Swift protocol for string interpolation

You need to implement the Printable protocol:

This protocol should be adopted by types that wish to customize their textual representation. This textual representation is used when objects are written to an OutputStreamType.

protocol Printable {
    var description: String { get }
}

There's also the DebugPrintable protocol when it's only for debugging purposes:

This protocol should be adopted by types that wish to customize their textual representation used for debugging purposes. This textual representation is used when objects are written to an OutputStreamType.

protocol DebugPrintable {
    var debugDescription: String { get }
}

Documentation (Thanks @MartinR)

Note: As @Antonio and @MartinR mentioned in the comments, this doesn't work in the playground (as of Xcode6 GM anyway); that's a known bug. It does work in compiled apps.

From the Xcode6 GM Release Notes:

In Playgrounds, println() ignores the Printable conformance of user-defined types. (16562388)

As of Swift 2.0 Printable has now become CustomStringConvertible. Everything stays the same as before, you still need to implement

 var description: String { get }

But now its called CustomStringConvertible. And debug is CustomDebugStringConvertible

In Swift 5 Apple introduced Custom String Interpolation.

Suppose you have person struct with two properties name and age.

struct Person {
  var name: String
  var age: Int
} 

If you wanted to add a special string interpolation for that so that we can print persons in descriptive way, we can add an extension to String.StringInterpolation with a new appendInterpolation() method.

 extension String.StringInterpolation {
       mutating func appendInterpolation(_ person: Person) {
       appendInterpolation("My name is \(person.name) and I'm \(person.age) years old.")
    }
}

Now If we print the person details like:

 let person = Person(name: "Yogendra", age: 28)
 print("Person Details: \(person)")

Output will be:

Person Details: My name is Yogendra and I'm 28 years old.