swift protocol 'weak' cannot be applied to non-class type

Swift >= 4:

protocol A : AnyObject { ... {

Swift < 4:

protocol A : class { ... }

defines a "class-only protocol": Only class types (and not structures or enumerations) can adopt this protocol.

Weak references are only defined for reference types. Classesare reference types, structures and enumerations are value types.(Closures are reference types as well, but closures cannot adopta protocol, so they are irrelevant in this context.)

Therefore, if the object conforming to the protocol needs to be stored in a weak property then the protocol must be a class-only protocol.

Here is another example which requires a class-only protocol:

protocol A {     var name : String { get set }}func foo(a : A) {    a.name = "bar" // error: cannot assign to property: 'a' is a 'let' constant}

This does not compile because for instances of structures and enumerations, a.name = "bar" is a mutation of a. If you definethe protocol as

protocol A : class {     var name : String { get set }}

then the compiler knows that a is an instance of a class type to thata is a reference to the object storage,and a.name = "bar" modifies the referenced object, but not a.

So generally, you would define a class-only protocol if you needthe types adopting the protocol to be reference types and not value types.

If you are using Swift 4 and later

protocol A : AnyObject { ... }

You can make the protocol derive from any class type like NSObject or AnyObject:

protocol TopNewsTableDelegate : AnyObject {    func topNewsTableDidLoadedStories()}