Objective-C Protocol Forward Declarations

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You cannot forward declare a superclass or a protocol that it conforms to. In those cases, you must include the header. This is because (in the case of superclass) the superclass’s instance variables and methods become part of your class; or (in the case of protocols) the protocol’s methods become methods declared in your class, without needing to declare them explicitly. (i.e. Now other people who include your class’s header will see that your class declares those methods, as if you declared them yourself.) The only way that that could be possible is if they were already defined in this scope, i.e. their headers are imported.

#import <SomeClass.h>
#import <SomeProtocol.h> // these two must be imported

@interface MyClass : SomeClass <SomeProtocol>
@end

Forward declarations are useful for things that just show up in the type of a variable (specifically, an object pointer variable). Object pointers are all the same size and there is no difference between different types of object pointers at runtime (the concept of type of object pointer is just a compile-time thing). So there is no real need to know exactly what’s in the classes of those types. Thus they can be forward declared.

@class SomeClass;
@protocol SomeProtocol; // you can forward-declare these

@interface MyClass {
    SomeClass *var1;
    id<SomeProtocol> var2;
}
@end

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