Re: [boost] Portable signbit macro/function?

From: Johan Råde (rade_at_[hidden])
Date: 2006-08-02 11:07:36

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;
     }

--Johan Råde