What protocol should be adopted by a Type for a generic function to take any number type as an argument in Swift?

Question

I want to make a function accept any number (Int, Float, Double, ...) in Swift

func myFunction <T : "What to put here"> (number : T) ->  {     //... } 

without using NSNumber

Answer 1

Update: The answer below still applies in principle, but Swift 4 completed a redesign of the numeric protocols, such that adding your own is often unnecessary. Take a look at the standard library's numeric protocols before you build your own system.

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This actually isn't possible out of the box in Swift. To do this you'll need to create a new protocol, declared with whatever methods and operators you're going to use inside your generic function. This process will work for you, but the exact details will depend a little on what your generic function does. Here's how you'd do it for a function that gets a number n and returns (n - 1)^2.

First, define your protocol, with the operators and an initializer that takes an Int (that's so we can subtract one).

protocol NumericType {     func +(lhs: Self, rhs: Self) -> Self     func -(lhs: Self, rhs: Self) -> Self     func *(lhs: Self, rhs: Self) -> Self     func /(lhs: Self, rhs: Self) -> Self     func %(lhs: Self, rhs: Self) -> Self     init(_ v: Int) } 

All of the numeric types already implement these, but at this point the compiler doesn't know that they conform to the new NumericType protocol. You have to make this explicit -- Apple calls this "declaring protocol adoption with an extension." We'll do this for Double, Float, and all the integer types:

extension Double : NumericType { } extension Float  : NumericType { } extension Int    : NumericType { } extension Int8   : NumericType { } extension Int16  : NumericType { } extension Int32  : NumericType { } extension Int64  : NumericType { } extension UInt   : NumericType { } extension UInt8  : NumericType { } extension UInt16 : NumericType { } extension UInt32 : NumericType { } extension UInt64 : NumericType { } 

Now we can write our actual function, using the NumericType protocol as a generic constraint.

func minusOneSquared<T : NumericType> (number : T) -> T {     let minusOne = number - T(1)     return minusOne * minusOne }  minusOneSquared(5)              // 16 minusOneSquared(2.3)            // 1.69 minusOneSquared(2 as UInt64)    // 1