Cocoa Programming for OS X: The Big Nerd Ranch Guide

Making Types Printable

If you use a Vector value in string interpolation, you will not get a very pleasing result:

println("Gravity is \(gravity).")             "Gravity is __lldb_expr_247.Vector."

You can improve this by conforming to the Printable protocol, which looks like this:

protocol Printable {
    var description: String { get }
}

We will cover protocols in more detail in Chapter 6, but the short version is that a protocol defines a set of properties or methods. In order to conform to a protocol, your type must implement the required properties and methods.

To conform to Printable, you must implement a read-only computed property called description to return a String. Start by declaring that Vector conforms to Printable:

struct ...