"Cannot find interface declaration for <class>"

Question

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I'm having quite a hard time setting up a category on a class I made. From what I've read, Objective-C allows you to create a category on any class, not just closed-source ones. (It honestly wouldn't make sense any other way.)

Of course I can add the category messages to the actual class file, but I want to keep them separate (as the category is an uncommonly special use of a class that can be used very generally). I want to share the class, but keep the category private... anyway.

I've stripped down the category to just show the issue at hand. I (currently) get four errors on the first category message. The number of errors I receive on that line is directly proportional to how many times it is references, but it is not an even rise. Does anyone know what could be causing this?

Answer 1

Your Resources.h file, which is imported by ByteCollection.h, imports ByteCollection+words.h. So when ByteCollection+words.h imports ByteCollection.h, this results in a circular dependency†. The simplest way to break a circular dependency is to move one of the imports to the implementation file rather than the header. It looks like this should be possible with Resources.h.

† You might be wondering, why is it a problem if you have a circular dependency? Well, the #import directive literally just textually includes the file you specify, just like if you copy-pasted. It also intelligently doesn't include a file twice, because that would create duplicate code. But this means that when File A says "I want File B to go before me" and File B says "I want File A to go before me," one of them is going to be disappointed, and that leads to errors like the one you're getting here.