Objective-C : BOOL vs bool

I saw the "new type" BOOL (YES, NO).

I read that this type is almost like a char.

For testing I did :

NSLog(@"Size of BOOL %d", sizeof(BOOL));
NSLog(@"Size of bool %d", sizeof(bool));

Good to see that both logs display "1" (sometimes in C++ bool is an int and its sizeof is 4)

So I was just wondering if there were some issues with the bool type or something ?

Can I just use bool (that seems to work) without losing speed?

From the definition in objc.h:

#if (TARGET_OS_IPHONE && __LP64__)  ||  TARGET_OS_WATCH
typedef bool BOOL;
#else
typedef signed char BOOL; 
// BOOL is explicitly signed so @encode(BOOL) == "c" rather than "C" 
// even if -funsigned-char is used.
#endif

#define YES ((BOOL)1)
#define NO  ((BOOL)0)

So, yes, you can assume that BOOL is a char. You can use the (C99) bool type, but all of Apple's Objective-C frameworks and most Objective-C/Cocoa code uses BOOL, so you'll save yourself headache if the typedef ever changes by just using BOOL.

As mentioned above, BOOL is a signed char. bool - type from C99 standard (int).

BOOL - YES/NO. bool - true/false.

See examples:

bool b1 = 2;
if (b1)	printf("REAL b1 \n");
if (b1 != true) printf("NOT REAL b1 \n");

BOOL b2 = 2;
if (b2)	printf("REAL b2 \n");
if (b2 != YES) printf("NOT REAL b2 \n");

And result is > REAL b1
REAL b2
NOT REAL b2

Note that bool != BOOL. Result below is only ONCE AGAIN - REAL b2

b2 = b1;
if (b2)	printf("ONCE AGAIN - REAL b2 \n");
if (b2 != true) printf("ONCE AGAIN - NOT REAL b2 \n");

If you want to convert bool to BOOL you should use next code

BOOL b22 = b1 ? YES : NO; //and back - bool b11 = b2 ? true : false;

So, in our case:

BOOL b22 = b1 ? 2 : NO;
if (b22)	printf("ONCE AGAIN MORE - REAL b22 \n");
if (b22 != YES) printf("ONCE AGAIN MORE- NOT REAL b22 \n");

And so.. what we get now? :-)

At the time of writing this is the most recent version of objc.h:

/// Type to represent a boolean value.
#if (TARGET_OS_IPHONE && __LP64__)  ||  TARGET_OS_WATCH
#define OBJC_BOOL_IS_BOOL 1
typedef bool BOOL;
#else
#define OBJC_BOOL_IS_CHAR 1
typedef signed char BOOL; 
// BOOL is explicitly signed so @encode(BOOL) == "c" rather than "C" 
// even if -funsigned-char is used.
#endif

It means that on 64-bit iOS devices and on WatchOS BOOL is exactly the same thing as bool while on all other devices (OS X, 32-bit iOS) it is signed char and cannot even be overridden by compiler flag -funsigned-char

It also means that this example code will run differently on different platforms (tested it myself):

int myValue = 256;
BOOL myBool = myValue;
if (myBool) {
    printf("i'm 64-bit iOS");
} else {
    printf("i'm 32-bit iOS");
}

BTW never assign things like array.count to BOOL variable because about 0.4% of possible values will be negative.

The Objective-C type you should use is BOOL. There is nothing like a native boolean datatype, therefore to be sure that the code compiles on all compilers use BOOL. (It's defined in the Apple-Frameworks.

Yup, BOOL is a typedef for a signed char according to objc.h.

I don't know about bool, though. That's a C++ thing, right? If it's defined as a signed char where 1 is YES/true and 0 is NO/false, then I imagine it doesn't matter which one you use.

Since BOOL is part of Objective-C, though, it probably makes more sense to use a BOOL for clarity (other Objective-C developers might be puzzled if they see a bool in use).

Another difference between bool and BOOL is that they do not convert exactly to the same kind of objects, when you do key-value observing, or when you use methods like -[NSObject valueForKey:].

As everybody has said here, BOOL is char. As such, it is converted to an NSNumber holding a char. This object is indistinguishable from an NSNumber created from a regular char like 'A' or '\0'. You have totally lost the information that you originally had a BOOL.

However, bool is converted to an CFBoolean, which behaves the same as NSNumber, but which retains the boolean origin of the object.

I do not think that this is an argument in a BOOL vs. bool debate, but this may bite you one day.

Generally speaking, you should go with BOOL, since this is the type used everywhere in the Cocoa/iOS APIs (designed before C99 and its native bool type).

The accepted answer has been edited and its explanation become a bit incorrect. Code sample has been refreshed, but the text below stays the same. You cannot assume that BOOL is a char for now since it depends on architecture and platform. Thus, if you run you code at 32bit platform(for example iPhone 5) and print @encode(BOOL) you will see "c". It corresponds to a char type. But if you run you code at iPhone 5s(64 bit) you will see "B". It corresponds to a bool type.

As mentioned above BOOL could be an unsigned char type depending on your architecture, while bool is of type int. A simple experiment will show the difference why BOOL and bool can behave differently:

bool ansicBool = 64;
if(ansicBool != true) printf("This will not print\n");

printf("Any given vlaue other than 0 to ansicBool is evaluated to %i\n", ansicBool);

BOOL objcBOOL = 64;
if(objcBOOL != YES) printf("This might print depnding on your architecture\n");

printf("BOOL will keep whatever value you assign it: %i\n", objcBOOL);

if(!objcBOOL) printf("This will not print\n");

printf("! operator will zero objcBOOL %i\n", !objcBOOL);

if(!!objcBOOL) printf("!! will evaluate objcBOOL value to %i\n", !!objcBOOL);

To your surprise if(objcBOOL != YES) will evaluates to 1 by the compiler, since YES is actually the character code 1, and in the eyes of compiler, character code 64 is of course not equal to character code 1 thus the if statement will evaluate to YES/true/1 and the following line will run. However since a none zero bool type always evaluates to the integer value of 1, the above issue will not effect your code. Below are some good tips if you want to use the Objective-C BOOL type vs the ANSI C bool type:

• Always assign the YES or NO value and nothing else.
• Convert BOOL types by using double not !! operator to avoid unexpected results.
• When checking for YES use if(!myBool) instead of if(myBool != YES) it is much cleaner to use the not ! operator and gives the expected result.

I go against convention here. I don't like typedef's to base types. I think it's a useless indirection that removes value.

1. When I see the base type in your source I will instantly understand it. If it's a typedef I have to look it up to see what I'm really dealing with.
2. When porting to another compiler or adding another library their set of typedefs may conflict and cause issues that are difficult to debug. I just got done dealing with this in fact. In one library boolean was typedef'ed to int, and in mingw/gcc it's typedef'ed to a char.

Also, be aware of differences in casting, especially when working with bitmasks, due to casting to signed char:

bool a = 0x0100;
a == true;  // expression true

BOOL b = 0x0100;
b == false; // expression true on !((TARGET_OS_IPHONE && __LP64__) || TARGET_OS_WATCH), e.g. MacOS
b == true;  // expression true on (TARGET_OS_IPHONE && __LP64__) || TARGET_OS_WATCH

If BOOL is a signed char instead of a bool, the cast of 0x0100 to BOOL simply drops the set bit, and the resulting value is 0.