Is comparing a BOOL against YES dangerous?

Question

I found a comment today in a source file:

//  - no longer compare BOOL against YES (dangerous!)

Is comparing BOOL against

Answer 1

You should never compare booleans against anything in any of the C based languages. The right way to do it is to use either:

if (b)

or:

if (!b)

This makes your code much more readable (especially if you're using intelligently named variables and functions like isPrime(n) or childThreadHasFinished) and safe. The reason something like:

if (b == TRUE)

is not so safe is that there are actually a large number of values of b which will evaluate to true, and TRUE is only one of them.

Consider the following:

#define FALSE 0
#define TRUE  1

int flag = 7;
if (flag)         printf ("number 1\n");
if (flag == TRUE) printf ("number 2\n");

You should get both those lines printed out if it were working as expected but you only get the first. That's because 7 is actually true if treated correctly (0 is false, everything else is true) but the explicit test for equality evaluates to false.

Update:

In response to your comment that you thought there'd be more to it than coder stupidity: yes, there is (but I still wouldn't discount coder stupidity as a good enough reason - defensive programming is always a good idea).

I also mentioned readability, which is rather high on my list of desirable features in code.

A condition should either be a comparison between objects or a flag (including boolean return values):

if (a == b) ...
if (c > d) ...
if (strcmp (e, "Urk") == 0) ...
if (isFinished) ...
if (userPressedEsc (ch)) ...

If you use (what I consider) an abomination like:

if (isFinished == TRUE) ...

where do you stop:

if (isFinished == TRUE) ...
if ((isFinished == TRUE) == TRUE) ...
if (((isFinished == TRUE) == TRUE) == TRUE) ...

and so on.

The right way to do it for readability is to just use appropriately named flag variables.