Why use two '!' operators on a boolean value?

Question

Possible Duplicate:
Double Negation in C++ code

When I scanned the Webkit source code, I found a strange use of the boolean "not" operator !:

BOOL enabled;
if (SUCCEEDED(sharedPreferences->continuousSpellCheckingEnabled(&enabled)))
    continuousSpellCheckingEnabled = !!enabled;
if (SUCCEEDED(sharedPreferences->grammarCheckingEnabled(&enabled)))
    grammarCheckingEnabled = !!enabled;

Why don't they use enabled directly instead of !!enabled?

Nothing here mentions it explicitly, but that `BOOL` is most likely an integer of some sort. Judging by `SUCCEEDED`, I presume winapi, in which it's `int`. — chris, Dec 12 '12 at 08:44

score 14 · Answer 1 · answered Dec 12 '12 at 08:42

14

It's a C++ trick to convert anything to 1 or 0.

For example, 42 is logically true, but is not 1, so applying !! to it, you turn it into 1.

answered Dec 12 '12 at 08:42

Luchian Grigore

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Grimace of Despair · Answer 2 · 2012-12-12T11:48:51.323

5

It's meant to force a value to a boolean. So if the value is evaluating to something, you'll get true.

edited Dec 12 '12 at 11:48

answered Dec 12 '12 at 08:39

Grimace of Despair

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score 4 · Answer 3 · answered Dec 12 '12 at 08:42

It forces a true value to be exactly 1.

In a boolean context (if (x), while (x), x ? a : b, for (; x; )), a value which is 0 means false, while any other value means true.

If your function accepts a truth value, but you need exactly 1 there, !! is fine.

In other words, !!x is the same as x ? 1 : 0.

score 1 · Answer 4 · answered Dec 12 '12 at 08:41

1

Most probably continuousSpellCheckingEnabled is of type bool. BOOL is defined as int, so:

continuousSpellCheckingEnabled = enabled;

issues a warning, while:

continuousSpellCheckingEnabled = !!enabled;

does not.

answered Dec 12 '12 at 08:41

Stefan

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+1 for explaining *why* it might be necessary to convert to {0,1} – OrangeDog Dec 12 '12 at 12:43

score 1 · Answer 5 · answered Dec 12 '12 at 08:42

1

why don't they directly use "enabled"

Double negation only cancels out for boolean values (0 and 1) so !!(0) == 0 for non-boolean values the value will be converted to boolean !!(100) == 1

answered Dec 12 '12 at 08:42

iabdalkader

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Why use two '!' operators on a boolean value?

5 Answers5