Why use !! when converting int to bool?

Question

What can be a reason for converting an integer to a boolean in this way?

bool booleanValue = !!integerValue;

instead of just

bool booleanValue = integerValue;

All I know is that in VC++7 the latter will cause C4800 warning and the former will not. Is there any other difference between the two?

Answer 1

Because !integerValue means integerValue == 0 and !!integerValue thus means integerValue != 0, a valid expression returning a bool. The latter is a cast with information loss.

Answer 2

After the double negation, the value is guaranteed to be 0 or 1; the original value might be any int value.

Answer 3

Historically, the !! metaphor was used to ensure that your bool really contained one of the two values expected in a bool-like variable, because C and C++ didn't have a true bool type and we faked it with ints. This is less of an issue now with "real" bools.

But using !! is an efficient means of documenting (for both the compiler and any future people working in your code) that yes, you really did intend to cast that int to a bool.

Answer 4

A bool can only have two states, 0, and 1. An integer can have any state from -2147483648 to 2147483647 assuming a signed 32-bit integer. The unary ! operator outputs 1 if the input is 0 and outputs 0 if the input is anything except 0. So !0 = 1 and !234 = 0. The second ! simply switches the output so 0 becomes 1 and 1 becomes 0.

So the first statement guarantees that booleanValue will be be set equal to either 0 or 1 and no other value, the second statement does not.

Answer 5

The problems with the "!!" idiom are that it's terse, hard to see, easy to mistake for a typo, easy to drop one of the "!'s", and so forth. I put it in the "look how cute we can be with C/C++" category.

Just write bool isNonZero = (integerValue != 0); ... be clear.

Answer 6

No big reason except being paranoid or yelling through code that its a bool.

for compiler in the end it wont make difference .

Answer 7

I've never like this technique of converting to a bool data type - it smells wrong!

Instead, we're using a handy template called boolean_cast found here. It's a flexible solution that's more explicit in what it's doing and can used as follows:

bool IsWindow = boolean_cast< bool >(::IsWindow(hWnd));