When does Objective-C implicitly cast to BOOL?
boolean, objective-c, type-conversion
Solution
When does Objective-C implicitly cast to BOOL?
Never, since this phrase is semantically incorrect. A cast is, by definition, explicit. What you're asking about is called an "implicit type conversion" (coercion - thanks, Josh!).
Also, strictly speaking, the conditionals in Objective-C are not special: they're inherited from C. So expressions, when needed to evaluated as booleans, aren't treated as `BOOL`, but as `int`.
// Does this break when foo is 'dangerous'?`
self.titleLabel.hidden = foo ? YES : NO;
No, it doesn't. `(expr)` when used as a "boolean" expression, is equivalent to `(expr != 0)`. No risks here.
// I'm pretty sure it breaks now
self.titleLabel.hidden = ((BOOL)foo) ? YES : NO;
Now this can break, since `BOOL` is just typedeffed to `signed char`, which is 8 bit on iOS. Thus, if `foo`, which is a (32-bit) pointer, gets truncated to 8 bits, and the lower 8 bits of the pointer were all zero, but the pointer itself wasn't `nil`, this will incorrectly report false. You don't want to do this superfluous, ugly and dangerous cast. If you want to be more explicit, write
var = (obj != nil) ? valueOne : valueTwo;
instead.
Problem
I read Mike Ash's Objective-C pitfalls page, and now I'm paranoid about implicitly casting variables of type `id` to `BOOL`. Assume I have a 'dangerous' pointer with the lower bits zeroed out, so that casting it to a `BOOL` would produce `NO` even though it points to a valid object. Let's call this 'dangerous' pointer `foo`. How do simple if statements work? Does the if statement cast `foo` to a `BOOL` when it evaluates the condition? Or does it cast to a pure boolean? ``` if( foo ) { [myArray addObject:foo]; } ``` Does it work the same way in ternary expressions? ``` // Does this break when foo is 'dangerous'? self.titleLabel.hidden = foo ? YES : NO ; ``` Or will the ternary only break if I do this: ``` // I'm pretty sure it breaks now self.titleLabel.hidden = ((BOOL)foo) ? YES : NO ; ``` I feel like I'm missing something basic about logical operations in C, and how they relate to `BOOL`. Please help enlighten me.