如何在可变参数模板中有多个参数包？

Question

函数 one() 接受一个参数包。函数 two() 接受两个。每个包都被限制在类型 A 和 B 中。为什么无法实例化 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);
}

尝试使用 gcc 和 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.

原文由 Samuel Danielson 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

two<>();
two<int>(a);
two<int>(a, b);

见 https://www.youtube.com/watch?v=va9I2qivBOA 29:00-35:00

原文由 Estin 发布，翻译遵循 CC BY-SA 4.0 许可协议