递归可迭代模板函数 C++

以下是我如何处理它的草图：

#include <iostream>
#include <algorithm>
#include <type_traits>
#include <sstream>
#include <vector>
#include <tuple>
// this allows us to test if a element should be recursed into
template <typename iterable, typename tag = void>
struct is_iterable
{
    static constexpr bool value = false;
};
template <typename iterable>
struct is_iterable <iterable, std::void_t <decltype (std::begin (std::declval <iterable> ()))>>
{
    static constexpr bool value = true;
};
template <typename iterable>
constexpr bool is_iterable_v = is_iterable <iterable>::value;
// visit elements of a tuple
template <size_t Index, typename visitor, typename ...types>
void visit_tuple (std::tuple <types...> const & Tuple, visitor && Visitor)
{
    Visitor (std::get <Index> (Tuple));
    constexpr auto nextIndex = Index + 1;
    if constexpr (nextIndex < sizeof... (types))
        visit_tuple <nextIndex> (Tuple, Visitor);
}
// this function generically walks the elements of a container, with an overload for tuples and pairs
template <typename container, typename visitor>
std::enable_if_t <is_iterable_v <container>>
visit_elements (container const & Container, visitor && Visitor)
{
    for (auto && Element: Container)
        Visitor (Element);
}
template <typename visitor, typename ...element_types>
void visit_elements (std::tuple <element_types...> const & Tuple, visitor && Visitor)
{
    if constexpr (sizeof... (element_types) > 0)
        visit_tuple <0> (Tuple, Visitor);
}
template <typename visitor, typename first_type, typename second_type>
void visit_elements (std::pair  <first_type, second_type> const & Pair, visitor && Visitor)
{
    Visitor (Pair.first);
    Visitor (Pair.second);
}
// type trait for testing if a value is "visitable"
struct dummy_visitor { template <typename type> void operator () (type &&); };
template <typename container, typename tag = void>
struct is_visitable
{
    static constexpr bool value = false;
};
template <typename container>
struct is_visitable <container, std::void_t <decltype (visit_elements (std::declval <container> (), dummy_visitor()))>>
{
    static constexpr bool value = true;
};
template <typename container>
constexpr bool  is_visitable_v =  is_visitable <container>::value;
// this function walks each item and either emits it or recurses into it
template <typename iterable>
bool join_impl (std::ostream & os, char delim, bool emitted_anything, iterable const & Iterable)
{
  using std::begin;
  using std::end;
  visit_elements (Iterable, [&] (auto && Element) {
    if constexpr (!is_visitable_v <decltype (Element)>)
    {
        if (emitted_anything)
          os << delim;
        os << Element;
        emitted_anything = true;
    }
    else
    {
      emitted_anything = join_impl (os, delim, emitted_anything, Element);
    }
  });
  return emitted_anything;
}
// these are wrappers to adapt join_impl for different use cases
template <typename container>
struct joiner
{
    char delim;
    container const & Container;
    operator std::string () const { return to_string <char> (); }
    template <typename char_t>
    std::basic_string <char_t> to_string () const;
};
template <typename container>
std::ostream & operator << (std::ostream & os, joiner <container> j)
{
    bool emitted_anything = false;
    join_impl (os, j.delim, emitted_anything, j.Container);
    return os;
}
template <typename container>
template <typename char_t>
std::basic_string <char_t> joiner <container>::to_string () const
{
    std::ostringstream os;
    os << *this;
    return os;
}
template <typename container>
std::enable_if_t <is_visitable_v <container>, joiner <container>>
join (char delim, container const & Container)
{
    return joiner <container> { delim, Container };
}
// test the streaming use case
int main ()
{
    std::vector <std::vector <std::tuple <int, float>>> x {
        { { 1, 1.0f }, { 2, 2.0f }},
        { { 3, 3.0f }, { 4, 4.0f }},
    };
    std::cout << join (',', x) << std::endl;
}

这应该只适用于支持标准迭代协议的任何递归。它还支持元组和对。In 可以通过定义访问每个公开字段的重载来扩展到用户定义结构visit_elements。is_iterable型特征需要在其预期的界面中更精确地工作。

我设法清理了错误。 我只是误称函子。 我这样做的初衷是让它也适用于数组，但它没有。 无论如何，这基本上是一个任意维度的 csv 编写器。

#include <iostream>
#include <typeinfo>
#include <vector>
template <typename RAI>
std::string string_join(RAI begin, RAI end, std::string delimiter) {
    if (begin == end) { return ""; }
    std::string joint = std::to_string(*begin);
    begin++;
    for (; begin != end; begin++) {
        joint += delimiter + std::to_string(*begin);
    }
    return joint;
}
template <typename RAI, int depth>
struct string_join_recursive {
    std::string operator()(RAI iterator, std::string *delimiters, int *dimensions) {
        typedef typename std::iterator_traits<RAI>::value_type::iterator value_iterator_type;
        std::string joint = string_join_recursive<value_iterator_type, depth-1>()(std::begin(*iterator), delimiters+1, dimensions+1);
        iterator++;
        for (int i=1; i<*dimensions; i++) {
            joint += *delimiters + string_join_recursive<value_iterator_type, depth-1>()(std::begin(*iterator), delimiters+1, dimensions+1);
            iterator++;
        }
        return joint;
    }
};
template <typename RAI>
struct string_join_recursive<RAI,1> {
    std::string operator()(RAI iterator, std::string *delimiters, int *dimensions) {
        return string_join(iterator, iterator + *dimensions, *delimiters);
    }
};
int main() {
    std::vector<int> a = {1,2,3};
    std::vector<int> b = {4,5,6};
    std::vector<int> c = {7,8,9};
    std::vector<int> d = {10,11,12};
    std::vector<std::vector<int>> e = {a,b};
    std::vector<std::vector<int>> f = {c,d};
    std::vector<std::vector<std::vector<int>>> g = {e,f};
    std::string delimiters[3] = {"n",";",","};
    int dimensions[3] = {2, 2, 3};
    std::cout << string_join_recursive<std::vector<std::vector<std::vector<int>>>::iterator, 3>()(g.begin(), delimiters, dimensions) << std::endl;
}