如何通过非常数变量访问元组元素？

Question

我有一个元组，我想通过一个变量访问元素，我如何才能做到这一点？现在我使用switch case来解决这个问题。如下所示：

unsigned short& indices(unsigned char samecnt) {
        
        switch (samecnt)
        {
        case 12:
            return std::get<12>(view).indices;
        case 11:
            return std::get<11>(view).indices;
        }
}

元组是view_rep：

template<unsigned char N>
struct poker_view
{
    unsigned short  indices = 0;
    constexpr static unsigned char count = N;
};

using wilds_view = poker_view<1>;
template<int N, class... Types>
struct poker_view_rep :public poker_view_rep<N - 1, poker_view<N - 1>, Types...> {

};
template<class... Types>
struct poker_view_rep<1, Types...>
{
    using type = std::tuple<wilds_view, Types...>;
};
using view_rep = poker_view_rep<13>::type;

Answer 1

实际上，您不能将非常数表达式用作模板参数，因此您必须通过一些分派将运行时值“转换”为constexpr。switch是另一种选择。

在您的示例中，您可以使用array：

unsigned short& indices(std::index_sequence<Is...>, unsigned char samecnt) {
    std::array<unsigned short*, sizeof...(Is)> arr{{&std::get<Is>(view).indices...}};
    return *arr[samecnt];
}

unsigned short& indices(std::index_sequence<Is...>, unsigned char samecnt) {
    return indices(std::make_index_sequence<13>(), samecnt);
}

或使用std::apply

unsigned short& indices(std::index_sequence<Is...>, unsigned char samecnt) {
    return std::apply([](auto&... args) -> unsigned short&
               { return std::array{{&args.indices...}}[samecnt]; }, view);
}