Re: [boost] Portable signbit macro/function?

From: Johan Råde (rade_at_[hidden])
Date: 2006-08-02 11:24:14

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.

Or how about this

     template<class T>
     bool is_negative(const T& x)
     {
         return x < T(0);
     }

     template<>
     bool is_negative(const 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<const int&>(x) & sign_mask;
     }

     template<>
     bool is_negative(const double& x)
     {
         return is_negative(static_cast<float>(x));
     }

     template<>
     bool is_negative(const long double& x)
     {
         return is_negative(static_cast<float>(x));
     }

--Johan Råde