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

Answer 1

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.