UIButton target/action function must not be private?

SwiftyButtonViewController.swift

//
//  SwiftyButtonViewController.swift
//  SwiftyButton
//
//  Created by Simone Ferrini on 11/03/16.
//  Copyright © 2016 Simone Ferrini. All rights reserved.
//

import UIKit

class SwiftyButtonViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let button = UIButton(frame: CGRect(x: 50, y: 50, width: 50, height: 50))
        button.backgroundColor = UIColor.redColor()
        button.addTarget(self, action: "buttonPressed:", forControlEvents: UIControlEvents.TouchUpInside)
        
        self.view.addSubview(button)
    }
    
    // As far as I can see, the target/action function must not be private,
    // otherwise will be thrown an exception like: unrecognized selector sent to instance 0x7f98a04174f0
    // I guess because the "button" instance doesn't have access to the function.
    // In Objective-C, visibility is controlled by the headers, but in Swift how we hide this function from the other classes?
    private func buttonPressed(sender: UIButton) {
        print("buttonPressed")
    }
    
}