What is the usage of “!!” (negating twice)? [duplicate]

Answer 1

Its use is to make sure the value is either 0 or 1. I think it's superfluous with C++'s bool type.

Answer 2

!! guarantees that the result will end up as a 1 or a 0, rather than just the value of _Expression or 0. In C, it's unlikely to matter, but in C++ I think it turns the result of the expression into a bool type, which might be useful in some cases. If you did have some API that required a literal 1 or 0 be passed to it, using !! would be a way to make it happen.

Answer 3

It's possible that you might want an int variable that's either 1 or 0.

So you can't for example pass a 5, instead the double negation would turn that 5 into a 1.

Also, have a look at how TRUE is defined:

#ifndef TRUE
#define TRUE                1
#endif

Therefore, an expression like:

int x = 5;
if ( x == TRUE )
{
   //....
}

would not pass, whereas

if ( x )
{
   //....
}

would.