else {
        selectedItems.append(selectedItem)
    }
}

where items is an array defined in my view controller:

private let items = ["Dog", "Cat", "Elephant", "Fox", "Ant", "Dolphin", "Donkey", "Horse", "Frog", "Cow", "Goose", "Turtle", "Sheep"] 

OPTION 2. Handling it using closures

I've decided to come back and show you another way of handling these type of situations. Using a closure in this case will result in less code and you'll achieve your goal.

1. Declare a closure variable inside your cell class:

var tapped: ((CustomCell) -> Void)?

2. Invoke the closure inside your button handler.

@IBAction func buttonTapped(sender: AnyObject) {
    tapped?(self)
}

3. In tableView(_:cellForRowAtIndexPath:) in the containing view controller class :

let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! CustomCell       
cell.tapped = { [unowned self] (selectedCell) -> Void in
    let path = tableView.indexPathForRowAtPoint(selectedCell.center)!
    let selectedItem = self.items[path.row]

    println("the selected item is \(selectedItem)")
}

Swift 3 I just get solution for access cell in @IBAction function using superview of button tag.

let cell = sender.superview?.superview as! ProductCell
var intQty = Int(cell.txtQty.text!);
intQty = intQty! + 1
let  strQty = String(describing: intQty!);
cell.txtQty.text = strQty

Since you have 1 section in the table view you can get the cell object as below.

let indexPath = NSIndexPath(forRow: tag, inSection: 0)
let cell = tableView.cellForRowAtIndexPath(indexPath) as! CustomCell!

where tag you will get from button tag.