c++ - 多次解包参数包

Question

是否可以多次解压一个参数包？

例如:我想获得一个包含 4 个 vector 的元组 - 2 个为 int 类型，2 个为 float 类型。要创建这样一个元组，我想使用如下语法:

ExampleClass<2, int, float> class;

是否可以创建这样的类？我在想这样的事情:

template <int numUnpacks, typename ... Types>
class ExampleClass
{
     using Types = std::tuple<std::vector<Types>...>; // here i don't know how to unpack "std::vector<Types>...>" as often as "numUnpacks">
}

Answer 1

这可以使用 C++14 的 integer_sequence 来完成.如果您没有可用的，这里有一个 implementation通过 Jonathan Wakely .

template <int numUnpacks, typename... Types>
struct ExampleClass
{
    template<typename T, size_t>
    using T_alias = T;

    template<typename T, size_t N, typename I = std::make_index_sequence<N>>
    struct repeat;

    template<typename T, size_t N, size_t... I>
    struct repeat<T, N, std::index_sequence<I...>> {
        using type = decltype(std::tuple_cat(std::declval<T_alias<T, I>>()...));
    };

    using type = typename repeat<std::tuple<std::vector<Types>...>, numUnpacks>::type;
};

ExampleClass<2, int, float>::type是std::tuple<std::vector<int>, std::vector<float>, std::vector<int>, std::vector<float>>

Live demo