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c++boostc++14boost-hana

boost::hana tuple unpacking for variadic template instantiation

Related to this question I was wondering if something like this would be achievable in a straightforward way using boost::hana:

#include <boost/hana.hpp>
#include <boost/hana/unpack.hpp>

namespace hana = boost::hana;

template<typename ... T>
struct A {};

int main() {

  auto my_tuple = hana::tuple_t<int, double, float>;

  // Is there any way to unpack my_tuple as template arguments for A?
  // Something like
  using MyStruct = A<hana::unpack_types(my_tuple)...>;

  static_assert(std::is_same<MyStruct, A<int, double, float>>::value, "Ooops!");
}
Solution

  • Use template_ to lift A into a metafunction, then call unpack:

    using MyStruct = decltype(hana::unpack(my_tuple, hana::template_<A>))::type;

    Example.