Cast an instance of a class to a @protocol in Objective-C

Question

I have an object (a UIViewController) which may or may not conform to a protocol I've defined.

I know I can determine if the object conforms to the protocol, then safely call the method:

if([self.myViewController conformsToProtocol:@protocol(MyProtocol)]) {
    [self.myViewController protocolMethod]; // <-- warning here
}

However, XCode shows a warning:

warning 'UIViewController' may not respond to '-protocolMethod'

What's the right way to prevent this warning? I can't seem to cast self.myViewController as a MyProtocol class.

score 182 · Accepted Answer · edited Mar 21 '17 at 21:08

182

The correct way to do this is to do:

if ([self.myViewController conformsToProtocol:@protocol(MyProtocol)])
{
        UIViewController <MyProtocol> *vc = (UIViewController <MyProtocol> *) self.myViewController;
        [vc protocolMethod];
}

The UIViewController <MyProtocol> * type-cast translates to "vc is a UIViewController object that conforms to MyProtocol", whereas using id <MyProtocol> translates to "vc is an object of an unknown class that conforms to MyProtocol".

This way the compiler will give you proper type checking on vc - the compiler will only give you a warning if any method that's not declared on either UIViewController or <MyProtocol> is called. id should only be used in the situation if you don't know the class/type of the object being cast.

edited Mar 21 '17 at 21:08

Alexander Abakumov

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answered Nov 26 '10 at 01:47

Nick Forge

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2

When using protocols you really shouldn't care about the object type -- the whole point of a protocol is that any object type can adopt it and be used without having to cast to the specific object. So, I would recommend using the answer by @andy anywhere you are casting to a protocol instead of the above -- `id p = (id)self.myViewController;` This answer and @andys are both correct, but his is _more_ correct. – memmons Nov 30 '13 at 17:13
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@Answerbot your comment is incorrect, and misses the point I made in the last paragraph of my answer. You might or might not care about the object type, it depends on the situation. What happens if you want to send a message declared on `UIViewController` to `vc` in the example in my answer, and it's declared as `id `? – Nick Forge Dec 01 '13 at 15:41
Not sure what in regards to my comment is incorrect? In any case, if you are checking if an object conforms to a protocol why would you then call some other method unrelated to the protocol? I can't recall ever needing to do this or seeing this in code I've reviewed. Seems like a code smell to me. – memmons Dec 01 '13 at 16:14
Just because you haven't seen/used it, doesn't mean it's a code smell. Here's a code snippet showing one example of where throwing away type information by using `id` is a problem: https://gist.github.com/nsforge/7743616 – Nick Forge Dec 02 '13 at 01:43

Andy · Answer 2 · 2010-04-15T11:09:29.763

65

You can cast it like this:

if([self.myViewController conformsToProtocol:@protocol(MyProtocol)])
{
    id<MyProtocol> p = (id<MyProtocol>)self.myViewController;
    [p protocolMethod];
}

This threw me for a bit, too. In Objective-C, the protocol isn't the type itself, so you need to specify id (or some other type, such as NSObject) along with the protocol that you want.

edited Apr 15 '10 at 11:09

answered Mar 06 '09 at 03:38

Andy

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Ah, cool, thanks. I just checked and saw that casting it as `(id)` works too. Is that bad form? – Ford Mar 06 '09 at 04:03
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If you cast it as id then the compiler will warn you if you use methods that aren't defined in that protocol. – dreamlax Mar 06 '09 at 04:09
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@dreamlax - This is how the compiler does type checking against protocols. See http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/chapter_7_section_11.html#//apple_ref/doc/uid/TP30001163-CH15-TPXREF151 for more info. – Andy Mar 06 '09 at 17:50
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@Ford - it would be better to use the the protocol specifically, since that way the compiler can perform some type checking for you. – Andy Mar 06 '09 at 18:56
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@Andy, I don't think you need the '*' since 'id' is already a pointer. So: id p = (id)self.myViewController; [p protocolMethod]; Or just: [(id)self.myViewController protocolMethod]; – Ford Mar 07 '09 at 01:43
anyone knows why this puts in me in an infinite loop when called on a subclass of `UICollectionView` with `if([self conformsToProtocol:@protocol(MyProtocol)])`? – nambatee Oct 26 '17 at 12:57

Cast an instance of a class to a @protocol in Objective-C

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