Difference between Printable and DebugPrintable in Swift

Question

Looking for a Swift equivalent of Cocoa's description, I found the following protocols in Swift: Printable and DebugPrintable.

What's the difference between these two protocols and when should I use each one?

Answer 1

Here is an example class

class Foo: Printable, DebugPrintable {
    var description: String {
        return "Foo"
    }
    var debugDescription: String {
        return "debug Foo"
    }
}

This is how to use it.

println(Foo())
debugPrintln(Foo())

Here is the output with no surprises:

Foo
debug Foo

I didn't try this in a Playground. It works in an actual project.

The answer above was for Swift 1. It was correct at the time.

Update for Swift 2.

println and debugPrintln are gone and the protocols have been renamed.

class Foo: CustomStringConvertible, CustomDebugStringConvertible {
    var description: String {
        return "Foo"
    }
    var debugDescription: String {
        return "debug Foo"
    }
}

print(Foo())
debugPrint(Foo())

Answer 2

In Xcode 6 Beta (Version 6.2 (6C101)) I find that both println and debugPrintln use description if-and-only-if the class descends from NSObject. I don't see that either uses debugDescription at all but when run in a Playground debugPrintln outputs only to the Console and doesn't appear in the playground itself.

import Foundation

class Tdesc: NSObject, Printable, DebugPrintable {
    override var description: String {return "A description"}
    override var debugDescription: String {return "A debugDescription"}
}

class Xdesc: Printable, DebugPrintable {
    var description: String {return "A description"}
    var debugDescription: String {return "A debugDescription"}
}

let t = Tdesc()
let x = Xdesc()

t.description

let z: String = "x\(t)"

println(t)      // Displays "A description" in the Playground and Console

debugPrintln(t) // Displays nothing in the Playground but "A description" in the Console

x.description

let y: String = "x\(x)"

println(x)      // Displays "__lldb_expr_405.Xdesc" in the Playground and Console

debugPrintln(x)

Answer 3

I believe the main difference is the property that's used to print. Printable has description and DebugPrintable has debugDescription:

https://stackoverflow.com/a/24254220/887210

https://developer.apple.com/library/prerelease/ios/documentation/General/Reference/SwiftStandardLibraryReference/Printable.html#//apple_ref/doc/uid/TP40014608-CH11-SW1

Edit:

Apparently print() and println() don't work properly with Printable and DebugPrintable:

struct TestPrintable : Printable {
  var description: String { return "Testing Printable" }
}

struct TestDebugPrintable : DebugPrintable {
  var debugDescription: String { return "Testing DebugPrintable" }
}

println(TestPrintable())      // -> "__lldb_expr_42.TestPrintable"
println(TestDebugPrintable()) // -> "__lldb_expr_42.TestDebugPrintable"

More information about this:

http://vperi.com/2014/06/04/textual-representation-for-classes-in-swift/