Search code examples
swiftswift3option-typeanyobjectswift-dictionary

Optional AnyObject values in Swift dictionary

When I add a value of type 'AnyObject?' to a dictionary of Type '[String : AnyObject]' in the following way, the value can not by added, which is actually what I've expected. Assigning an optional type to a non-optional type should fail in my opinion.

var myDict : [String : AnyObject] = [ "Key1" : "Value1" as AnyObject? ]

But why does this procedure work if I first initialize an empty dictionary and then add the value to it?

var myDict = [String : AnyObject]()
myDict["Key1"] = "Value1" as AnyObject?

I've seen this approach in the GenericKeychain example from Apple https://developer.apple.com/library/content/samplecode/GenericKeychain/Introduction/Intro.html

Solution

  • Your second line works because the subscript override for Dictionary uses an Optional:

    subscript(key: Key) -> Value? { get set }

    Whereas the initializer for a Dictionary of type [String : AnyObject] requires concrete types:

    Dictionary init