Generic type constraint for numerical type only

Question

I'm trying to figure out how to implement a type constraint for a generic class (in Swift) that will limit the generic types to numeric types only. For instance Double, Int, etc., but not string. Thanks for any help.

Answer 1

You can specify type constraints (using both classes and protocols) for a generic class (same syntax applies to functions) using angle brackets:

class Foo<T: Equatable, U: Comparable> { } 

To specify more than one requirement on a single type, use a where clause:

class Foo<T: UIViewController where T: UITableViewDataSource, T: UITextFieldDelegate> { } 

However, it doesn't look like you can specify optional requirements in a generic parameter clause, so one possible solution is to create a protocol that all the numeric types implement via extensions and then constrain your class on that requirement:

protocol Numeric { }  extension Float: Numeric {} extension Double: Numeric {} extension Int: Numeric {}   class NumberCruncher<C1: Numeric> {     func echo(num: C1)-> C1 {         return num     } }  NumberCruncher<Int>().echo(42) NumberCruncher<Float>().echo(3.14) 

Answer 2

Strideable is the smallest standard protocol that every standard number type conforms to, but it also has a few more types conforming to it. http://swiftdoc.org/protocol/Strideable/hierarchy/

Or you can use IntegerType and FloatingPointType.