2048 casted to BOOL returns 0

Question

Consider this code

NSInteger q = 2048;
BOOL boolQ = q;

NSLog(@"%hhd",boolQ);

After execution boolQ is equal 0. Could someone explain why is this so?

Answer 1

BOOL probably is implemented as char or uint8_t/int8_t, as "hh" prints half of the half of an integer. which typically is a byte.

Converting to char is taking the lowest 8bit of 2048 (=0x800) and gives you 0.

The proper way to convert any integer to a boolean value is:

NSInteger q = some-value;
BOOL b = !!q;

Answer 2

Casting an integer value to a type too small to represent the value being converted is undefined behaviour in C (C11 standard Annex J.2), and therefore also in the part of Objective-C which deals with C-level matters. Since it's undefined behaviour it can represent the result however it wants, expected value or not.

As per 6.3.1.4, any integer can be used as a boolean value without casting, in which case it will show the expected behaviour (0 is 0, everything else is 1), giving rise to the !! idiom suggested by alk; perhaps counterintuitively, you convert the value by not explicitly converting the value (instead, the conversion is correctly handled by the implicit conversion operation inserted by the ! operator).