Enum bitwise masking limitations

Question

I was trying to enumerate filetypes with bitmasking for fast and easy distinguishing on bitwise OR:

typedef enum {

    FileTypeDirectory = 1,
    FileTypePIX = 2,
    FileTypeJPG = 4,
    FileTypePNG = 8,
    FileTypeGIF = 16,
    FileTypeHTML = 32,
    FileTypeXML = 64,
    FileTypeTXT = 128,
    FileTypePDF = 256,  
    FileTypePPTX = 512,

    FileTypeAll = 1023

} FileType;

My OR operations did work until 128, afterwards it failed. Are enums on a 64 Bit Mac OSX limited to Byte Datatypes? (2^7=128)

See [this old SO answer](http://stackoverflow.com/a/366026/440558). — Some programmer dude, Aug 17 '12 at 12:40
The enum from your code is guaranteed to be large enough to hold `1023`, so the problem most likely has something to do with your "afterwards it failed" check. — Sergey Kalinichenko, Aug 17 '12 at 12:42
Hmmm....might be useful to post the code where the OR'ing is failing.... — David W, Aug 17 '12 at 12:54
@JoachimPileborg, the answer there is misleading, I think, because it speaks about the storage size of objects of the enumeration type and not of the constants. — Jens Gustedt, Aug 17 '12 at 13:18

score 0 · Accepted Answer · answered Aug 17 '12 at 13:17

All enum constants in C are of type int and not of the type of the enumeration itself. So the restriction is not in the storage size for enum variables, but only in the number of bits for an int.

I don't know much of objective-c (as this is tagged also) but it shouldn't deviate much from C.

score 0 · Answer 2 · answered Aug 17 '12 at 13:36

0

I'm not quite sure how you used the OR operator but it works for me well with your typedef.

FileType _fileType = FileTypeGIF | FileTypePDF | FileTypePPTX;
NSLog(@"filetype is : %d", _fileType);

the result is:

filetype is : 784

which is correct values because 16 + 256 + 512 is precisely 784.

(it has been tested on real device only.)

answered Aug 17 '12 at 13:36

holex

23,961
7
62
76

I tested it too and saw that the failure has to be somewhere else. the enumerations works properly! – Low Noise Aug 17 '12 at 13:47
@LowNoise, then how did you experience, as you've mentioned, _My OR operations did work until 128, afterwards it failed_? – holex Aug 17 '12 at 13:57
It actually occured somewhere else, my mentioning was not correct. – Low Noise Aug 17 '12 at 14:06

Enum bitwise masking limitations

2 Answers2