Re: [boost] A C++ puzzler

From: JOAQUIN M. LOPEZ MUÑOZ (joaquin_at_[hidden])
Date: 2008-04-13 08:30:16

________________________________________
De: boost-bounces_at_[hidden] [boost-bounces_at_[hidden]] En nombre de Daniel Frey [d.frey_at_[hidden]]
Enviado el: domingo, 13 de abril de 2008 13:43
Para: boost_at_[hidden]
Asunto: Re: [boost] A C++ puzzler

On Sun, 2008-04-13 at 12:51 +0200, Daniel Frey wrote:
> > The new version (attached) has only 4 overloads instead of N, but it
> > still uses typeof.
>
> OK, another version. No typeof and O(1), due to the fact that in the
> good case the common_type_helper must always be instantiated with the
> same T, so the number of template instantiations doesn't grow, it has a
> constant limit. The bad case will fail anyway, but it might need some
> instantiations before finding out. I hope that this solution is
> acceptable for you.

Instead of having nested common_type_helper invokations in the
common type calculation formula, you can do it linearly like this:

  template<
    typename T0, typename T1, typename T2, typename T3,
    typename T4, typename T5, typename T6, typename T7 >
  struct common_type
  {
    typedef typename common_type_helper<T0,T1>::type t0;
    typedef typename common_type_helper<t0,T2>::type t1;
    typedef typename common_type_helper<t1,T3>::type t2;
    typedef typename common_type_helper<t2,T4>::type t3;
    typedef typename common_type_helper<t3,T5>::type t4;
    typedef typename common_type_helper<t4,T6>::type t5;
    typedef typename common_type_helper<t5,T7>::type t6;
    typedef t6 type;
  };

which has the advantage of being much more easily automatable
via Boost.Preprocessor. Boost.PP-generated modified version
attached.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo