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From: Yitzchak Scott-Thoennes <sthoenna AT efn DOT org>
Subject:  Re: cygwin source question
Date: Mon, 22 Jan 2007 08:19:19 +0000 (UTC)
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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).

-- 
I'm looking for a job: http://perlmonks.org/?node=ysth#looking




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