What does double not (!!) used on a non-boolean variable do?

Question

Possible Duplicate:
Double Negation in C++ code.

I am working with production code where I have run across statements like this a few times:

Class.func(!!notABool);

The first couple of times I dismissed it as a programmer quirk (maybe to emphasize that it is a conditional statement rather than a number being passed into func?) but I have run across several statements that use the above and now I am wondering whether it actually makes a difference or not. In most cases notABool is a number(int, float, double... I have seen all 3) My initial guess was that it is akin to typing:

Class.func((bool)notABool);

but I am not entirely sure?

@jonsca: Doh! Searched for all other wordings except for negation. Agreed, that is a duplicate. — Samaursa, Jul 20 '11 at 15:14
Not to worry. I think the title of the duplicate is misleading, but it popped up on the "Related" list ----> — jonsca, Jul 20 '11 at 15:16

score 6 · Answer 1 · answered Jul 20 '11 at 15:15

Yes, functionally it is exactly the same as doing (bool) notABool.

By definition, in C++ language the operand of ! is implicitly converted to bool type, so !!notABool is really the same as !! (bool) notABool, i.e. the same as just (bool) notABool.

In C language the !! was a popular trick to "normalize" a non-1/0 value to 1/0 form. In C++ you can just do (bool) notABool. Or you can still use !!notABool if you so desire.

score 4 · Accepted Answer · answered Jul 20 '11 at 15:13

4

For primitive types, yes, it's essentially equivalent to:

!(notABool != 0)

which in turn is equivalent to:

(bool)notABool

For non-primitive types, it will be a compiler error, unless you've overloaded operator!, in which case, it might do anything.

answered Jul 20 '11 at 15:13

Oliver Charlesworth

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score 3 · Answer 3 · answered Jul 20 '11 at 15:16

3

It's a legacy idiom from C, where it meant "normalize to 0 or 1". I don't think there's a reason to use it in C++ other than habit.

answered Jul 20 '11 at 15:16

Rafał Dowgird

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score 1 · Answer 4 · answered Jul 20 '11 at 15:18

It's converting BOOL (define for int) to c++ bool. BOOL is a define that in some cases for true can contain different integer values. So for example BOOL a = (BOOL)1; and BOOL b =(BOOL)2; both pass check for true. But if you'll try to compare you'll find that a not equals b. But after conversion !!a equals !!b.

(bool)notABoo - is not akin, because you'll convert type of variable byte still'll have different values. !! converts not only type but in some cases values too.

What does double not (!!) used on a non-boolean variable do?

4 Answers4