Unpacking a std tuple into pointers?

Question

Suppose I have a tuple

std::tuple<A, B, C> myFavoriteTuple;

I can do this:

A a;
B b;
C c;
std::tie(a, b, c) = myFavoriteTuple

But if some of these tuples are really expensive to copy, what I'd really like is to take a reference or a pointer to the right spots in my tuple. I can do this:

A* a = &std::get<0>(myFavoriteTuple);
B* b = &std::get<1>(myFavoriteTuple);
C* c = &std::get<2>(myFavoriteTuple);

But that seems so lame compared to how awesome the tie syntax is. Is there any other way to get pointers/references to the tuple components?

Answer 1

Given the usual indices infrastructure:

template<int... Is>
struct seq { };

template<int N, int... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...> { };

template<int... Is>
struct gen_seq<0, Is...> : seq<Is...> { }; 

You could create a function that returns a tuple of pointers, each element being a pointer to the corresponding element of the input tuple:

template<typename... Args, int... Is>
auto make_pointer_tuple(std::tuple<Args...>& t, seq<Is...>)
    -> std::tuple<typename std::add_pointer<Args>::type...>
{
    return std::make_tuple(&std::get<Is>(t)...);
}

template<typename... Args>
auto make_pointer_tuple(std::tuple<Args...>& t)
    -> std::tuple<typename std::add_pointer<Args>::type...>
{
    return make_pointer_tuple(t, gen_seq<sizeof...(Args)>());
}

And this is how you could use it:

std::tuple<int, bool, std::string> myFavoriteTuple{0, false, ""};

int* pInt = nullptr;
bool* pBool = nullptr;
std::string* pString = nullptr;
tie(pInt, pBool, pString) = make_pointer_tuple(myFavoriteTuple);

Here is a live example.