What happens when you print a Swift object (po) in lldb?

Question

In Objective-C, when you NSLog an object or po it in lldb, the object receives the description message.

In Swift however, the behaviour seems to be different. I implemented both Printable (requires description property) and DebugPrintable (which requires in turn a property named debugDescription). If I try to println() an object or po it, none of those properties gets called.

What's going on? What are those protocols for then??

Answer 1

There's a known issue that Printable is ignored by the Swift REPL (i.e., anything in a Playground or run by xcrun swift at the command line) but recognized by the compiler (compiled apps in the simulator or xcrun swiftc).

For example, here's the code of "foo.swift":

struct Person : Printable {
    let name: String

    var description: String {
        return "\(name)"
    }
}

let me = Person(name: "Nate")
println(me)

If I run it using the REPL, I get this:

$ xcrun swift foo.swift 
foo.Person

But if I compile it first, and then run it, it uses the description computed property:

$ xcrun -sdk macosx swiftc foo.swift ; ./foo
Nate

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The DebugPrintable protocol is useful if you want to be able to use the debugPrint and debugPrintln functions -- in compiled code, they print out an instance's debugDescription property.