Question

我有一个可调用类型的元组。

std::tuple<std::function<int(int)>, std::function<std::string(double)>> funcs;

我想创建另一个具有每个callable结果类型的元组。例如，funcs包含int->int和double->std::string

如何创建依赖于results中每个元素的funcs元组，它们可能如下所示。

std::tuple<int, std::string> results;

Answer 1

#include <tuple>
#include <functional>
#include <string>


// takes an arbitrary tuple of std::functions and creates a
//   tuple of the return types of those std::functions
template<class T, class... Types>
struct TupleHandler;

// handles the recursive inheritance base case when all the
//   elements of the tuple have been processed
template<class... Types>
  struct TupleHandler<std::tuple<>, Types...> {
    using ReturnTypeTuple = std::tuple<Types...>;
  };

// Strips off the first std::function in the tuple, determines
//   its return type and passes the remaining parts on to have the next element
//   processed
template<class Return, class... Rest, class... Tail, class... Types>
struct TupleHandler<std::tuple<std::function<Return(Rest...)>, Tail...>, Types...> : TupleHandler<std::tuple<Tail...>, Types..., Return> {
     using ReturnTypeTuple = typename TupleHandler<std::tuple<Tail...>, Types...,  Return>::ReturnTypeTuple;
};

int main()
{
    std::tuple<std::function<int(int)>, std::function<std::string(double)>> funcs;

    // Of course for this simple use case you could have just used std::make_tuple, but it still demonstrates the solution
    TupleHandler<decltype(funcs)>::ReturnTypeTuple return_value_tuple(std::get<0>(funcs)(1), std::get<1>(funcs)(4.4));

    // added per comment
    auto x = [](auto funcs){ typename TupleHandler<decltype(funcs)>::ReturnTypeTuple results; };

}

Answer 2

非递归方式：

JAVA_HOME=C:\jdk1.5.0_06
PATH=%JAVA_HOME%\bin;%PATH%
CLASSPATH=.;%JAVA_HOME%\lib;%JAVA_HOME%\lib\tools.jar

Demo

Answer 3

std::function有一个名为result_type的成员typedef。只需使用它。

template<class... Funcs>
auto tuple_function_ret_impl(std::tuple<Funcs...>)
    -> std::tuple<typename Funcs::result_type ...>;

template<class Tuple>
using tuple_function_ret = decltype(tuple_function_ret_impl(Tuple()));

Demo：

using func_tuple = std::tuple<std::function<int(int)>,
                              std::function<std::string(double)>>;

using ret_tuple = tuple_function_ret<func_tuple>;
using ret_tuple = std::tuple<int, std::string>; // OK

从可调用元组的结果创建一个元组

3 个答案: