c++ - 具有许多基本情况的模板递归

Question

目标是通过在位置 START 获取其组件然后跳过每个 INTERVAL 组件，从另一个元组创建一个元组(并提取其类型)。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、...一直到 -INTERVAL+1。如何做到这一点？

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);
}

如何处理具有任意值 INTERVAL 的所有基本情况？代码以其他方式工作，并测试了最大 10 的 INTERVAL 大小(是的，目前我正在使用宏来覆盖 0、-1、-2、...、-10，但当然这很糟糕)。

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<>();
    }
};

但它不会编译，因为它以某种方式与 alternating_tuple_type 冲突(这是为了提取类型)。也许只有那部分需要修复。但我不确定到底是什么问题。

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 粘贴到实现细节命名空间中，并将公共(public)接口(interface)转发到那里。