Problem z boost::variant
jest to, że nie jest prawdziwe o zmiennej liczbie argumentów szablonu tylko jedną symulację dodając szereg parametrów szablonu z domyślnej wartości boost::detail::variant::void_
. Aby pracować z nim tak jak przy szablonie variadic, trzeba go najpierw zmienić na pakiet z odpowiednimi typami. Pełna gotowość do użycia C++ 11 do robienia tego, co chcesz, może wyglądać następująco. Podejście uwzględnia zarówno płaskość, jak i założenie unikalności dla typu wyniku. integer_sequence
część będzie konieczne, jeśli użytkownik zdecyduje się użyć C++ 14:
#include <boost/variant.hpp>
#include <tuple>
#include <type_traits>
template <class T, T... Vs>
struct integer_sequence { };
template <class T, class, class, class = integer_sequence<T>, class = integer_sequence<T, 0>, class = void>
struct make_integer_sequence_impl;
template <class T, T ICV1, T... Res, T... Pow>
struct make_integer_sequence_impl<T, std::integral_constant<T, ICV1>, std::integral_constant<T, 0>, integer_sequence<T, Res...>, integer_sequence<T, Pow...>, typename std::enable_if<(ICV1 > 0)>::type>: make_integer_sequence_impl<T, std::integral_constant<T, ICV1/2>, std::integral_constant<T, ICV1%2>, integer_sequence<T, Res...>, integer_sequence<T, Pow..., (Pow + sizeof...(Pow))...>> { };
template <class T, T ICV1, T... Res, T... Pow>
struct make_integer_sequence_impl<T, std::integral_constant<T, ICV1>, std::integral_constant<T, 1>, integer_sequence<T, Res...>, integer_sequence<T, Pow...>, void>: make_integer_sequence_impl<T, std::integral_constant<T, ICV1/2>, std::integral_constant<T, ICV1%2>, integer_sequence<T, Pow..., (Res + sizeof...(Pow))...>, integer_sequence<T, Pow..., (Pow + sizeof...(Pow))...>> { };
template <class T, class Res, class Pow>
struct make_integer_sequence_impl<T, std::integral_constant<T, 0>, std::integral_constant<T, 0>, Res, Pow, void> {
using type = Res;
};
template <class T, T V>
using make_integer_sequence = typename make_integer_sequence_impl<T, std::integral_constant<T, V/2>, std::integral_constant<T, V%2>>::type;
template <class>
struct is_variant_void {
static constexpr size_t value = 0;
};
template <>
struct is_variant_void<boost::detail::variant::void_> {
static constexpr size_t value = 1;
};
constexpr size_t sum(size_t v){
return v;
}
template <class... Args>
constexpr size_t sum(size_t v, Args... vs){
return v + sum(vs...);
}
template <class Variant>
struct variant_size;
template <class... Ts>
struct variant_size<boost::variant<Ts...>> {
static constexpr size_t value = sizeof...(Ts) - sum(is_variant_void<Ts>::value...);
};
template <class...>
struct pack { };
template <class V, class=make_integer_sequence<size_t, variant_size<V>::value>>
struct variant_to_pack;
template <class... Ts, size_t... Is>
struct variant_to_pack<boost::variant<Ts...>, integer_sequence<size_t, Is...>> {
using type = pack<typename std::tuple_element<Is, std::tuple<Ts...>>::type...>;
};
template <class>
struct unique_opaque { };
template <class... Ts>
struct unique_pack: unique_opaque<Ts>... { };
template <class, class, class = void>
struct unique;
template <class... Res, class T, class... Ts>
struct unique<unique_pack<Res...>, pack<T, Ts...>, typename std::enable_if<std::is_base_of<unique_opaque<T>, unique_pack<Res...>>::value>::type>:
unique<unique_pack<Res...>, pack<Ts...>> { };
template <class... Res, class T, class... Ts>
struct unique<unique_pack<Res...>, pack<T, Ts...>, typename std::enable_if<!std::is_base_of<unique_opaque<T>, unique_pack<Res...>>::value>::type>:
unique<unique_pack<Res..., T>, pack<Ts...>> { };
template <class... Res>
struct unique<unique_pack<Res...>, pack<>, void> {
using type = boost::variant<Res...>;
};
template <class...>
struct flatten;
template <class... Res, class T, class... Rest>
struct flatten<pack<Res...>, T, Rest...>: flatten<pack<Res..., T>, Rest...> { };
template <class... Res, class... Vs, class... Rest>
struct flatten<pack<Res...>, boost::variant<Vs...>, Rest...>: flatten<pack<Res...>, typename variant_to_pack<boost::variant<Vs...>>::type, Rest...> {};
template <class... Res, class... Vs, class... Rest>
struct flatten<pack<Res...>, pack<Vs...>, Rest...>: flatten<pack<Res...>, Vs..., Rest...> { };
template <class Res>
struct flatten<Res>:unique<unique_pack<>, Res> { };
int main() {
boost::variant<int, boost::variant<float, int, boost::variant<char, bool>, bool>> vif;
static_assert(std::is_same<flatten<pack<>, decltype(vif)>::type, boost::variant<int, float, char, bool>>::value, "Test");
}
można używać C++ 17/14/11? –
Mogę użyć co najmniej C++ 11. – Aleph0
@FrankSimon zobacz edytowaną odpowiedź –