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

Subject: Re: [Boost-users] [math] Warning in math/common_factor_rt.hpp
From: John Maddock (boost.regex_at_[hidden])
Date: 2010-07-01 13:50:43

> 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".

Actually I think in this case the code will work correctly, but in any case 
this patch should fix things: 
https://svn.boost.org/trac/boost/changeset/63496/trunk/boost/math/common_factor_rt.hpp

HTH, John.