Re: [Boost-users] [math] Warning in math/common_factor_rt.hpp

Subject: Re: [Boost-users] [math] Warning in math/common_factor_rt.hpp
From: Gabriel Redner (gredner_at_[hidden])
Date: 2010-07-01 12:14:53

Investigating a bit further, I discovered that we are actually passing
-funsigned-char to the compiler.  This causes a problem because:

common_factor_rt.hpp:329
    BOOST_PRIVATE_GCD_SF( char, unsigned char ); // should work even if unsigned

This macro expands into:
common_factor_rt.hpp:317
#define BOOST_PRIVATE_GCD_SF( St, Ut )                            \
    template < >  struct gcd_optimal_evaluator<St>                \
    {  St  operator ()( St a, St b ) const  { Ut const  a_abs =   \
    static_cast<Ut>( a < 0 ? -a : +a ), b_abs = static_cast<Ut>(  \
    b < 0 ? -b : +b ); return static_cast<St>(                    \
    gcd_optimal_evaluator<Ut>()(a_abs, b_abs) ); }  }

The expression 'a < 0'  is always false if 'a' is unsigned, so the
compiler issues a warning (if warnings are turned high enough).

My impression is that this code should be considered incorrect -
making assumptions about the signed-ness of char is not portable.  I
do not understand why the comment suggests that this "should work".

-Gabe