You don't actually need to write any code for this. There are many ways to do this:
Select the subview and drag over the view entry in the Objects Explorer of storyboard. Once you leave the mouse hold, the parent view will have a triangle indicating your subview has become its child and the subview will have a bigger indent than your view
Or you can use the "Embed in View" menu as shown in the below pic [source:http://codesheriff.blogspot.co.il/2014/03/8-tips-for-working-effectively-with.html]