Create tuple of vectors from a Typelist

Question

I have a simple typelist implementation;

template<typename... Ts> 
struct Typelist
{
  static constexpr size_t count{sizeof...(Ts)};
};

What I want to do with it, is for generate an std::tuple of std::vector> for every type in the typelist; for example:

struct A {};
struct B {};
struct C {};

using myStructs = typelist<A,B,C>;
using myList = tupleOfVectorTypes<myStructs>; tuple<vector<A>, vector<B>, vector<C>>

This is what I've been playing around with:

template<template<typename... Ts> class T>
struct List
{
  using type = std::tuple<std::vector<Ts>...>;
};

However, it keeps spitting back that it expects a type. I've tried wrapping Ts in decltype, like so:

using type = std::tuple<std::vector<decltype(Ts)>...>;

But that's wrong as well, and I'm guessing I'm using decltype incorrectly as well. So, how can I create a tuple of vectors of types, based off the typelist I throw it?

Answer 1

The trick is to use specialization to drill down to the template parameters.

Tested with gcc 5.3.1 in -std=c++1z mode:

#include <vector>
#include <tuple>

template<typename... Ts>
struct Typelist{
};

// Declare List
template<class> class List;

// Specialize it, in order to drill down into the template parameters.
template<template<typename...Args> class t, typename ...Ts>
struct List<t<Ts...>> {
    using type = std::tuple<std::vector<Ts>...>;
};

// Sample Typelist

struct A{};
struct B{};
struct C{};

using myStructs = Typelist<A,B,C>;

// And, the tuple of vectors:

List<myStructs>::type my_tuple;

// Proof

int main()
{
    std::vector<A> &a_ref=std::get<0>(my_tuple);
    std::vector<B> &b_ref=std::get<1>(my_tuple);
    std::vector<C> &c_ref=std::get<2>(my_tuple);
    return 0;
}

Answer 2

Here is another way to achieve what you want. It relies on the power of functions:

#include <cstddef>
#include <tuple>
#include <vector>
#include <utility>

template<typename... Ts> 
struct Typelist
{
  static constexpr size_t count{sizeof...(Ts)};
};

template<class... ARGS>
std::tuple<std::vector<ARGS>... > typelist_helper(Typelist<ARGS...>);

template<class T> 
using vectorOfTuples = decltype(typelist_helper(std::declval<T>()));

struct A{};
struct B{};
struct C{};

using testlist = Typelist<A, B, C>;
vectorOfTuples<testlist> vec;