Swift - Generic not able to append to array of super class

Question

Ok here's the problem:

say we have a parent class that holds an array of ChildClasses

class ParentClass { var list: [ChildClass<UITableViewCell>] = [] func append<T>(cell: T) where T: UITableViewCell { let child = ChildClass<T>() list.append(child) } }

and the child class

class ChildClass<T> where T: UITableViewCell { var obj: T! }

both of the classes are generic and the Type(T) is alway of the type UITableViewCell

now if you try to build it you'll get this error:

Cannot convert value of type ChildClass< T > to expected argument type ChildClass< UITableViewCell >

but if the T is a subclass of UITableViewCell, shouldn't it be able to convert the T???
thanks in advance

Strongly related (dupe?): How do I store a value of type Class<ClassImplementingProtocol> in a Dictionary of type [String:Class<Protocol>] in Swift? — Hamish
– Hamish, Commented Mar 6, 2017 at 7:54
it's really hard to find that question, still if you think it's duplicate, i agree — Mohsen Shakiba
– Mohsen Shakiba, Commented Mar 6, 2017 at 7:58

Community · Accepted Answer · 2017-05-23 11:53:53Z

1

ChildClass<T> is not a subclass of ChildClass<UITableViewCell>, even if T is a subclass of UITableViewCell.

My answer here provides an example of what can go wrong if such a covariance was established: https://stackoverflow.com/a/42615736/3141234

edited May 23, 2017 at 11:53

CommunityBot

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answered Mar 6, 2017 at 7:36

Alexander

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5 Comments

Mohsen Shakiba Over a year ago

Ok, but how can I store items with different type of the same subclass in a list with generics??

Alexander Over a year ago

You have to define child as ChildClass<UITableViewCell>(), and assign cell to its obj. Note of course, that this upcasts the cell to be UITableViewCell, losing type information.

Mohsen Shakiba Over a year ago

so I will lose the type information and I have to add additional cast for it to work. well guess you are right, thanks anyway

Alexander Over a year ago

@MohsenShakiba Yup, there's no way around that. As Sweeper pointed out, ChildClass<UITableViewCell> is a completely unrelated type to ChildClass<SomeSubclassOfUITableViewCell>`

Mohsen Shakiba Over a year ago

I've voted up both, but which one of the answers do you think deserves the mark??

Sweeper · Accepted Answer · 2017-03-06 07:38:01Z

Swift is very strict with generics. ChildClass<UITableViewCell> is not compatible with ChildClass<SomeSubclassOfUITableViewCell>.

One workaround for this is to convert the ChildClass<SomeSubclassOfUITableViewCell> to ChildClass<UITableViewCell>, since logically, they should be compatible. I also noticed that you have not used the cell parameter, so maybe this is how you want your method to be:

func append<T>(cell: T) where T: UITableViewCell { let child = ChildClass<UITableViewCell>() child.obj = cell list.append(child) }

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