是否可以使用元组中传递的参数的所有可能的 K 组合（带重复）来调用方法？

所需的行为可以如下所示：

void foo(int x, int y) {
    std::cout << x << " " << y << std::endl;
}

int main() {
    all_combinations<2>(foo, std::make_tuple(1, 2)); // K = 2 
    // to run: 
    // foo(1, 1)
    // foo(1, 2)
    // foo(2, 1)
    // foo(2, 2)
}

c++14 版本可能如下所示：

#include <tuple>
#include <utility>
#include <iostream>
#include <initializer_list>


template <class Foo, class Tuple, size_t... Is, size_t... Is2>
int all_combinations_impl(Foo foo, Tuple t, std::index_sequence<Is...> , std::integral_constant<size_t, 0>, std::index_sequence<Is2...>) {
   foo(std::get<Is>(t)...);
   std::cout << std::endl;
   return 0;
}

template <class Foo, class Tuple, size_t... Is, size_t K, size_t... Is2>
int all_combinations_impl(Foo foo, Tuple t, std::index_sequence<Is...>, std::integral_constant<size_t, K>, std::index_sequence<Is2...> is) {
   std::initializer_list<int> all = {all_combinations_impl(foo, t, std::index_sequence<Is..., Is2>{}, std::integral_constant<size_t, K-1>{}, is)...};
   (void)all;
}

template <size_t K, class Foo, class Tuple>
void all_combinations(Foo foo, Tuple t) {
   all_combinations_impl(foo, t, std::index_sequence<>{}, std::integral_constant<size_t, K>{}, std::make_index_sequence<std::tuple_size<Tuple>::value>{});
}

int main() {
   all_combinations<2>([](auto... args) { std::forward_as_tuple((std::cout << args)...); }, std::make_tuple("1 ", "2 ", "3 "));
}

不幸的是c++11没有附带std::integer_sequence，所以我们需要做额外的实现：

#include <tuple>
#include <utility>
#include <iostream>
#include <initializer_list>

template <class T, T... Vs>
struct integer_sequence { };

template <class T, class, class, class = integer_sequence<T>, class = integer_sequence<T, 0>, class = void>
struct make_integer_sequence_impl;

template <class T, T ICV1, T... Res, T... Pow>
struct make_integer_sequence_impl<T, std::integral_constant<T, ICV1>, std::integral_constant<T, 0>, integer_sequence<T, Res...>, integer_sequence<T, Pow...>, typename std::enable_if<(ICV1 > 0)>::type>: make_integer_sequence_impl<T, std::integral_constant<T, ICV1/2>, std::integral_constant<T, ICV1%2>, integer_sequence<T, Res...>, integer_sequence<T, Pow..., (Pow + sizeof...(Pow))...>> { };

template <class T, T ICV1, T... Res, T... Pow>
struct make_integer_sequence_impl<T, std::integral_constant<T, ICV1>, std::integral_constant<T, 1>, integer_sequence<T, Res...>, integer_sequence<T, Pow...>, void>: make_integer_sequence_impl<T, std::integral_constant<T, ICV1/2>, std::integral_constant<T, ICV1%2>, integer_sequence<T, Pow..., (Res + sizeof...(Pow))...>, integer_sequence<T, Pow..., (Pow + sizeof...(Pow))...>> { };

template <class T, class Res, class Pow>
struct make_integer_sequence_impl<T, std::integral_constant<T, 0>, std::integral_constant<T, 0>, Res, Pow, void> {
   using type = Res;
};

template <class T, T V>
using make_integer_sequence = typename make_integer_sequence_impl<T, std::integral_constant<T, V/2>, std::integral_constant<T, V%2>>::type;

template <size_t V>
using make_index_sequence = make_integer_sequence<size_t, V>;

template <size_t... V>
using index_sequence = integer_sequence<size_t, V...>;

template <class Foo, class Tuple, size_t... Is, size_t... Is2>
int all_combinations_impl(Foo foo, Tuple t, index_sequence<Is...> , std::integral_constant<size_t, 0>, index_sequence<Is2...>) {
   foo(std::get<Is>(t)...);
   std::cout << std::endl;
   return 0;
}

template <class Foo, class Tuple, size_t... Is, size_t K, size_t... Is2>
int all_combinations_impl(Foo foo, Tuple t, index_sequence<Is...>, std::integral_constant<size_t, K>, index_sequence<Is2...> is) {
   std::initializer_list<int> all = {all_combinations_impl(foo, t, index_sequence<Is..., Is2>{}, std::integral_constant<size_t, K-1>{}, is)...};
   (void)all;
}

template <size_t K, class Foo, class Tuple>
void all_combinations(Foo foo, Tuple t) {
   all_combinations_impl(foo, t, index_sequence<>{}, std::integral_constant<size_t, K>{}, make_index_sequence<std::tuple_size<Tuple>::value>{});
}

struct s {
   template <class... Args>
   void operator()(Args... args) const {
      std::forward_as_tuple((std::cout << args)...);
   }
};

int main() {
   all_combinations<3>(s{}, std::make_tuple("1 ", "2 ", "3 "));
}