How to get the index of an element in a tuple via address?

Question

For example:

std::tuple<int, double> t;
void* p = &std::get<1>(t);

Now I want to get p's index by some function like

template<typename... Ts>
size_t index(void* p, std::tuple<Ts...> const& t)
{
   ...
}

Sure the result is 1 for this example. I am interested in how to implement the function to get the index when p is obtained by ways other than the explicit index.

Answer 1

Something you do not need in C++14, a mini indexes library:

template<unsigned...>struct indexes{using type=indexes;};
template<unsigned Cnt,unsigned...Is>
struct make_indexes:make_indexes<Cnt-1,Cnt-1,Is...>{};
template<unsigned...Is>
struct make_indexes<0,Is...>:indexes<Is...>{};
template<unsigned Cnt>
using make_indexes_t=typename make_indexes<Cnt>::type;

Function that does actual work. Creates an array pointers-to-elements. Then searches for p. The nullptr and -1 make it work for empty tuples.

template<unsigned...Is,class Tuple>
unsigned index_of(indexes<Is...>,void const* p, Tuple const&t){
  void const* r[]={ nullptr, &std::get<Is>(t)... };
  auto it = std::find( std::begin(r), std::end(r), p );
  if (it==std::end(r))
    return -1;
  else
    return (it-std::begin(r))-1;
}

you can put that in a details namespace.

The final function:

template<class...Ts>
unsigned index_of( void const*p, std::tuple<Ts...> const& t ){
  return index_of( make_indexes_t<sizeof...(Ts)>{}, p, t );
}

return unsigned(-1) on failure.