Mail Archives: cygwin/2007/01/22/03:19:56
Dave Korn writes:
> On 21 January 2007 14:38, Christopher Layne wrote:
> > I notice in some places, there are double-negates, like:
> >
> > me->read_ready |= ret || !!(events & (FD_READ | FD_ACCEPT | FD_CLOSE));
> >
> > What's the rationale for these? To enforce either a 0 or 1, to be directly
> > in line with boolean, rather than a zero or non-zero result?
>
> It's a standard C idiom for that, yeh, it normalizes zero/non-zero into 0/1.
Also protects against accidentally truncating true values to false (e.g. if
events is an int and events |(FD_READ...) is 0x80000000 and me->read_ready is a
short, the implicit cast to short in the assignment turns 0x80000000 into 0).
This is also a Perl idiom (though Perl doesn't suffer the above problem).
Some people prefer (expr) ? 1 : 0, which looks a lot worse to me than !!(expr).
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