Swift double to string

It is not casting, it is creating a string from a value with a format.

let a: Double = 1.5
let b: String = String(format: "%f", a)

print("b: \(b)") // b: 1.500000

With a different format:

let c: String = String(format: "%.1f", a)

print("c: \(c)") // c: 1.5

You can also omit the format property if no formatting is needed.

let double = 1.5 
let string = double.description

update Xcode 7.1 • Swift 2.1:

Now Double is also convertible to String so you can simply use it as you wish:

let double = 1.5
let doubleString = String(double)   // "1.5"

Swift 3 or later we can extend LosslessStringConvertible and make it generic

Xcode 11.3 • Swift 5.1 or later

extension LosslessStringConvertible { 
    var string: String { .init(self) } 
}

let double = 1.5 
let string = double.string  //  "1.5"

For a fixed number of fraction digits we can extend FloatingPoint protocol:

extension FloatingPoint where Self: CVarArg {
    func fixedFraction(digits: Int) -> String {
        .init(format: "%.*f", digits, self)
    }
}

If you need more control over your number format (minimum and maximum fraction digits and rounding mode) you can use NumberFormatter:

extension Formatter {
    static let number = NumberFormatter()
}

extension FloatingPoint {
    func fractionDigits(min: Int = 2, max: Int = 2, roundingMode: NumberFormatter.RoundingMode = .halfEven) -> String {
        Formatter.number.minimumFractionDigits = min
        Formatter.number.maximumFractionDigits = max
        Formatter.number.roundingMode = roundingMode
        Formatter.number.numberStyle = .decimal
        return Formatter.number.string(for: self) ?? ""
    }
}

2.12345.fractionDigits()                                    // "2.12"
2.12345.fractionDigits(min: 3, max: 3, roundingMode: .up)   // "2.124"

In addition to @Zaph's answer, you can create an extension on Double:

extension Double {
    func toString() -> String {
        return String(format: "%.1f",self)
    }
}

Usage:

var a:Double = 1.5
println("output: \(a.toString())")  // output: 1.5