bool versus BOOL

Question

Possible Duplicates:
Objective-C : BOOL vs bool
Is there any difference between BOOL and Boolean in Objective-C?

I noticed from the autocomplete in XCode that there is a bool and a BOOL in Objective-C. Are these different? Why are there two different kinds of bool?

Are they interchangeable?

Answer 1

BOOL is actually a signed char (thanks Yuji), while bool is a true boolean from the ISO C99 standard.

See here: http://iphonedevelopertips.com/objective-c/of-bool-and-yes.html

Answer 2

BOOL is defined in Objective-C as typedef signed char BOOL, while bool is the datatype defined in C99.

Answer 3

You should use bool unless you need to interoperate with code that uses BOOL, because bool is a real Boolean type and BOOL isn't. What do I mean "real Boolean type"? I mean that code like this does what you expect it to:

#define FLAG_A 0x00000001
#define FLAG_B 0x00000002
...
#define FLAG_F 0x00000020
struct S
{
    // ...
    unsigned int flags;
};

void doSomething(S* sList, bool withF)
{
    for (S* s = sList; s; s = s->next)
    {
        if ((bool)(s->flags & FLAG_F) != withF)
            continue;
        // actually do something
    }
}

because (bool)(s->flags & FLAG_F) can be relied upon to evaluate to either 0 or 1. If that were a BOOL instead of a bool in the cast, it wouldn't work, because withF evaluates to 0 or 1, and (BOOL)(s->flags & FLAG_F) evaluates to 0 or the numeric value of FLAG_F, which in this case is not 1.

This example is contrived, yeah, but real bugs of this type can and do happen all too often in old code that doesn't use the C99/C++ genuine boolean types.

Answer 4

Yes they are different.

• C++ has bool has a true boolean type. It is guaranteed to be 0 or 1 within integer context.
• C99 has _Bool as a true boolean type, and if <stdbool.h> is included, then bool becomes a typedef for _Bool (this header also defines true and false).
• Cocoa has BOOL as a type, but it is just a typedef for signed char.
• Carbon has Boolean as a type, but it is just a typedef for unsigned char.