Question

In one header file, I have something like:

// PasscodeInputViewController.h

typedef enum {
    PasscodeInputModeOn,     // set passcode
    PasscodeInputModeEnter,  // enter passcode
    PasscodeInputModeChange, // change passcode
    PasscodeInputModeOff     // turn off passcode
} PasscodeInputMode;

In another header file, I declare a method that takes an argument of type PasscodeInputMode:

#import "PasscodeInputViewController.h"

- (void)presentPasscodeInputWithMode:(PasscodeInputMode)mode;

As you can see, I use #import "PasscodeInputViewController.h" as above so that PasscodeInputMode is recognized, but is there a @class equivalent for typedef enum?

Answer 1

No, there isn’t an equivalent.

Enumerations in Objective-C are the same as enumerations in C. Since the underlying type of an enumeration is implementation-dependent (e.g., it could be char or int), the compiler must know the complete declaration of the enumeration.

That said, a type specifier

enum nameOfEnum

without listing the enumeration members is valid C provided it appears after the type it specifies is complete, i.e., enum nameOfEnum { … } must appear beforehand in the translation unit.

In summary: There’s no forward declaration of enumerations, only backward references.