这是一种方法。我实现了归并排序，它非常适合函数式编程。不到200行。其中大部分基于我在an earlier answer 中使用的元函数。这是基于许多其他人在 SO 问题中使用的东西，与 typelist 的基本操作等相关......
一种改进方法是减少所需的模板递归深度，目前是O(n)。我猜它可能是O(log n)，但我不确定，这取决于你是否能找到重写merge 元函数的方法。 （与 Yakk 在另一个问题中指出的类似。）
#include <type_traits>
template <class... T>
struct TypeList {
static constexpr const std::size_t size = sizeof...(T);
};
/***
* Concat metafunction
*/
template <typename A, typename B>
struct Concat;
template <class... As, class... Bs>
struct Concat<TypeList<As...>, TypeList<Bs...>> {
typedef TypeList<As..., Bs...> type;
};
template <typename A, typename B>
using Concat_t = typename Concat<A, B>::type;
/***
* Split metafunction
*/
template <int i, typename TL>
struct Split;
template <int k, typename... TL>
struct Split<k, TypeList<TL...>> {
private:
typedef Split<k / 2, TypeList<TL...>> FirstSplit;
typedef Split<k - k / 2, typename FirstSplit::R> SecondSplit;
public:
typedef Concat_t<typename FirstSplit::L, typename SecondSplit::L> L;
typedef typename SecondSplit::R R;
};
template <typename T, typename... TL>
struct Split<0, TypeList<T, TL...>> {
typedef TypeList<> L;
typedef TypeList<T, TL...> R;
};
template <typename T, typename... TL>
struct Split<1, TypeList<T, TL...>> {
typedef TypeList<T> L;
typedef TypeList<TL...> R;
};
template <int k>
struct Split<k, TypeList<>> {
typedef TypeList<> L;
typedef TypeList<> R;
};
// Metafunction Subdivide: Split a typelist into two roughly equal typelists
template <typename TL>
struct Subdivide : Split<TL::size / 2, TL> {};
/***
* Ordered tuple
*/
template <int X>
using int_t = std::integral_constant<int, X>;
template <class... T>
struct Ordered_List : TypeList<T...> {};
template <class... As, class... Bs>
struct Concat<Ordered_List<As...>, Ordered_List<Bs...>> {
typedef Ordered_List<As..., Bs...> type;
};
/***
* Merge metafunction
*/
template <typename A, typename B>
struct Merge;
template <typename B>
struct Merge<Ordered_List<>, B> {
typedef B type;
};
template <int a, class... As>
struct Merge<Ordered_List<int_t<a>, As...>, Ordered_List<>> {
typedef Ordered_List<int_t<a>, As...> type;
};
template <int a, class... As, int b, class... Bs>
struct Merge<Ordered_List<int_t<a>, As...>, Ordered_List<int_t<b>, Bs...>> {
typedef Ordered_List<int_t<a>, As...> A;
typedef Ordered_List<int_t<b>, Bs...> B;
typedef typename std::conditional<a <= b,
Concat_t<Ordered_List<int_t<a>>, typename Merge<Ordered_List<As...>, B>::type>,
Concat_t<Ordered_List<int_t<b>>, typename Merge<A, Ordered_List<Bs...>>::type>
>::type type;
};
template <typename A, typename B>
using Merge_t = typename Merge<A, B>::type;
/***
* Mergesort metafunction
*/
template <typename TL>
struct MergeSort;
// Boilerplate base-cases
template <>
struct MergeSort<TypeList<>> {
typedef Ordered_List<> type;
};
template <int X>
struct MergeSort<TypeList<int_t<X>>> {
typedef Ordered_List<int_t<X>> type;
};
// Inductive step
template <int X, class... Xs>
struct MergeSort<TypeList<int_t<X>, Xs...>> {
typedef TypeList<int_t<X>, Xs...> input_t;
typedef Subdivide<input_t> subdivide_t;
typedef typename MergeSort<typename subdivide_t::L>::type left_sort_t;
typedef typename MergeSort<typename subdivide_t::R>::type right_sort_t;
typedef Merge_t<left_sort_t, right_sort_t> type;
};
template <typename T>
using MergeSort_t = typename MergeSort<T>::type;
/***
* Make ordered tuple impl
*/
#include <tuple>
template <typename T>
struct MakeStdTuple;
template <class... Ts>
struct MakeStdTuple<Ordered_List<Ts...>> {
typedef std::tuple<Ts...> type;
};
template <typename T>
using MakeStdTuple_t = typename MakeStdTuple<T>::type;
template <class... T>
constexpr MakeStdTuple_t<MergeSort_t<TypeList<T...>>> make_ordered_tuple(T&&...) {
return {};
}
static_assert(std::is_same<Ordered_List<int_t<1>, int_t<2>, int_t<3>>,
MergeSort_t<TypeList<int_t<1>, int_t<2>, int_t<3>>>>::value, "Failed a unit test");
static_assert(std::is_same<Ordered_List<int_t<1>, int_t<2>, int_t<3>>,
MergeSort_t<TypeList<int_t<2>, int_t<1>, int_t<3>>>>::value, "Failed a unit test");
static_assert(std::is_same<Ordered_List<int_t<1>, int_t<2>, int_t<3>>,
MergeSort_t<TypeList<int_t<3>, int_t<2>, int_t<1>>>>::value, "Failed a unit test");
int main() {}