What is the Swift equivalent of -[NSObject description]?

Question

In Objective-C, one can add a description method to their class to aid in debugging:

@implementation MyClass
- (NSString *)description
{
    return [NSString stringWithFormat:@"<%@: %p, foo = %@>", [self class], foo _foo];
}
@end

Then in the debugger, you can do:

po fooClass
<MyClass: 0x12938004, foo = "bar">

What is the equivalent in Swift? Swift's REPL output can be helpful:

  1> class MyClass { let foo = 42 }
  2> 
  3> let x = MyClass()
x: MyClass = {
  foo = 42
}

But I'd like to override this behavior for printing to the console:

  4> println("x = \(x)")
x = C11lldb_expr_07MyClass (has 1 child)

Is there a way to clean up this println output? I've seen 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 `OutputStream`.
protocol Printable {
    var description: String { get }
}

I figured this would automatically be "seen" by println but it does not appear to be the case:

  1> class MyClass: Printable {
  2.     let foo = 42
  3.     var description: String { get { return "MyClass, foo = \(foo)" } }
  4. }   
  5> 
  6> let x = MyClass()
x: MyClass = {
  foo = 42
}
  7> println("x = \(x)")
x = C11lldb_expr_07MyClass (has 1 child)

And instead I have to explicitly call description:

 8> println("x = \(x.description)")
x = MyClass, foo = 42

Is there a better way?

drewag · Accepted Answer · 2018-03-14T18:08:03.130

142

To implement this on a Swift type you must implement the CustomStringConvertible protocol and then also implement a string property called description.

For example:

class MyClass: CustomStringConvertible {
    let foo = 42

    var description: String {
        return "<\(type(of: self)): foo = \(foo)>"
    }
}

print(MyClass()) // prints: <MyClass: foo = 42>

Note: type(of: self) gets the type of the current instances instead of explicitly writing ‘MyClass’.

edited Mar 14 '18 at 18:08

answered Jun 08 '14 at 17:38

drewag

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3

Great find! I'm going to file a radar -- println output of "swift -i sample.swift" and "swift sample.swift && sample" differ. – Jason Jun 08 '14 at 17:54
Thanks for the info on that. I was trying out Printable in a playground and indeed it doesn't work right now. Good it hear it works in a app. – Tod Cunningham Jun 23 '14 at 16:05
Printable does work in the playground, but iff the class descends from NSObject – dar512 Feb 24 '15 at 17:21
6

In Swift 2.0 it has changed to CustomStringConvertible and CustomDebugStringConvertible – Mike Vosseller Sep 12 '15 at 04:18
Also, there is no issue using CustomStringConvertible and CustomDebugStringConvertible in Playground with Xcode 7.2 – Nicholas Credli Jan 02 '16 at 14:35

King-Wizard · Answer 2 · 2015-09-19T07:12:01.977

Example of using CustomStringConvertible and CustomDebugStringConvertible protocols in Swift:

PageContentViewController.swift

import UIKit

class PageContentViewController: UIViewController {

    var pageIndex : Int = 0

    override var description : String { 
        return "**** PageContentViewController\npageIndex equals \(pageIndex) ****\n" 
    }

    override var debugDescription : String { 
        return "---- PageContentViewController\npageIndex equals \(pageIndex) ----\n" 
    }

            ...
}

ViewController.swift

import UIKit

class ViewController: UIViewController
{

    /*
        Called after the controller's view is loaded into memory.
    */
    override func viewDidLoad() {
        super.viewDidLoad()

        let myPageContentViewController = self.storyboard!.instantiateViewControllerWithIdentifier("A") as! PageContentViewController
        print(myPageContentViewController)       
        print(myPageContentViewController.description)
        print(myPageContentViewController.debugDescription)
    }

          ...
}

Which print out:

**** PageContentViewController
pageIndex equals 0 ****

**** PageContentViewController
pageIndex equals 0 ****

---- PageContentViewController
pageIndex equals 0 ----

Note: if you have a custom class which does not inherit from any class included in UIKit or Foundation libraries, then make it inherit of NSObject class or make it conform to CustomStringConvertible and CustomDebugStringConvertible protocols.

the function must declared as public – Karsten Mar 16 '18 at 08:50 — Karsten, Mar 16 '18 at 08:50

Peter Ahlberg · Answer 3 · 2017-07-23T14:13:58.793

Just use CustomStringConvertible and var description: String { return "Some string" }

works in Xcode 7.0 beta

class MyClass: CustomStringConvertible {
  var string: String?


  var description: String {
     //return "MyClass \(string)"
     return "\(self.dynamicType)"
  }
}

var myClass = MyClass()  // this line outputs MyClass nil

// and of course 
print("\(myClass)")

// Use this newer versions of Xcode
var description: String {
    //return "MyClass \(string)"
    return "\(type(of: self))"
}

score 21 · Answer 4 · answered Jan 11 '16 at 11:05

The answers relating to CustomStringConvertible are the way to go. Personally, to keep the class (or struct) definition as clean as possible, I would also separate out the description code into a separate extension:

class foo {
    // Just the basic foo class stuff.
    var bar = "Humbug!"
}

extension foo: CustomStringConvertible {
    var description: String {
        return bar
    }
}

let xmas = foo()
print(xmas)  // Prints "Humbug!"

score 10 · Answer 5 · answered Mar 15 '18 at 16:51

As described here, you can also use Swift's reflection capabilities to make your classes generate their own description by using this extension:

extension CustomStringConvertible {
    var description : String {
        var description: String = "\(type(of: self)){ "
        let selfMirror = Mirror(reflecting: self)
        for child in selfMirror.children {
            if let propertyName = child.label {
                description += "\(propertyName): \(child.value), "
            }
        }
        description = String(description.dropLast(2))
        description += " }"
        return description
    }
}

Peter Ahlberg · Answer 6 · 2017-07-23T14:16:22.057

class SomeBaseClass: CustomStringConvertible {

    //private var string: String = "SomeBaseClass"

    var description: String {
        return "\(self.dynamicType)"
    }

    // Use this in newer versions of Xcode
    var description: String {
        return "\(type(of: self))"
    }

}

class SomeSubClass: SomeBaseClass {
    // If needed one can override description here

}


var mySomeBaseClass = SomeBaseClass()
// Outputs SomeBaseClass
var mySomeSubClass = SomeSubClass()
// Outputs SomeSubClass
var myOtherBaseClass = SomeSubClass()
// Outputs SomeSubClass

score 5 · Answer 7 · answered Dec 30 '15 at 00:52

struct WorldPeace: CustomStringConvertible {
    let yearStart: Int
    let yearStop: Int

    var description: String {
        return "\(yearStart)-\(yearStop)"
    }
}

let wp = WorldPeace(yearStart: 2020, yearStop: 2040)
print("world peace: \(wp)")

// outputs:
// world peace: 2020-2040

What is the Swift equivalent of -[NSObject description]?

7 Answers7

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