c++ - Plantillas variadas variadas

c++11 metaprogramming

Question

Actualmente estoy luchando con el siguiente código, cuyo objetivo es implementar plantillas de plantilla variadic variadic:

template
<
  template <typename... HeadArgs> class Head,
  template <typename... TailArgs> class...
>
struct join<Head<typename HeadArgs...>, Head<typename TailArgs...>...>
{
  typedef Head<typename HeadArgs..., typename TailArgs......> result;
};

Idealmente, podría usar esta metafunción de plantilla para lograr lo siguiente:

template <typename...> struct obj1 {};
template <typename...> struct obj2 {};

typedef join
<
  obj1<int, int, double>, 
  obj1<double, char>,
  obj1<char*, int, double, const char*>
>::result new_obj1;

typedef join
<
  obj2<int, int, double>, 
  obj2<double, char>,
  obj2<char*, int, double, const char*>
>::result new_obj2;

/* This should result in an error, because there are 
   different encapsulating objects
typedef join
<
  obj1<int, int, double>, 
  obj1<double, char>,
  obj2<char*, int, double, const char*>
>::result new_obj;
*/

Se espera que la salida de lo anterior cree new_obj1 y new_obj2 en la template<int, int, double, double, char, char*, int, double, const char*> struct new_obj[1|2] {}; formulario template<int, int, double, double, char, char*, int, double, const char*> struct new_obj[1|2] {};

Estoy usando gcc 4.6.2 en Windows, que genera un "paquete de parámetros esperado antes de ''...''" para la expansión de " Head<typename TailArgs...> ...".

Este error es reproducible con gcc 4.5.1 .

Answer 1

Intenta algo como esto:

template <typename...> struct join;

template <template <typename...> class Tpl,
          typename ...Args1,
          typename ...Args2>
struct join<Tpl<Args1...>, Tpl<Args2...>>
{
    typedef Tpl<Args1..., Args2...> type;
};

template <template <typename...> class Tpl,
          typename ...Args1,
          typename ...Args2,
          typename ...Tail>
struct join<Tpl<Args1...>, Tpl<Args2...>, Tail...>
{
     typedef typename join<Tpl<Args1..., Args2...>, Tail...>::type type;
};