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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?
709
That treats 2 as a boolean value (which is considered true). To ask if x is 1 or 2, say x ==1 || x == 2. In English, is sometimes means "is equal to", and sometimes means "has the property".
A Boolean variable has only two possible values: true or false. It is common to use Booleans with control statements to determine the flow of a program.
Show activity on this post. According to the R language definition, the difference between & and && (correspondingly | and || ) is that the former is vectorized while the latter is not.
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.
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