Tristate values -- is there a convention?

Question

I've been thrown a bit of a poser, and I'm not sure what the answer is.

Basically - is there a convention for what values to use in tristate data-types? Doing some googling around, it doesn't look like there is: I've seen:

• -1 = False, 0 = Not known/undefined, +1 = True
• 0 = False, +1 = True, +2 = Not known/undefined
• -1 =Not known/undefined, 0 = False, +1 = True

..amongst others. I'd rather use a well-known convention if there is one. Otherwise I'll make one up :-) It may well be there is no right answer, but just thought I'd dig a bit deeper...

Edit
Found this one as well that Microsoft seem to use in recent code: -1 = true, 0 = false, 2 = not known. I assume having 2 == unknown means it removes the ambiguity over interpreting +1/-1 when just looking at the raw values in a debugger/dump/memory etc. Oddly enough, this option appeals just for this reason alone (removes chance of forgetting which variation of 1 means 'true').

Answer 1

To my knowledge, there is no convention for that.

There isn't even a single convention for a boolean value, the most common are:

• 0 = False, -1 = True
• 0 = False, 1 = True

Another common (but not universal) convention that would be relevant is the usage of -1 for undefined values.

If you choose to use -1 for undefined, you could use 0/1 for False/True:

• -1 = Undefined
• 0 = False
• 1 = True

And naturally I have to mention that the third state should of course not be undefined/unknown, but FileNotFound. ;)