Variadic模板中的函数顺序

Question

The following code is working! But my question is theoretical if we inverse the two version of index_of in the namespace detail, the compiler says that no function template is matching but why?

#include <iostream>
using namespace std;


template <typename... Args>
class Pack{
public:
  Pack(){}
};

template <typename T, typename Arg, typename... Args>
bool contains(Pack<Arg, Args...> p)
{
  Pack<Args...> n;
  return std::is_same_v<T,Arg> || contains<T>(n);
}

template <typename T>
bool contains(Pack<> p){
  return false;
}

namespace detail{


  template <typename T>
  int index_of(Pack<> p, int index){
    return -1;
  }

  template <typename T, typename Arg, typename... Args>
  int index_of(Pack<Arg, Args...> p, int index = 0){
    if(is_same_v<T,Arg>) return index;
    Pack<Args...> rest;
    return index_of<T>(rest, index+1);
  }

}

template <typename T, typename... Ts>
int index_of(Pack<Ts...> p)
{
  return detail::index_of<T>(p);
}

int main()
{
  Pack<int,string,double> p;
  cout << contains<int>(p) << endl;
  cout << contains<char>(p) << endl;
  cout << index_of<int>(p) << endl;
}

Answer 1

The second index_of itself calls index_of . If it's listed first, then the other index_of is not visible, so it can only be a recursive call; but the recursive call is not viable once only one type remains.

When it's listed second, both versions of index_of are in scope for that call.

Basically, it boils down to the fact that a function has to be declared before it can be called.