Why do nonzero numbers fail to set a BOOL property to YES?

Question

It is my understanding that in Objective C, which is based off of C, all BOOLs are basically shorts (-127 to 128), with zero being the only value for "FALSE", or "NO". However, when I recently tried to set a button's selected value based off of a bitmask, it fails. Why?

NSInteger bitfield = 127;
NSInteger bitmask = 1 << 6; // 64

myButton.selected = bitfield & bitmask; // selected will remain NO

Answer 1

That's because BOOL is not bool.

BOOL is a just a non-standard (Objective-C-specific) typedef for a (non-bool) integral type (as far as I know, it's always signed char but I might be wrong). As such, it does not behave as a true Boolean data type, but rather as its underlying integral type. So, if you assign 64 to it, it will store 64 (and not true or 1). It is possible that, as a result of this, an operation that always assumes the true value to be 1 (i. e. the LSB set) will fail to recognize 64 as such.

In contrast, if you replaced BOOL with the true C99 Boolean type, which is _Bool or bool, then you would experience the expected behavior. I. e., assigning any non-zero value to the variable would have it store true or 1, regardless to whether that value was really 1.