swift - Is it possible to specify the object type allowed in a Dictionary?

Question

In Swift 3.x is it possible to specify the object type allowed in a Dictionary?

It would be great if at declaration time we could pass a set of object types allowed.

Answer 1

You can achieve this via a custom protocol that is implemented only by the types you want to allow within the dictionary:

protocol AllowedDictionaryValue: {}

extension String: AllowedDictionaryValue {}
extension Int: AllowedDictionaryValue {}
extension MyClass: AllowedDictionaryValue {}
extension MyEnum: AllowedDictionaryValue {}

let dictionary: [String:AllowedDictionaryValue] = ...

The above dictionary will hold only String's, Int's, MyClass instances, and MyEnum values.

This way you can have only the values you want in the dictionary, while keeping the dictionary heterogenous.