具有许多基本情况的模板递归

template recursion with many base cases

本文关键字：递归 基本情况 许多      更新时间：2023-10-16

目标是从另一个元组创建一个元组(并提取其类型)，方法是在位置START处获取其组件，然后跳过每个INTERVAL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<>();}
};

//……基本情况必须涵盖0，-1，-2，…降至-INTERVAL+1。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);
}

如何处理任意值INTERVAL的所有基本情况?代码可以正常工作，并且测试INTERVAL大小为10(是的，目前我正在使用宏来覆盖0，-1，-2，…)， -10，但这当然很臭)。

我试着

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<>();
    }
};

但是它不会编译，因为它与alternating_tuple_type在某种程度上冲突(这是提取类型)。也许只有那部分需要修理。

您可以使用以下技术创建一个逻辑测试:

#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};
};

生活例子

要美化模板签名，您可以将void hack粘贴到实现细节命名空间中，并将公共接口转发到那里。

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