如何将 boost::hana::tuple 转换为 std::variant

Question

I have the following code where I want to convert a boost::hana::tuple into a std::variant

namespace hana = boost::hana;

template <typename Tuple>
struct to_variant;

template <typename... Ts>
struct to_variant<std::tuple<Ts...>>
{
    using type = std::variant<Ts...>;
};

auto my_tuple = hana::make_tuple(hana::type_c<int>, hana::type_c<char>, hana::type_c<float>);

using my_variant = typename to_variant<my_tuple>::type;

But I always get the error message

error: type/value mismatch at argument 1 in template parameter list for 'template<class Tuple> struct to_variant'
using my_variant = typename to_variant<my_tuple>::type;

I tried to replace std::tuple with hana::tuple with the same result.

Answer 1

my_tuple is an object, but not type . (And its type is not std::tuple but boost::hana::tuple which should be used for the specialization.)

I think you want

template <typename Tuple>
struct to_variant;

template <typename... Ts>
struct to_variant<boost::hana::tuple<Ts...>>
//                ^^^^^^^^^^^^^^^^^^
{
    using type = std::variant<Ts...>;
};

auto my_tuple = hana::make_tuple(hana::type_c<int>, hana::type_c<char>, hana::type_c<float>);

using my_variant = typename to_variant<decltype(my_tuple)>::type;
//                                     ^^^^^^^^^        ^

Answer 2

I suggest three corrections in your code (two are already addressed in the other answer):

• Use decltype(my_tuple) to pass a type into to_variant
• Inside to_variant , use using type = std::variant<typename Ts::type...> to extract the types out of the Hana type_c 's.
• Use a hana::tuple in the specialization, not a std::tuple . More in general, you could also use a template template parameter here.

Here is the corresponding code:

namespace hana = boost::hana;

template <typename Tuple>
struct to_variant;

template <typename... Ts>
struct to_variant<hana::tuple<Ts...>>
{
    using type = std::variant<typename Ts::type...>;
};

auto my_tuple = hana::make_tuple(hana::type_c<int>, hana::type_c<char>, hana::type_c<float>);

using my_variant = typename to_variant<decltype(my_tuple)>::type;

Answer 3

With Boost.Mp11 , this is pretty short (as always):

template <typename T> using unwrap = typename T::type;

template <typename T>
using to_variant = mp_rename<mp_transform<unwrap, T>, std::variant>;

Where you would use this as to_variant<decltype(my_tuple)> .

Basically, we first unwrap all the types (this gets us from something like hana::tuple<hana::type_impl<int>::_, hana::type_impl<char>::_, ...> to just hana::tuple<int, char, ...> ) and then we rename the top-level template from hana::tuple to std::variant .