Swift Generic array Equatable casting

Question

I have a class AAA which contains an generic array. Since Item could be not Equatable, so I do not code it as class AAA<Item: Equatable>.

I would like to add an remove function in class AAA which is available when Item is Equatable. By calling this function, it will call a function in the Array Extension.

Since Item is not Equatable by default, the following code is not able to compile.

class AAA<Item>
{
    var items = [Item]()

    func remove(item: Item)
    {
        items.remove(object: item)
    }    
}

extension Array where Element: Equatable
{
    // Remove first collection element that is equal to the given `object`:
    mutating func remove(object: Element)
    {
        if let index = index(of: object)
        {
            remove(at: index)
        }
    }
}

In the next step, I try to convert the items and item into Equatable. However, I cannot find any way to check and cast the items array into a array with Equatable elements.

func remove<ItemEquatable: Item> (item: ItemEquatable) where ItemEquatable: Equatable
{
    items.remove(object: item)
}

Any idea or suggestion? Thanks

Martin R · Accepted Answer · 2017-06-09T09:46:16.507

To define the function only for AAA<Item> if Item is equatable, simply put the definition into a restricted extension of the class:

class AAA<Item> {
    var items = [Item]()
}

extension AAA where Item: Equatable {

    func remove(item: Item) {
        items.remove(object: item)
    }
}

Your approach cannot work because you could still call the remove method on any AAA<Item> instance, without Item being Equatable.

In other words: You need to restrict the generic type Item of the class, not the type of the argument of remove().

Swift Generic array Equatable casting

1 Answers1