【发布时间】：2016-07-05 09:48:31
【问题描述】：
考虑这个输出：
int foo (int, char) {std::cout << "foo\n"; return 0;}
double bar (bool, double, long ) {std::cout << "bar\n"; return 3.5;}
bool baz (char, short, float) {std::cout << "baz\n"; return true;}
int main() {
const auto tuple = std::make_tuple(5, 'a', true, 3.5, 1000, 't', 2, 5.8);
multiFunction<2,3,3> (tuple, foo, bar, baz); // foo bar baz
}
所以multiFunction<2,3,3> 获取tuple 的前2 个元素并将它们传递给foo，接下来的3 个tuple 元素并将它们传递给bar，等等......我得到了这个工作（除了当函数有重载时，这是一个单独的问题）。但是每个调用的函数的返回值都会丢失。我希望将这些返回值存储在某个地方，例如
std::tuple<int, double, bool> result = multiFunction<2,3,3> (tuple, foo, bar, baz);
但我不知道如何实现。对于那些想要帮助完成这项工作的人，这是我迄今为止的（更新的）工作代码，它仅将输出存储到字符串流中。取回所有值并不容易，特别是如果保存在流中的对象是复杂的类。
#include <iostream>
#include <tuple>
#include <utility>
#include <sstream>
template <std::size_t N, typename Tuple>
struct TupleHead {
static auto get (const Tuple& tuple) { // The subtuple from the first N components of tuple.
return std::tuple_cat (TupleHead<N-1, Tuple>::get(tuple), std::make_tuple(std::get<N-1>(tuple)));
}
};
template <typename Tuple>
struct TupleHead<0, Tuple> {
static auto get (const Tuple&) { return std::tuple<>{}; }
};
template <std::size_t N, typename Tuple>
struct TupleTail {
static auto get (const Tuple& tuple) { // The subtuple from the last N components of tuple.
return std::tuple_cat (std::make_tuple(std::get<std::tuple_size<Tuple>::value - N>(tuple)), TupleTail<N-1, Tuple>::get(tuple));
}
};
template <typename Tuple>
struct TupleTail<0, Tuple> {
static auto get (const Tuple&) { return std::tuple<>{}; }
};
template <typename Tuple, typename F, std::size_t... Is>
auto functionOnTupleHelper (const Tuple& tuple, F f, const std::index_sequence<Is...>&) {
return f(std::get<Is>(tuple)...);
}
template <typename Tuple, typename F>
auto functionOnTuple (const Tuple& tuple, F f) {
return functionOnTupleHelper (tuple, f, std::make_index_sequence<std::tuple_size<Tuple>::value>{});
}
template <typename Tuple, typename... Functions> struct MultiFunction;
template <typename Tuple, typename F, typename... Fs>
struct MultiFunction<Tuple, F, Fs...> {
template <std::size_t I, std::size_t... Is>
static inline auto execute (const Tuple& tuple, std::ostringstream& oss, const std::index_sequence<I, Is...>&, F f, Fs... fs) {
const auto headTuple = TupleHead<I, Tuple>::get(tuple);
const auto tailTuple = TupleTail<std::tuple_size<Tuple>::value - I, Tuple>::get(tuple);
// functionOnTuple (headTuple, f); // Always works, though return type is lost.
oss << std::boolalpha << functionOnTuple (headTuple, f) << '\n'; // What about return types that are void???
return MultiFunction<std::remove_const_t<decltype(tailTuple)>, Fs...>::execute (tailTuple, oss, std::index_sequence<Is...>{}, fs...);
}
};
template <>
struct MultiFunction<std::tuple<>> {
static auto execute (const std::tuple<>&, std::ostringstream& oss, std::index_sequence<>) { // End of recursion.
std::cout << std::boolalpha << oss.str();
// Convert 'oss' into the desired tuple? But how?
return std::tuple<int, double, bool>(); // This line is just to make the test compile.
}
};
template <std::size_t... Is, typename Tuple, typename... Fs>
auto multiFunction (const Tuple& tuple, Fs... fs) {
std::ostringstream oss;
return MultiFunction<Tuple, Fs...>::execute (tuple, oss, std::index_sequence<Is...>{}, fs...);
}
// Testing
template <typename T> int foo (int, char) {std::cout << "foo<T>\n"; return 0;}
double bar (bool, double, long ) {std::cout << "bar\n"; return 3.5;}
template <int...> bool baz (char, short, float) {std::cout << "baz<int...>\n"; return true;}
int main() {
const auto tuple = std::make_tuple(5, 'a', true, 3.5, 1000, 't', 2, 5.8);
std::tuple<int, double, bool> result = multiFunction<2,3,3> (tuple, foo<bool>, bar, baz<2,5,1>); // foo<T> bar baz<int...>
}
【问题讨论】：
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只需更改
functionOnTuple以返回单个元素tuple。对于void，返回tuple<>（或者可能是tuple<this_function_returns_void_t>）。然后你可以一起tuple_cat他们。显然这不是最有效的，但你的其余代码也不是。 -
该问题的解决方案很有趣且内容丰富，但我很难理解“多重呼叫”是最优雅的解决方案的问题。似乎有点……无法维护？
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@Richard Hodges 只是另一层间接性，有一天可能会派上用场。
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@T.C.谢谢你的建议。我使用您的想法修改了我的解决方案并将其发布。所有返回值现在都存储在所需的元组中（包括 void）。
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标签： c++ templates tuples c++14 variadic