How can I print an object?

Question

I want to print an object in the console in Swift.

In Java I would override the public String toString() method.

How can I do that in Swift?

class MyClass {
    var i : Int
    var j : Int
// ...
//  method to return a String {
        return String(i) + ", " + String(j)
    }
}

var mc = MyClass(5, 10)
print(mc) // 5, 10

Your code isn't valid. I added a class implementation so it'll compile. If you want to avoid retain cycles, you may consider using a struct instead of a class and you can get rid of the initializer. — Adrian, Dec 20 '21 at 16:22

score 1 · Answer 1 · answered Dec 20 '21 at 16:17

You need your entity to conform to CustomStringConvertible and implement var description.

class MyClass: CustomStringConvertible {
    var i : Int // Integer doesn't exist in swift
    var j : Int

    var description: String {
        String(i) + ", " + String(j) // return is implicit
    }
}

In general, Swift classes don't inherit from any super class, so they don't have basic methods like toString, equal and so on.

Every implementation needs to conform to a specific protocol (a java interface).

Bjorn B. · Answer 2 · 2021-12-20T16:49:52.747

You need to conform to the CustomStringConvertible protocol. Then you implement the description property.

class MyClass: CustromStringConvertible {
    var i: Int
    var j: Int

    // ...

    var description: String {
        return "\(i), \(j)"
    }
}

var mc = MyClass(5, 10)
print(mc) // 5, 10

From the Apple documentation:

Types that conform to the CustomStringConvertible protocol can provide their own representation to be used when converting an instance to a string. The String(describing:) initializer is the preferred way to convert an instance of any type to a string. If the passed instance conforms to CustomStringConvertible, the String(describing:) initializer and the print(_:) function use the instance’s custom description property.

Accessing a type’s description property directly or using CustomStringConvertible as a generic constraint is discouraged.

For debugging purposes, you might want to conform to CustomDebugStringConvertible and implement debugDescription instead (documentation).

This would look like this (Playground file):

import Foundation

struct Point {
    let x: Int, y: Int
}

extension Point: CustomStringConvertible {
    var description: String {
        return "NON-DEBUG - (\(x), \(y))"
    }
}

extension Point: CustomDebugStringConvertible {
    var debugDescription: String {
        return "DEBUG - (\(x), \(y))"
    }
}

let p = Point(x: 21, y: 30)

print(p)
// NON-DEBUG - (21, 30)

print(String(reflecting: p))
// DEBUG - (21, 30)

Adrian · Answer 3 · 2021-12-20T16:32:35.997

You'll want to use CustomStringConvertible or CustomDebugStringConvertible:

class MyClass {
    var i: Int
    var j: Int
    
    init(i: Int, j: Int) {
        self.i = i
        self.j = j
    }
}

extension MyClass: CustomStringConvertible {
    var description: String {
        "\(i), \(j)"
    }
}

extension MyClass: CustomDebugStringConvertible {
    var debugDescription: String {
        "\(i), \(j)"
    }
}

As a suggestion, if you're just going to be passing this around and don't need to hang onto it, you could change it to a value type like a struct and you'd get an initializer for free.

I'd recommend extending classes to utilize protocols like CustomStringConvertible and CustomDebugStringConvertible to keep your code neat.

How can I print an object?

3 Answers3