Re: [boost] Userfriendly interaction of tuples with mpl containers

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2004-01-15 06:04:46

Daniel Krügler wrote:
> Hi boosters,

Hi Daniel,

>
> I am just doing first steps in using the boost::mpl library, which is
> really great.
> Obviously due to my ignorance of knowing it not better, I tried the
> following:
>
> #include <boost/mpl/vector.hpp>
> #include <boost/mpl/size.hpp>
> #include <boost/mpl/void.hpp>
> #include <boost/tuple/tuple.hpp>
>
> template<
>     typename T0 = boost::mpl::void_, typename T1 = boost::mpl::void_,
>     typename T2 = boost::mpl::void_, typename T3 = boost::mpl::void_,
>     typename T4 = boost::mpl::void_, typename T5 = boost::mpl::void_,
>     typename T6 = boost::mpl::void_, typename T7 = boost::mpl::void_,
>     typename T8 = boost::mpl::void_, typename T9 = boost::mpl::void_
>     >
> class Test
> {
>     typedef boost::mpl::vector<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9>
>         InternalTypeContainer;
>
>     typedef boost::tuples::tuple<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9>
> Tuples; // Fragile code!!
>     Tuples m_Tuple;
> };
>
> typedef Test<double, char, bool> MyTypes;
>
> As you will belief, that will not compile successfully, because
> boost::mpl::void_ is
> an undefined type (similar to void) and I am trying to fill my tuple
> partially with undefined
> types. So my problem is:
>
> Does there exist a boost::mpl way (?) to define my
> boost::tuples::tuple<> inside the given
> Test class template until the last non-mpl::void_ is filled into it?
> (Iteration until end??)

This won't give you an exact equivalent of
'boost::tuples::tuple<T0,...,T9>',
but for many practical purposes it's close enough:

    template< typename Types > struct tuple_gen
        : mpl::fold_backward<
              Types
            , boost::tuples::null_type
            , boost::tuples::cons<_2,_1>
            >
    {
    };

    template<
        typename T0 = boost::mpl::void_, typename T1 = boost::mpl::void_,
        typename T2 = boost::mpl::void_, typename T3 = boost::mpl::void_,
        typename T4 = boost::mpl::void_, typename T5 = boost::mpl::void_,
        typename T6 = boost::mpl::void_, typename T7 = boost::mpl::void_,
        typename T8 = boost::mpl::void_, typename T9 = boost::mpl::void_
        >
    class Test
    {
        typedef mpl::vector<T0,T1,T2,T3,T4,T5,T6,T7,T8,T9> types_;

        typedef typename tuple_gen<
              mpl::filter_view<
                  types_
                , mpl::not_< mpl::is_void_<_1> >
                >
            >::type Tuples;

        Tuples m_Tuple;
    };

>
> Or does their exist another tuple class, which I should try to use at
> this place?

Joel de Guzman's Fusion library (http://tinyurl.com/yvxzm) has the one that
is about to replace the current implementation, and much more. In general,
Fusion is definitely a must use tool if you are doing anything more than a
very basic metaprogramming with tuples.

HTH,
--
Aleksey Gurtovoy
MetaCommunications Engineering