In Objective-c, safe and good way to compare 2 BOOL values?

Question

I want to compare 2 BOOL values in objective-c.

I found out that (3)-(6) of the following code works.
(1)-(2) doesn't work because BOOL is just signed char.

(3) works and is very readable but I think bool isn't objective-c.
Using bool in objective-c code is good?

Which is the safe and good way to compare 2 BOOL values in objective-c?
Are there other better ways to compare?

BOOL b = YES;
BOOL c = 2;

NSLog(@"(1) %d", b == c); // not work
NSLog(@"(2) %d", (BOOL)b == (BOOL)c); // not work
NSLog(@"(3) %d", (bool)b == (bool)c);
NSLog(@"(4) %d", !b == !c);
NSLog(@"(5) %d", !!b == !!c);
NSLog(@"(6) %d", (b != 0) == (c != 0));

results:

(1) 0
(2) 0
(3) 1
(4) 1
(5) 1
(6) 1

Answer 1

Comparing two boolean values should be handled with an XOR operation.

Trying to compare the two booleans directly is a misuse of the fundamentals of Boolean Algebra: http://en.wikipedia.org/wiki/Boolean_algebra_(logic)

When you are doing

BOOL a = (b == c);

then this value may return false even if both b and c are true. However the expression b && c will always return YES if both b and c are true, i.e. greater than 0, and NO otherwise.

Instead this is what you are actually trying to do:

BOOL xor = b && !c || !b && c;
BOOL equal = !xor;

equivalent with

BOOL equal = !(b && !c || !b && c);

or

BOOL equal = (b && c) || (!b && !c)

If you have to spend time making sure that your BOOL values are normalized (i.e. set to either 1 or 0) then your doing something wrong.

Answer 2

It's perfectly valid to use bool in Objective-C as it's part of the C99 standard (§7.16). In my opinion it's also the best way to handle safe comparisons of boolean types.

The only reason not to use bool everywhere is that BOOL is omnipresent in Objective-C and the frameworks.