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#include <iostream>
#include <future>
#include <numeric>
#include <vector>
#include <list>
 
template <typename It, typename Func>
void map(It first, It last, Func f)
{
	while (first != last)
	{
		f(*first++);
	}
}
 
template <typename It, typename MapFunc, typename FoldFunc>
auto map_reduce(It first, It last, MapFunc f1, FoldFunc f2, size_t num_threads)
	-> decltype(f2(f1(*first), f1(*first)))
{
	size_t block_size = std::distance(first, last) / num_threads;
 
	std::vector<std::future<int>> results(num_threads);
	for (size_t i = 0; i < num_threads; ++i)
	{
		results[i] = std::async(map, first, last, f1);
	}
 
	return 0;
}
 
 
int main()
{
	std::list<int> ls = { 1, 2, 3 };
	auto sum = map_reduce(ls.begin(), ls.end(),
		[](int i) {return i; },
		std::plus<int>(),
		3);
 
	return 0;
}

prog.cpp: In instantiation of ‘decltype (f2(f1((* first)), f1((* first)))) map_reduce(It, It, MapFunc, FoldFunc, size_t) [with It = std::_List_iterator<int>; MapFunc = main()::<lambda(int)>; FoldFunc = std::plus<int>; decltype (f2(f1((* first)), f1((* first)))) = int; size_t = long unsigned int]’:
prog.cpp:38:4:   required from here
prog.cpp:25:26: error: no matching function for call to ‘async(<unresolved overloaded function type>, std::_List_iterator<int>&, std::_List_iterator<int>&, main()::<lambda(int)>&)’
   results[i] = std::async(map, first, last, f1);
                ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
In file included from prog.cpp:2:0:
/usr/include/c++/6/future:1709:5: note: candidate: template<class _Fn, class ... _Args> std::future<typename std::result_of<typename std::decay<_Tp>::type(typename std::decay<_BoundArgs>::type ...)>::type> std::async(std::launch, _Fn&&, _Args&& ...)
     async(launch __policy, _Fn&& __fn, _Args&&... __args)
     ^~~~~
/usr/include/c++/6/future:1709:5: note:   template argument deduction/substitution failed:
prog.cpp:25:26: note:   cannot convert ‘map’ (type ‘<unresolved overloaded function type>’) to type ‘std::launch’
   results[i] = std::async(map, first, last, f1);
                ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
In file included from prog.cpp:2:0:
/usr/include/c++/6/future:1739:5: note: candidate: template<class _Fn, class ... _Args> std::future<typename std::result_of<typename std::decay<_Tp>::type(typename std::decay<_BoundArgs>::type ...)>::type> std::async(_Fn&&, _Args&& ...)
     async(_Fn&& __fn, _Args&&... __args)
     ^~~~~
/usr/include/c++/6/future:1739:5: note:   template argument deduction/substitution failed:
prog.cpp:25:26: note:   couldn't deduce template parameter ‘_Fn’
   results[i] = std::async(map, first, last, f1);
                ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

Standard output is empty