proper way of setting up a packaged_task - c++

Question

When compiling the program below, I get the error message:
Error 1 error C2228: left of '.get_future' must have class/struct/union c:\users\haliaga\documents\visual studio 2010\projects\test\test\accumulateexceptionsafe.cpp 62 1 Test
which is not actually the real problem.
if you comment the lines:
//futures[i]=task.get_future();
//threads[i]=std::thread(std::move(task),block_start,block_end);
//block_start=block_end;
you'll get the warning below, that says that "tasK" was not called:
*warning C4930: 'std::packaged_task<> task(accumulate_block (__cdecl )(void))': prototyped function not called (was a variable definition intended?)
1> with
1> [
1> =int (std::_List_iterator>>,std::_List_iterator>>),
1> Iterator=std::_List_iterator>>,
1> T=int
1> ]
what would be the proper way of specifying:
std::packaged_task<T(Iterator,Iterator)> task(accumulate_block<Iterator,T>());
?
Thank you
PS: find below the code:
#include <list>
#include <numeric>
#include <vector>
#include <thread>
#include <future>
using namespace std;
template<typename Iterator,typename T>
struct accumulate_block
{
T operator()(Iterator first, Iterator last)
{
std::thread::id id = std::this_thread::get_id();
return std::accumulate(first, last, T());
}
};
class join_threads
{
std::vector<std::thread>& threads;
public:
explicit join_threads(std::vector<std::thread>& threads_):
threads(threads_)
{
std::thread::id id = std::this_thread::get_id();
}
~join_threads()
{
std::thread::id id = std::this_thread::get_id();
for(unsigned long i=0;i<threads.size();++i)
{
if(threads[i].joinable())
threads[i].join();
}
}
};
template<typename Iterator,typename T>
T parallel_accumulate(Iterator first,Iterator last,T init)
{
std::thread::id id = std::this_thread::get_id();
unsigned long const length=std::distance(first,last);
if(!length)
return init;
unsigned long const min_per_thread=25;
unsigned long const max_threads=(length+min_per_thread-1)/min_per_thread;
unsigned long const hardware_threads=std::thread::hardware_concurrency();
unsigned long const num_threads=std::min(hardware_threads!=0?hardware_threads:2,max_threads);
unsigned long const block_size=length/num_threads;
std::vector<std::future<T> > futures(num_threads-1);
std::vector<std::thread> threads(num_threads-1);
join_threads joiner(threads);
Iterator block_start=first;
for(unsigned long i=0;i<(num_threads-1);++i)
{
Iterator block_end=block_start;
std::advance(block_end,block_size);
std::packaged_task<T(Iterator,Iterator)> task(accumulate_block<Iterator,T>());
futures[i]=task.get_future();
threads[i]=std::thread(std::move(task),block_start,block_end);
block_start=block_end;
}
T last_result=accumulate_block<Iterator, T>()(block_start,last);
T result=init;
for(unsigned long i=0;i<(num_threads-1);++i)
{
result+=futures[i].get();
}
result += last_result;
return result;
};
int main()
{
list<int> l;
for(int i=0; i<26; ++i)
l.push_back(i);
std::thread::id id = std::this_thread::get_id();
int res = ::parallel_accumulate(l.begin(), l.end(), 0);
return 0;
}

Answer 1

Most vexing parse.
std::packaged_task<T(Iterator,Iterator)> task(accumulate_block<Iterator,T>());
declares a function called task that takes a parameter of type pointer to function taking no arguments and returning a accumulate_block<Iterator,T> and returns a std::packaged_task<T(Iterator,Iterator)>.
Disambiguate using the uniform initialization syntax:
std::packaged_task<T(Iterator,Iterator)> task(accumulate_block<Iterator,T>{});
Or with an extra pair of parentheses for ancient compilers that doesn't have support for uniform initialization:
std::packaged_task<T(Iterator,Iterator)> task((accumulate_block<Iterator,T>()));
// ^ ^