c++ - How can I have multiple parameter packs in a variadic template?

Question

Function one() accepts one parameter pack. Function two() accepts two. Each pack is constrained to be wrapped in types A and B. Why is it impossible to instantiate two()?

template <typename T>
struct A {};

template <typename T>
struct B {};

template <typename... Ts>
void one(A<Ts> ...as) {
}

template <typename... Ts, typename... Us>
void two(A<Ts> ...as, B<Us> ...bs) {
}

int main() {
  auto a = A<int>();
  auto b = B<int>();

  // Just fine
  one();
  one(a);
  one(a, a);

  // All errors    
  two();
  two(a);
  two(a, b);
}

Tried with gcc and clang.

sam@wish:~/x/cpp$ gcc -std=c++0x variadic_templates.cpp 
variadic_templates.cpp: In function ‘int main()’:
variadic_templates.cpp:23:7: error: no matching function for call to ‘two()’
variadic_templates.cpp:23:7: note: candidate is:
variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...)
variadic_templates.cpp:24:8: error: no matching function for call to ‘two(A<int>&)’
variadic_templates.cpp:24:8: note: candidate is:
variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...)
variadic_templates.cpp:25:11: error: no matching function for call to ‘two(A<int>&, B<int>&)’
variadic_templates.cpp:25:11: note: candidate is:
variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...)
sam@wish:~/x/cpp$ clang -std=c++0x variadic_templates.cpp 
variadic_templates.cpp:23:3: error: no matching function for call to 'two'
  two();
  ^~~
variadic_templates.cpp:11:6: note: candidate function template not viable: requires at least 1 argument, but 0 were provided                                                                                                                 
void two(A<Ts> ...as, B<Us> ...bs) {}
     ^
variadic_templates.cpp:24:3: error: no matching function for call to 'two'                                                                                                                                                                   
  two(a);
  ^~~
variadic_templates.cpp:11:6: note: candidate function not viable: requires 0 arguments, but 1 was provided                                                                                                                                   
void two(A<Ts> ...as, B<Us> ...bs) {}
     ^
variadic_templates.cpp:25:3: error: no matching function for call to 'two'                                                                                                                                                                   
  two(a, b);
  ^~~
variadic_templates.cpp:11:6: note: candidate function not viable: requires 0 arguments, but 2 were provided                                                                                                                                  
void two(A<Ts> ...as, B<Us> ...bs) {}
     ^
3 errors generated.

Answer 1

Here is another way to have several parameters packs using template template parameters:

#include <iostream>

template <typename... Types>
struct foo {};

template < typename... Types1, template <typename...> class T
         , typename... Types2, template <typename...> class V
         , typename U >
void
bar(const T<Types1...>&, const V<Types2...>&, const U& u)
{
  std::cout << sizeof...(Types1) << std::endl;
  std::cout << sizeof...(Types2) << std::endl;
  std::cout << u << std::endl;
}

int
main()
{
  foo<char, int, float> f1;
  foo<char, int> f2;
  bar(f1, f2, 9);
  return 0;
}