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[英]Get number of 'nested' template arguments of a template type in c++
[英]In C++ template arguments, is it possible to specify the type and number of occurrence of that type?
我在C ++中有一個可變參數模板類(類似於std::tuple
)。 在實例化類時,我需要鍵入很多類型,例如
MyClass<int, int, double, double, double> my_obj;
當類型數量很少時,此方法有效。 但是,如果我有10個ints
后跟20個doubles
,那么輸入它會很麻煩並且容易出錯。
C ++中是否存在機制,在template <>
參數中指定類型后跟出現次數? 像這樣的東西:
MyClass<some_magic(int,2), some_magic(double, 3)> my_obj;
這將使它:
namespace detail{
template <class T, auto> using always_t = T;
template <class T, std::size_t... Idx>
auto repeat_impl(std::index_sequence<Idx...>) -> std::tuple<always_t<T, Idx>...>;
template <class T, class...>
struct pack
{
using type = T;
};
template <class... T, class... R, class... Tuple>
struct pack<std::tuple<T...>, std::tuple<R...>, Tuple...>
: pack<std::tuple<T..., R...>, Tuple...>
{ };
template <class> struct tuple_to_class;
template <class... T> struct tuple_to_class<std::tuple<T...>>
{
using type = MyClass<T...>;
};
}
template <class T, std::size_t N>
using repeat_t = decltype(detail::repeat_impl<T>(std::make_index_sequence<N>{}));
template <class... Repeats>
using ToMyClass = typename detail::tuple_to_class<typename detail::pack<Repeats...>::type>::type;
using C = MyClass<int, int, double, double, double>;
static_assert(std::is_same_v<C,
ToMyClass<repeat_t<int, 2>, repeat_t<double, 3>>
>);
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