How can I change the textual representation displayed for a type in Swift?

Question

How can I modify the textual output that gets displayed in string interpolation?

The Printable protocol looks the most obvious but it's ignored in both String Interpolation and when printing the instance, e.g:

struct Point : Printable
{
    var x = 0
    var y = 0

    var description : String {
        return "((x), (y))"
    }

    func toString() -> String {
        return description
    }
}

Likewise the toString() convention has no effect either:

var p = Point(x: 10, y: 20)

println(p)                   // V11lldb_expr_05Point (has 2 children)
println("(p)")              // V11lldb_expr_05Point (has 2 children)
println(p.description)       // (10, 20)
println("(p.description)")  // (10, 20)

The behavior is different again in PlayGround which uses its own String representation for structs, i.e:

p // {x 10, y 20}

Is there a way I can change how an instance is displayed?

Answer 1

Swift 2 - 4

Summary

Conform to the CustomStringConvertible protocol and add description:

var description: String {
    return "description here"
}

---

Example

You can create some structs:

struct Animal : CustomStringConvertible {
    let type : String

    var description: String {
        return type
    }
}

struct Farm : CustomStringConvertible {
    let name : String
    let animals : [Animal]

    var description: String {
        return "(name) is a (self.dynamicType) with (animals.count) animal(s)."
    }
}

If you initialize them:

let oldMajor = Animal(type: "Pig")
let boxer = Animal(type: "Horse")
let muriel = Animal(type: "Goat")

let orwellsFarm = Farm(name: "Animal Farm", animals: [oldMajor, boxer, muriel])

The custom descriptions will appear in your playground:

See also CustomDebugStringConvertible, which you can use for more verbose output during debugging.

---

Usage Note

You can initialize a String from any type without implementing this protocol. For example:

For this reason, the docs say:

Using CustomStringConvertible as a generic constraint, or accessing a conforming type's description directly, is therefore discouraged.