CODEWITHSUNDEEP
objective-cc
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Reputation: 81

NS_OPTIONS has a flaw?

In the objective-c variant of C, NS_OPTIONS exists to help validate bit masks. But it seems to have an inherent flaw. If I need to define a value representing a bitwise OR of all of the bits, e.g. FubarAllOptions some would say that the convention is to simply use INT_MAX. However this has a problem.

Imagine that I use NS_OPTIONS for the lower five bits of a uint8_t. e.g.

typedef NS_OPTIONS(uint8_t) {
 FubarA=1,
 FubarB=1<<1,
 FubarC=1<<2,
 FubarD=1<<3,
 FubarE=1<<4,
 FubarAllOptions=0xff // MAX
} FubarOptions;

If I bitwise clear each of the assigned bits of a FubarOptions variable, the remaining three upper bits will remain set. Therefore if I check for the NS_OPTIONS value being nonzero as a test of whether all the bits are cleared, it will appear that some bits are still set. Therefore a bug. FubarAllOptions includes bits that are not assigned.

Q: How do I define FubarAllOptions so that it only includes assigned bits, without laboriously typing out all of the potential options and Or'ing them? i.e. FubarA|FubarB|.... But this would be vulnerable to typo mistakes.

Sure I can take the largest, <<1 and subtract 1. But too this would be vulnerable to typo mistakes.

Upvotes: 1

Views: 219

Answers (1)

Sulthan
Sulthan

Reputation: 130122

You will have to set all options manually:

FubarAllOptions = (FubarA | FubarB | FubarC | FubarD | FubarE)

Of course, you can also fix the problem by always checking every option manually instead of masking them all and then comparing with zero.

You are too worried about typing mistakes when you should rather worry what will happen when you start using another bit.

Upvotes: 2

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