Difficulties to assign default value to a paramete

问题:

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
  private let DefaultValue = 10

  // Compiler error: Cannot use instance member 'DefaultValue' as a default parameter
  public func doTask(amount: Int = DefaultValue) {
    ...
  }
}

But I get compiler error: Cannot use instance member 'DefaultValue' as a default parameter.

Then, I also tried declare DefaultValue as private static:

class Foo {
      // static variable
      private static let DefaultValue = 10

      // Compiler error: Static let 'DefaultValue' is private and cannot be referenced from a default argument value
      public func doTask(amount: Int = DefaultValue) {
        ...
      }
    }

But I get new compiler error: Static let 'DefaultValue' is private and cannot be referenced from a default argument value

I need to keep DefaultValue private to this class & I would like to assign default value to the parameter of function with a private variable, whether this is achievable in Swift 4?

回答1:

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
        // ...
    }
}

回答2:

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.

回答3:

How about a top-level variable?

fileprivate let DefaultValue = 10

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