Question

Simple question really; is there a difference between these values (and is there a difference between BOOL and bool)? A co-worker mentioned that they evaluate to different things in Objective-C, but when I looked at the typedefs in their respective .h files, YES/TRUE/true were all defined as 1 and NO/FALSE/false were all defined as 0. Is there really any difference?

Answer 1

There is no practical difference provided you use BOOL variables as booleans. C processes boolean expressions based on whether they evaluate to 0 or not 0. So:

if(someVar ) { ... } if(!someVar) { ... } 

means the same as

if(someVar!=0) { ... } if(someVar==0) { ... } 

which is why you can evaluate any primitive type or expression as a boolean test (including, e.g. pointers). Note that you should do the former, not the latter.

Note that there is a difference if you assign obtuse values to a so-called BOOL variable and test for specific values, so always use them as booleans and only assign them from their #define values.

Importantly, never test booleans using a character comparison — it’s not only risky because someVar could be assigned a non-zero value which is not YES, but, in my opinion more importantly, it fails to express the intent correctly:

if(someVar==YES) { ... } // don't do this! if(someVar==NO ) { ... } // don't do this either! 

In other words, use constructs as they are intended and documented to be used and you’ll spare yourself from a world of hurt in C.