How can I convert data into types like Doubles, Ints and Strings in Swift?

Xcode 11 • Swift 5.1 or later

To convert from String or any Numeric type to Data:

extension StringProtocol {
    var data: Data { .init(utf8) }
}

---

extension Numeric {
    var data: Data {
        var source = self
        // This will return 1 byte for 8-bit, 2 bytes for 16-bit, 4 bytes for 32-bit and 8 bytes for 64-bit binary integers. For floating point types it will return 4 bytes for single-precision, 8 bytes for double-precision and 16 bytes for extended precision.
        return .init(bytes: &source, count: MemoryLayout<Self>.size)
    }
}

---

To convert from Data (bytes) back to String

extension DataProtocol {
    var string: String? { String(bytes: self, encoding: .utf8) }
}

---

To convert from Data back to generic Numeric value

extension Numeric {
    init<D: DataProtocol>(_ data: D) {
        var value: Self = .zero
        let size = withUnsafeMutableBytes(of: &value, { data.copyBytes(to: $0)} )
        assert(size == MemoryLayout.size(ofValue: value))
        self = value
    }
}

---

extension DataProtocol {
    func value<N: Numeric>() -> N { .init(self) }
}

---

let value = 12.34                      // implicit Double 12.34
let data = value.data                  // double data - 8 bytes
let double = Double(data)              // implicit Double 12.34
let double1: Double = .init(data)      // explicit Double 12.34
let double2: Double = data.value()     // explicit Double 12.34
let double3 = data.value() as Double   // casting to Double 12.34

---

Now we can easily add a property for each Numeric type:

extension DataProtocol {
    var integer: Int { value() }
    var int32: Int32 { value() }
    var float: Float { value() }
    var cgFloat: CGFloat { value() }
    var float80: Float80 { value() }
    var double: Double { value() }
    var decimal: Decimal { value() }
}

---

Playground testing

let intData = 1_234_567_890_123_456_789.data    // 8 bytes (64 bit Integer)
let dataToInt: Int = intData.integer                 // 1234567890123456789

let intMinData = Int.min.data                   // 8 bytes (64 bit Integer)
let backToIntMin = intMinData.integer           // -9223372036854775808

let intMaxData = Int.max.data                   // 8 bytes (64 bit Integer)
let backToIntMax = intMaxData.integer           // 9223372036854775807

---

let myInt32Data = Int32(1_234_567_890).data     // 4 bytes (32 bit Integer)
let backToInt32 = myInt32Data.int32             // 1234567890

let int32MinData = Int32.min.data               // 4 bytes (32 bit Integer)
let backToInt32Min = int32MinData.int32         // -2147483648

let int32MaxData = Int32.max.data               // 4 bytes (32 bit Integer)
let backToInt32Max = int32MaxData.int32         // 2147483647

---

let myFloatData = Float.pi.data                 // 4 bytes (32 bit single=precison FloatingPoint)
let backToFloat = myFloatData.float             // 3.141593
backToFloat == .pi      // true

let myCGFloatData = CGFloat.pi.data                 // 4 bytes (32 bit single=precison FloatingPoint)
let backToCGFloat = myCGFloatData.cgFloat             // 3.141593
backToCGFloat == .pi      // true

let myDoubleData = Double.pi.data               // 8 bytes (64 bit double-precision FloatingPoint)
let backToDouble = myDoubleData.double          // 3.141592653589793
backToDouble == .pi     // true

let myFloat80Data = Float80.pi.data             // 16 bytes (128 bit extended-precision FloatingPoint)
let backToFloat80 = myFloat80Data.float80       // 3.141592653589793116
backToFloat80 == .pi    // true

let decimalData = Decimal.pi.data             // 20 bytes Decimal type
let backToDecimal = decimalData.decimal       // 3.14159265358979323846264338327950288419
backToDecimal == .pi    // true

---

let stringBytes = "Hello World !!!".data.prefix(4)  // 4 bytes
let backToString = stringBytes.string               //  "Hell"