NS_ENUM gives compiler warning about forward references

Question

Im using the spiffy new NS_ENUM to try and define an enum in my objective-c iOS project.

I'm declaring the NS_ENUM in the header of a class like so:

NS_ENUM(int, SomeEnumType){
    SomeEnumType1,
    SomeEnumType2,
    SomeEnumType3,
    SomeEnumType4
};

@interface Issue : NSObject
....

And im getting the compiler warning:

ISO C forbids forward references to 'enum' types

Now if i define the enum the (slightly) older traditional way like so:

typedef enum{
    SomeEnumType1,
    SomeEnumType2,
    SomeEnumType3,
    SomeEnumType4
}SomeEnumType;

@interface Issue : NSObject
....

in exactly the same place in the code the issue goes away. What am i doing wrong with NS_ENUM?

EDIT:

I corrected it by adding the typedef but its still giving a warning.

I have turned on the pedantic compiler warnings. Is this just a case where its being too pedantic or is there a correct way that im missing?

Put them in a header so that all files that need them can access the header and use the declarations from it. — Rushi, Jan 28 '13 at 11:26
it is in a header. Its in the header file of the Issue class. — jackslash, Jan 28 '13 at 11:28
May be this http://stackoverflow.com/questions/7480217/c-forward-declaration-of-enums will help you — Rushi, Jan 28 '13 at 11:29

WDUK · Accepted Answer · 2013-01-28T11:52:05.380

Try:

typedef NS_ENUM(int, SomeEnumType){
    SomeEnumType1,
    SomeEnumType2,
    SomeEnumType3,
    SomeEnumType4
};

NS_ENUM won't do the typedef for you to declare the SomeEnumType type, you have to do it yourself.

Update: The reason why the warning shows up is due to the implementation of NS_ENUM. Let's see what it tries to do:

#define NS_ENUM(_type, _name) enum _name : _type _name; enum _name : _type

The problem line (I believe) is this:

enum _name : _type _name;

This is performing a forward declaration within the macro itself. Hence, with pedantic warnings, it's flagging the use of this up.

The pedantic warning is simply stating if you wanted to transition this to pure C, it would not be portable as it does not follow the standardisation of no forward declarations of enums. Within the realm of Xcode, Clang and LLVM (and the fact NS_ENUM is provided by Apple), you should be pretty safe.

good catch but im still getting the warning. Ive got the pedantic compiler warnings on. Is there still something im missing or is it being too pedantic? — jackslash, Jan 28 '13 at 11:39
Yes. Thanks very much for the extra detail. I can see why its causing the issue now. I think im gonna give up on the pedantic warnings. Sorry I had already accepted the other answer but I upvoted all of your contributions to make up for it. — jackslash, Jan 28 '13 at 11:55

score 5 · Answer 2 · 2013-01-28T11:46:24.890

5

You missed off the typedef:

typedef NS_ENUM(int, SomeEnumType){
    SomeEnumType1,
    SomeEnumType2,
    SomeEnumType3,
    SomeEnumType4
};

You mentioned that you're using pedantic warnings. The compiler is correct: the fixed-type enums are part of the C++ standard, not ISO C.

edited Jan 28 '13 at 11:46

answered Jan 28 '13 at 11:33

Well, thankyou for both your suggestions. I think I'll either `clang diagnostic push` it away as its valid objective-c or ill just turn off the pedantic warnings. I dont know if im hardcore enough to keep it up. – jackslash Jan 28 '13 at 11:49

score 3 · Answer 3 · answered Jun 26 '13 at 19:44

As others have pointed out, the pedantic warning is correct. However, you don't have to use the NS_ENUM macro to take advantage of strictly typed enums. Just declare your enum like this and the warning will go away while you retain the strict typing:

typedef enum : int {
    SomeEnumType1,
    SomeEnumType2,
    SomeEnumType3,
    SomeEnumType4
} SomeEnumType;

NS_ENUM gives compiler warning about forward references

3 Answers3