How to get the indexpath.row when an element is activated?

giorashc almost had it with his answer, but he overlooked the fact that cell's have an extra contentView layer. Thus, we have to go one layer deeper:

guard let cell = sender.superview?.superview as? YourCellClassHere else {
    return // or fatalError() or whatever
}

let indexPath = itemTable.indexPath(for: cell)

This is because within the view hierarchy a tableView has cells as subviews which subsequently have their own 'content views' this is why you must get the superview of this content view to get the cell itself. As a result of this, if your button is contained in a subview rather than directly into the cell's content view, you'll have to go however many layers deeper to access it.

The above is one such approach, but not necessarily the best approach. Whilst it is functional, it assumes details about a UITableViewCell that Apple have never necessarily documented, such as it's view hierarchy. This could be changed in the future, and the above code may well behave unpredictably as a result.

As a result of the above, for longevity and reliability reasons, I recommend adopting another approach. There are many alternatives listed in this thread, and I encourage you to read down, but my personal favourite is as follows:

Hold a property of a closure on your cell class, have the button's action method invoke this.

class MyCell: UITableViewCell {
    var button: UIButton!

    var buttonAction: ((Any) -> Void)?

    @objc func buttonPressed(sender: Any) {
        self.buttonAction?(sender)
    }
}

Then, when you create your cell in cellForRowAtIndexPath, you can assign a value to your closure.

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("Cell") as! MyCell
    cell.buttonAction = { sender in
        // Do whatever you want from your button here.
    }
    // OR
    cell.buttonAction = buttonPressed(closure: buttonAction, indexPath: indexPath) // <- Method on the view controller to handle button presses.
}

By moving your handler code here, you can take advantage of the already present indexPath argument. This is a much safer approach that the one listed above as it doesn't rely on undocumented traits.

My approach to this sort of problem is to use a delegate protocol between the cell and the tableview. This allows you to keep the button handler in the cell subclass, which enables you to assign the touch up action handler to the prototype cell in Interface Builder, while still keeping the button handler logic in the view controller.

It also avoids the potentially fragile approach of navigating the view hierarchy or the use of the tag property, which has issues when cells indexes change (as a result of insertion, deletion or reordering)

CellSubclass.swift

protocol CellSubclassDelegate: class {
    func buttonTapped(cell: CellSubclass)
}

class CellSubclass: UITableViewCell {

@IBOutlet var someButton: UIButton!

weak var delegate: CellSubclassDelegate?

override func prepareForReuse() {
    super.prepareForReuse()
    self.delegate = nil
}

@IBAction func someButtonTapped(sender: UIButton) {
    self.delegate?.buttonTapped(self)
}

ViewController.swift

class MyViewController: UIViewController, CellSubclassDelegate {

    @IBOutlet var tableview: UITableView!

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {

        let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! CellSubclass

        cell.delegate = self

        // Other cell setup

    } 

    //  MARK: CellSubclassDelegate

    func buttonTapped(cell: CellSubclass) {
        guard let indexPath = self.tableView.indexPathForCell(cell) else {
            // Note, this shouldn't happen - how did the user tap on a button that wasn't on screen?
            return
        }

        //  Do whatever you need to do with the indexPath

        print("Button tapped on row \(indexPath.row)")
    }
} 

UPDATE: Getting the indexPath of the cell containing the button (both section and row):

Using Button Position

Inside of your buttonTapped method, you can grab the button's position, convert it to a coordinate in the tableView, then get the indexPath of the row at that coordinate.

func buttonTapped(_ sender:AnyObject) {
    let buttonPosition:CGPoint = sender.convert(CGPoint.zero, to:self.tableView)
    let indexPath = self.tableView.indexPathForRow(at: buttonPosition)
}

NOTE: Sometimes you can run into an edge case when using the function view.convert(CGPointZero, to:self.tableView) results in finding nil for a row at a point, even though there is a tableView cell there. To fix this, try passing a real coordinate that is slightly offset from the origin, such as:

let buttonPosition:CGPoint = sender.convert(CGPoint.init(x: 5.0, y: 5.0), to:self.tableView)

Previous Answer: Using Tag Property (only returns row)

Rather than climbing into the superview trees to grab a pointer to the cell that holds the UIButton, there is a safer, more repeatable technique utilizing the button.tag property mentioned by Antonio above, described in this answer, and shown below:

In cellForRowAtIndexPath: you set the tag property:

button.tag = indexPath.row
button.addTarget(self, action: "buttonClicked:", forControlEvents: UIControlEvents.TouchUpInside)

Then, in the buttonClicked: function, you reference that tag to grab the row of the indexPath where the button is located:

func buttonClicked(sender:UIButton) {
    let buttonRow = sender.tag
}

I prefer this method since I've found that swinging in the superview trees can be a risky way to design an app. Also, for objective-C I've used this technique in the past and have been happy with the result.