Reputation: 1961
I'd like to make a template that takes a pack of templates and instantiates them with the same argument pack.
Unfortunately I can't seem to figure out how to expand a parameter pack inside a parameter pack of templates.
How can I make this compile?
#include<type_traits>
#include <tuple>
template <template <typename...> typename... Args>
struct TupleTupleMaker
{
template <typename... InstantiateWith>
using NewTupleTuple = typename std::tuple<Args<InstantiateWith...>...>;
};
template<typename a, typename b>
using tuple1 = std::tuple<int,a,b>;
template<typename a, typename b>
using tuple2 = std::tuple<a,b,b>;
using expected = std::tuple<
std::tuple<int,int,double>,
std::tuple<int,double,double>>;
using actual = TupleTupleMaker<tuple1,tuple2>::NewTupleTuple<int,double>;
static_assert(std::is_same_v<actual,expected>, "Should be same");
Upvotes: 4
Views: 819
Reputation: 137414
Per the direction of core issue 1430, you can't pack expand into an alias template with a fixed parameter list. The workaround is to rewrite tuple1
and tuple2
by routing them through a class template:
template<class a, class b>
struct tuple1_impl { using type = std::tuple<int,a,b>; };
template<typename... a>
using tuple1 = typename tuple1_impl<a...>::type;
template<class a, class b>
struct tuple2_impl { using type = std::tuple<a,b, b>; };
template<typename... a>
using tuple2 = typename tuple2_impl<a...>::type;
Upvotes: 5