[boost] [mpl] rational_c

From: Andy Little (andy_at_[hidden])
Date: 2005-03-07 15:57:41

Hi again,

The problem:  For  some Integral Constants values  mpl::divides<A.,B> truncates.

The solution:  A new operator... mpl::rational_divides<A,B> When it is desired 
not to truncate, this form will preserve the fractional part by if necessary 
converting to a rational number. With this operator in place it is also possible 
to 'reduce' rationals to integers where possible and  further to keep integral 
constants as integral types  by default, knowing that  conversion will be 
automatic if a fraction is required to prevent loss of precision.

No  mpl::divides<A.,B> is available if A or B is a rational types, because it is 
assumed that if you are using rationals you require the fraction preserving 
behaviour. A numeric_cast to an integral type  can be used to solve this where 
necessary.

    Errors of using divides<A,B> on rationals will show at compile time.

    Thanks to Alexey Gurtovoy for the conversion mechanism

    As an example:

int main()
{
    // current division operator
    typedef boost::mpl::divides<
            boost::mpl::int_<2>,
            boost::mpl::int_<3>
    >::type int_div;

    // error no mpl::divides allowed for rationals
    /* typedef boost::mpl::divides<
            boost::mpl::rational_c<int,2>,
            boost::mpl::rational_c<int,3>
    >::type  XXX;*/

    // when some type might be a rational and rational behaviour rather than 
truncation is required:

    typedef boost::mpl::rational_divides<
            boost::mpl::rational_c<long,2>,
            boost::mpl::int_<3>
    >::type  rat_div1;
    //also works on int_'s etc...
    typedef boost::mpl::rational_divides<
            boost::mpl::int_<2>,
            boost::mpl::int_<3>
    >::type  rat_div2;

    typedef boost::mpl::rational_divides<
            boost::mpl::rational_int<2>,
            boost::mpl::rational_int<3>
    >::type  rat_div3;

    // also 'promotes' to int_ where possible
    typedef boost::mpl::rational_divides<
            boost::mpl::rational_int<210>,
            boost::mpl::rational_int<3>
    >::type  rat_div4;

    // also 'promotes' to int_ where possible
    typedef boost::mpl::rational_divides<
            boost::mpl::int_<210>,
            boost::mpl::integral_c<long,3>
    >::type  rat_div5;

    std::ofstream os("output.txt");

    os << typeid(int_div).name() <<'\n';
    os << typeid(rat_div1).name() <<'\n';
    os << typeid(rat_div2).name() <<'\n';
    os << typeid(rat_div3).name() <<'\n';
    os << typeid(rat_div4).name() <<'\n';
    os << typeid(rat_div5).name() <<'\n';

}
output: (VC7.1)

struct boost::mpl::integral_c<int,0>
struct boost::mpl::rational_c<long,2,3>
struct boost::mpl::rational_c<int,2,3>
struct boost::mpl::rational_c<int,2,3>
struct boost::mpl::int_<70>
struct boost::mpl::long_<70>

    regards
Andy Little