![](/img/trans.png)
[英]C++ transform std::pair<std::pair<std::pair<A, B>, C>, D> to std::tuple<A, B, C, D>
[英]Transform std::pair to std::tuple with any number of elements
我需要將以下元代碼從std::pair
改編為具有任意數量元素的std::tuple
。 我不想為每個可能的元素數量單獨實現它。
template<typename A, typename B>
struct merge_pairs
{ typedef std::pair<
decltype(typename A::first() + typename B::first()),
decltype(typename A::second() + typename B::second())
> type;
};
#include <cstddef>
#include <type_traits>
#include <utility>
#include <tuple>
template <typename A, typename B>
struct merge_tuples
{
static_assert(std::tuple_size<A>::value == std::tuple_size<B>::value, "!");
template <std::size_t... Is>
static auto merge(std::index_sequence<Is...>) noexcept
-> std::tuple<typename std::decay<decltype(std::declval<typename std::tuple_element<Is, A>::type>()
+ std::declval<typename std::tuple_element<Is, B>::type>())
>::type...>;
using type = decltype(merge(std::make_index_sequence<std::tuple_size<A>::value>{}));
};
#include <type_traits>
#include <utility>
template <typename A, typename B>
struct merge_tuples;
template <template <typename...> class Tuple, typename... Ts, typename... Us>
struct merge_tuples<Tuple<Ts...>, Tuple<Us...>>
{
static_assert(sizeof...(Ts) == sizeof...(Us), "!");
using type = Tuple<typename std::decay<decltype(std::declval<Ts>()
+ std::declval<Us>())
>::type...>;
};
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