Re: [boost] Portable signbit macro/function?

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-08-02 13:43:00

|  -----Original Message-----
|  From: boost-bounces_at_[hidden] 
|  [mailto:boost-bounces_at_[hidden]] On Behalf Of Johan Råde
|  Sent: 02 August 2006 16:08
|  To: boost_at_[hidden]
|  Subject: Re: [boost] Portable signbit macro/function?
|  
|  Paul A Bristow wrote:
|  > Does anyone have a ***portable*** version of macro signbit (C99)
|  > 
|  > or can we combine versions that work on each 
|  compiler/hardware/endian...
|  > 
|  > or an equivalent C++ function, perhaps called is_negative?
|  > 
|  > (This would just test the signbit for the type(s)).
|  > 
|  > There are several places where this would be useful in 
|  dealing properly with
|  > NaNs and zero and infinity etc.
|  > 
|  > Thanks
|  > 
|  > Paul
|  > 
|  > PS Must be Boost license of course.
|  
|  How about
|  
|       bool is_negative(float x)
|       {
|           const float one = 1;
|           const float neg_one = -1;
|           const int one_bits = reinterpret_cast<const int&>(one);
|           const int neg_one_bits = reinterpret_cast<const 
|  int&>(neg_one);
|           const int sign_mask = one_bits ^ neg_one_bits;
|  
|           return reinterpret_cast<int&>(x) & sign_mask;
|       }

Sneaky - and looks plausible - but may be quicker to use a macro, if
available, as I suspect it is for many platforms.

Need to check it works for negative zero and NaNs too - I may get a moment
to check this out, for my sins - since I raised the issue ;-)

Paul

---
Paul A Bristow
Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB
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