CODEWITHSUNDEEP
genericsswift
IluTov
IluTov

Reputation: 6852

Cast of generics type to Any?

I have a class called Box with a generics parameter T.

For some reason, it's not valid in Swift to cast a Box<String> (or any other type, for that matter) to Box<Any>.

class Box<T> {}

var box: Box<Any>?
box = Box<String>() // 'String' is not identical to 'Any'

In Java there is a ? that represents any type. (Box<?>)

What can I do here in Swift?

Upvotes: 2

Views: 2240

Answers (2)

Bryan Chen
Bryan Chen

Reputation: 46578

Short answer is: You cannot cast Box<String> to Box<Any> because there is no relationship between them.

This page is for Java, but it also apply here

Given two concrete types A and B (for example, Number and Integer), MyClass<A> has no relationship to MyClass<B>, regardless of whether or not A and B are related. The common parent of MyClass<A> and MyClass<B> is Object.

Unless you explicit define a relationship. e.g. support implicit conversion operator:

class Box<T> {
    func __conversion<U>() -> Box<U> {
        // do whatever required for conversion
        // try reinterpretCast if you can't make type system happy
        // which will give you runtime error if you do it wrong...
        return Box<U>()
    }
}

Upvotes: 7

Zedenem
Zedenem

Reputation: 2559

First, why would you want your box variable to be of type Box<Any> but to contain an instance of Box<String>?

Swift being a type-safe language, you cannot assign to a variable an object that is not of the same type. It's as simple as you cannot do this:

var x: Float
x = 2 as Int

If you want your Box instance to be associated with any object of any class, just declare it like that:

var box = Box<AnyObject>?

And you will have the box instance that I guess you need.

See this link on type safety in Swift: https://developer.apple.com/library/prerelease/mac/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_443

And this one on the use of Any and AnyObject: https://developer.apple.com/library/prerelease/mac/documentation/Swift/Conceptual/Swift_Programming_Language/TypeCasting.html#//apple_ref/doc/uid/TP40014097-CH22-XID_492

Hope this helps,

Upvotes: 1

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