Difficulties to assign default value to a parameter of a function

Question

In a class, I defined a private constant, I try to use the constant as a default value of parameter of a function:

class Foo {
  // instance variable
  priva         


        

Answer 1

A somewhat complicated, but workable, solution, for hiding the default value you want within your class is to use a protocol and a conforming struct whose intimate details are known only by the file declaring Foo:

// enabling custom stuff
public protocol IntLike {
    var intValue: Int { get }
}

// allowing to pass Int's
extension Int: IntLike {
    public var intValue: Int { return self }
}

public class Foo {
    // the placeholder
    public struct FooIntLike: IntLike {
        // what we really want to hide
        fileprivate let realInt = 10

        public init() { }

        public var intValue: Int = Int.max // or 0, or whatever
    }

    public func doTask(amount: IntLike = FooIntLike()) {
        // default value will expand to a non-nil value for `realInt`
        let amount = (amount as? FooIntLike)?.realInt ?? amount.intValue
        // do your stuff with the amount
    }
}

Callers of doTask are able to pass Int's, while not knowing what the default value provides.

Answer 2

I don't think that is possible. The default value is inserted at the calling site, and therefore needs to be public, see also Access control in swift 4.

A possible workaround would be to make the parameter optional, and substitute nil by the default value locally:

class Foo {
    private static let DefaultValue = 10

    public func doTask(amount: Int? = nil) {
        let amount = amount ?? Foo.DefaultValue
        // ...
    }
}

Answer 3

How about a top-level variable?

fileprivate let DefaultValue = 10

class Foo {
    public func doTask(amount: Int = DefaultValue) {
        ...
    }
}