CODEWITHSUNDEEP
objective-c
mm_857
mm_857

Reputation: 171

Having to cast to id to invoke isKindOfClass

In the following code

id<SwiftProtocol> anotherInstanceAsProtocol = [[SomeObjectiveCClassImplementingOBJCSwiftProtocol alloc] init];
[anotherInstanceAsProtocol isKindOfClass:[SomeObjectiveCClassImplementingOBJCSwiftProtocol class]];

I get the warning "No known instance method for selector 'isKindOfClass:'"

If I modify the last line to

[(id)anotherInstanceAsProtocol isKindOfClass:[SomeObjectiveCClassImplementingOBJCSwiftProtocol class]]

It runs perfectly.

It also works if I assign to NSObject<SwiftProtocol> instead of id<SwiftProtocol>, but I think neither should be necessary.

Why is this cast necessary?

Upvotes: 1

Views: 236

Answers (1)

Sulthan
Sulthan

Reputation: 130162

The problem is that your SwiftProtocol does not inherit from NSObject(Protocol) therefore the Obj-C compiler does not know that there is a method called isKindOfClass:.

Using id basically means you don't want any type checking at compilation time. The real fix should be to make the protocol extend NSObjectProtocol, making sure that all instances conforming to it are normal Obj-C objects.

Note that the history of Objective-C is complicated and not all Objective-C objects have to inherit from NSObject and have isKindOfClass: available.

Upvotes: 3

Related Questions