Convert constexpr struct to a runtime one - c++

Question

I'm trying to use a template class (here Foo), with a basic type like:
hana::tuple<hana::pair<hana::type<int>, Runtime>> with Runtime a class which obiviously can't be constepxr.
But the type can be construct in several way that's why I use:
hana::tuple<hana::pair<hana::type<int>, hana::type<Runtime>>> to do the work at compile time.
So the question is basically how convert from the first tuple type to the second one. I wonder if there is something in hana that could help me. Or even better, some tips on that kind of "conversion".
namespace hana = boost::hana;
using namespace hana::literals;
struct Runtime { std::vector<int> data; };
template < typename T >
struct Foo {
T data;
};
constexpr decltype(auto) convertMap(auto storageMap) {
return hana::make_type(hana::transform(
storageMap,
[] (auto pair) {
return hana::make_pair(
hana::first(pair),
typename decltype(hana::typeid_(hana::second(pair)))::type {});
}));
}
int main() {
constexpr auto map = hana::make_tuple(
hana::make_pair(hana::type_c<int>, hana::type_c<Runtime>)
);
constexpr auto result = convertMap(map);
static_assert(result ==
hana::type_c<hana::tuple<hana::pair<hana::type<int>, Runtime>>>);
Foo<typename decltype(result)::type> test;
}
As you can see I tried some c++1z convertMap with hana::transform and lambdas, but the second tuple can't be constexpr, so I can't pass it to hana::make_type hoping to get a hana::type_c.
test.cpp: In function ‘int main()’:
test.cpp:70:41: error: ‘constexpr decltype(auto) convertMap(auto:27) [with auto:27 = boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, boost::hana::type_impl<Runtime>::_> >]’ called in a constant expression
constexpr auto result = convertMap(map);
^
test.cpp:53:27: note: ‘constexpr decltype(auto) convertMap(auto:27) [with auto:27 = boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, boost::hana::type_impl<Runtime>::_> >]’ is not usable as a constexpr function because:
constexpr decltype(auto) convertMap(auto storageMap) {
^~~~~~~~~~
test.cpp:53:27: error: temporary of non-literal type ‘boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, Runtime> >’ in a constant expression
In file included from /usr/include/boost/hana/detail/struct_macros.hpp:29:0,
from /usr/include/boost/hana/adapt_adt.hpp:15,
from lib/hana/include/boost/hana.hpp:59,
from test.cpp:1:
/usr/include/boost/hana/tuple.hpp:68:12: note: ‘boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, Runtime> >’ is not literal because:
struct tuple
^~~~~
/usr/include/boost/hana/tuple.hpp:68:12: note: ‘boost::hana::tuple<boost::hana::pair<boost::hana::type_impl<int>::_, Runtime> >’ has a non-trivial destructor

Answer 1

All you care about is a type - you can therefore hide the computation of the type in a non-constexpr implementation function, and afterwards call it with decltype(...){} to "force constexpr":
template <typename T>
decltype(auto) convertMapImpl(T storageMap)
{
return hana::make_type(hana::transform(storageMap, [](auto pair) {
return hana::make_pair(hana::first(pair),
typename decltype(hana::typeid_(hana::second(pair)))::type{});
}));
}
template <typename T>
constexpr decltype(auto) convertMap(T storageMap)
{
return decltype(convertMapImpl(storageMap)){};
}
live wandbox example
Also note that using auto in a function signature is a gcc extension - you should use a template parameter instead to be standard-compliant.