具有多个基本情况的模板递归

Question

目标是从另一个元组创建元组(并提取其类型)，方法是在位置开始时获取元组的组件，然后跳过每个间隔组件。甚至不需要参数包：

template <int NUM_LEFT, int START, int INTERVAL>
struct alternating_tuple_helper {
    template <typename TUPLE>
    auto operator() (const TUPLE& t) const
    -> decltype (std::tuple_cat (std::make_tuple (std::get<START>(t)), alternating_tuple_helper<NUM_LEFT - INTERVAL, START + INTERVAL, INTERVAL>()(t))) {
        return std::tuple_cat (std::make_tuple (std::get<START>(t)), alternating_tuple_helper<NUM_LEFT - INTERVAL, START + INTERVAL, INTERVAL>()(t));
    }
};

template <int START, int INTERVAL>
struct alternating_tuple_helper<0, START, INTERVAL> { // first base case   
    template <typename TUPLE>
    std::tuple<> operator() (const TUPLE& t) const {return std::tuple<>();}
};

template <int START, int INTERVAL>
struct alternating_tuple_helper<-1, START, INTERVAL> { // second base case
    template <typename TUPLE>
    std::tuple<> operator() (const TUPLE& t) const {return std::tuple<>();}
};

// ...基本情况必须包括0，-1，-2，...深入到-INTERVAL+1，该怎么做呢？

template <typename TUPLE, int START = 0, int INTERVAL = 2>  // extracting the type
struct alternating_tuple_type {
    using type = decltype (alternating_tuple_helper<std::tuple_size<TUPLE>::value - START, START, INTERVAL>()(std::declval<TUPLE>()));
};

template <typename TUPLE, int START = 0, int INTERVAL = 2>  // getting the new tuple itself
typename alternating_tuple_type<TUPLE, START, INTERVAL>::type alternating_tuple (const TUPLE& t) {
    return alternating_tuple_helper<std::tuple_size<TUPLE>::value - START, START, INTERVAL>()(t);
}

如何处理所有具有任意值区间的基本情况？代码在其他情况下工作，并测试间隔大小为10 (是的，目前我正在使用宏来覆盖0，-1，-2，...，-10，但这当然很糟糕)。

我试过了

template <int NUM_LEFT, int START, int INTERVAL>
struct alternating_tuple_helper {
    template <typename TUPLE>
        auto operator() (const TUPLE& t) const 
        -> std::conditional<(NUM_LEFT > 0), decltype (std::tuple_cat (std::make_tuple (std::get<START>(t)), alternating_tuple_helper<NUM_LEFT - INTERVAL, START + INTERVAL, INTERVAL>()(t))), std::tuple<>> {
            return (NUM_LEFT > 0) ? std::tuple_cat (std::make_tuple (std::get<START>(t)), alternating_tuple_helper<NUM_LEFT - INTERVAL, START + INTERVAL, INTERVAL>()(t))
                : std::tuple<>();
    }
};

但它无法编译，因为它以某种方式与alternating_tuple_type冲突(这是为了提取类型)。也许那时只需要修复这一部分。但我不确定问题到底出在哪里。

Answer 1

您可以使用此技术创建逻辑测试：

#include <utility>

template<int n, int m, class=void>
struct count{
  enum{value=1 +count<n-m, m>::value};
};
template<int n, int m>
struct count<n,m, typename std::enable_if<(n<m)>::type>{
  enum{value=0};
};

live example

为了美化模板签名，您可以将void hack放在一个实现细节命名空间中，并将公共接口转发到那里。