Swift: Casting a FloatingPoint conforming value to Double

问题

I'm writing an extension to a FloatingPoint protocol. I want to cast it to a Double in any possible way.

extension FloatingPoint {
    var toDouble: Double {
        return Double(exactly: self) ?? 0 //compile error
    }
}

I'd prefer not to tell what I'm trying to achieve so we can focus just on above problem. I'd prefer to hear that's impossible to do it this way instead of receiving a valid workaround for the bigger problem I'm trying to solve.

I tried to use different Double constructors but maybe I wasn't trying hard enough.

回答1:

The problem there is that you need to extend BinaryFloatingPoint instead of FloatingPoint protocol:

extension BinaryFloatingPoint {
    // Creates a new instance from the given value if possible otherwise returns nil
    var double: Double? { Double(exactly: self) }
    // Creates a new instance from the given value, rounded to the closest possible representation.
    var doubleValue: Double { .init(self) }
}

Playground testing

let cgFloat: CGFloat = .pi                    // 3.141592653589793

let exactlyDouble = cgFloat.double            // 3.141592653589793
let closestDoubleValue = cgFloat.doubleValue  // 3.141592653589793

You can also create generic methods to return any floating point type that conforms to the BinaryFloatingPoint protocol:

extension BinaryFloatingPoint {
    func floatingPoint<F: BinaryFloatingPoint>() -> F? { F(exactly: self) }
    func floatingPointValue<F: BinaryFloatingPoint>() -> F { .init(self) }
}

Playground testing

let float80pi: Float80 = .pi   // 3.1415926535897932385

let closestCGFloatPiValue: CGFloat = float80pi.floatingPointValue()  // 3.141592653589793
let closestDoublePiValue: Double = float80pi.floatingPointValue()    // 3.141592653589793
let closestFloatPiValue: Float = float80pi.floatingPointValue()      // 3.141593

let exactlyCGFloat: CGFloat? = float80pi.floatingPoint()  // nil
let exactlyDouble: Double? = float80pi.floatingPoint()  // nil
let exactlyFloat: Float? = float80pi.floatingPoint()    // nil

来源：https://stackoverflow.com/questions/60773207/swift-casting-a-floatingpoint-conforming-value-to-double

标签

swift

floating-point

double