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POHow to cast Class object to conformance witih protocol
 

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    <p>I'm trying to cast a Class object to a certain protocol, which defines class methods (+) that that class implements.</p> <p>I know how to do this with (id&lt; protocol>), as outlined <a href="https://stackoverflow.com/questions/617616/cast-an-instance-of-a-class-to-a-protocol-in-objective-c">in this question</a>, but I can't seem to figure out the right way for Class objects.</p> <p>The basic scenario is as follows.</p> <p>I have a protocol:</p> <pre><code>@protocol Protocol &lt;NSObject&gt; + (id)classMethod:(id)arg; @end </code></pre> <p>I then have a function which accepts a Class object, which it knows sometimes conforms to the protocol based on another argument (this is obviously very simplified):</p> <pre><code>- (id)someMethodWithClass:(Class)cls andUseArg:(BOOL)arg { id instance; if (arg != nil) { instance = [(Class&lt;Protocol&gt;)cls classMethod:arg]; } } </code></pre> <p>Now I don't get any warnings on this, and it looks right to me. (I'm not going to see any errors in any case, because I can guarantee that if arg != nil then the class conforms.)</p> <p>However, I'm not getting autocompletion in Xcode, which makes me wonder if this is the right way to do it. Any thoughts? (Note that I am not interested in instance being id&lt; Protocol>.)</p>
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    1. CORather than casting it to a protocol, you should change the class, itself, to just conform to that protocol (e.g. `@interface MyClass <MyProtocol>` in the .h, or `@interface MyClass () <MyProtocol>` in the .m. Then the compiler will allow autocomplete with that protocol's methods and no casting is needed. It's generally better to fix the object or class definition than it is to cast it. Also, by defining the class to conform to a particular protocol, the compiler will also warn you if you failed to implement some required methods of the protocol.
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    2. COThanks, but maybe I wasn't clear enough. cls is a class object (an object of type Class), not a class I've defined (in other words, it's passed in as [SomeClass class]). In this simple example, if arg != nil, then I know that cls is a class which implements Protocol. I just want Xcode to realise that.
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    3. COI'm very surprised you don't get compiler warnings here because, first that you're comparing a BOOL to a pointer (passing an `arg` of `nil` is the same as passing `FALSE`), and second that you're not returning anything from this method. Perhaps you're using an older version of the compiler which doesn't generate the warning, or perhaps you've turned off some warnings. Your construct makes sense if `arg` was `id` or a pointer, but not if `BOOL`. Also, you're not returning `instance`. When I compile your code, I get warnings for both of those.
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