Implications of int values not “fitting” – bool or BOOL for Objective-C?

问题

In the comments here -- https://stackoverflow.com/a/9393138/8047 -- I discovered that BOOL has some unexpected behavior when setting its value from an int value. Mostly, if the value is set to 0x1000 it gets evaluated as FALSE (surprisingly).

NSLog(@"All zero? %d %d", (BOOL)0, (bool)0);
NSLog(@"All one? %d %d %d", (BOOL)4095, (BOOL)4096, (BOOL)4097); // 4096=0x1000 or 8-bits
NSLog(@"All one? %d %d %d", (bool)4095, (bool)4096, (bool)4097);

Produces:
All zero? 0 0
All one? -1 0 1
All one? 1 1 1

I think this is odd, but then again, I don't cast from int to BOOL much anyway. However:

Does this imply that bool be preferred to BOOL? Why or why not?
Is it okay to use

if (thatBool) {

or should one prefer

if (thatBool ? YES : NO) {

And why?

Note: This is a more specific version of this question of this -- Objective-C : BOOL vs bool -- but I think it adds to it and is not a duplicate.

回答1:

I think that (BOOL)4096 being evaluated to 0 is a simple arithmetic overflow, just as (BOOL)256, since BOOL is an unsigned char. And I think that the !! casting trick ("double negation") works fine:

NSLog(@"%i", (BOOL)256); // 0
NSLog(@"%i", !!256); // 1

That means I’d use BOOL to keep the standard Cocoa coding style and simply watch for dangerous type casts. The thatBool ? YES : NO expression hurts my eyes, why would you want to do that? :)

回答2:

1) bool is the C++ type, BOOL is the Objective-C one. Casting from int to BOOL doesn't work properly because YES is just (BOOL)1 = (signed char)1 (= 0x001) and that isn't equal to (signed char)4 (= 0x100), for example.

2) Both will work, the second might be unreadable to somebody with little experience in programming. I prefer the good old c-style safe condition check with the constant on the left to prevent accidental omission of one of the equal signs.

if (YES == isEnabled) {

}

回答3:

When using an int, you can always be explicit when setting a BOOL by checking if the int is equal to a particular value, e.g.

BOOL yesNo = ((int)4096 > 0);
BOOL enableButton = (someInt >= 16);

In other words, don't pass the int to the BOOL directly; turn it into a true/false statement.

来源：https://stackoverflow.com/questions/9433121/implications-of-int-values-not-fitting-bool-or-bool-for-objective-c

标签

objective-c

primitive