using std::async with template functions

How can I, or, can I, pass a template function to async?

Here is the code:

//main.cpp
#include <future>
#include <vector>
#include <iostream>
#include <numeric>

int
main
    ()
{      
    std::vector<double> v(16,1);

    auto r0 =  std::async(std::launch::async,std::accumulate,v.begin(),v.end(),double(0.0));

    std::cout << r0.get() << std::endl;
    return 0;
}

Here are the error messages:

                                                                               ^
a.cpp:13:88: note: candidates are:
In file included from a.cpp:1:0:
/usr/include/c++/4.8/future:1523:5: note: template std::future::type> std::async(std::launch, _Fn&&, _Args&& ...)
     async(launch __policy, _Fn&& __fn, _Args&&... __args)
     ^
/usr/include/c++/4.8/future:1523:5: note:   template argument deduction/substitution failed:
a.cpp:13:88: note:   couldn't deduce template parameter ‘_Fn’
   auto r0 = std::async(std::launch::async,std::accumulate,v.begin(),v.end(),double(0.0));
                                                                                        ^
In file included from a.cpp:1:0:
/usr/include/c++/4.8/future:1543:5: note: template std::future::type> std::async(_Fn&&, _Args&& ...)
     async(_Fn&& __fn, _Args&&... __args)
     ^
/usr/include/c++/4.8/future:1543:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.8/future: In substitution of ‘template std::future::type> std::async(_Fn&&, _Args&& ...) [with _Fn = std::launch; _Args = {}]’:
a.cpp:13:88:   required from here
/usr/include/c++/4.8/future:1543:5: error: no type named ‘type’ in ‘class std::result_of’

---

The problem is that to pass the second argument to std::async the compiler has to turn the expression &std::accumulate into a function pointer, but it doesn't know which specialization of the function template you want. To a human it's obvious you want the one that can be called with the remaining arguments to async , but the compiler doesn't know that and has to evaluate each argument separately.

As PiotrS.'s answer says, you can tell the compiler which std::accumulate you want with an explicit template argument list or by using a cast, or alternatively you can just use a lambda expression instead:

std::async(std::launch::async,[&] { return std::accumulate(v.begin(), v.end(), 0.0); });

Inside the body of the lambda the compiler performs overload resolution for the call to std::accumulate and so it works out which std::accumulate to use.

---

You have to disambiguate between possible instantiations by either explicitly passing the template arguments or using static_cast , so:

auto r0 = std::async(std::launch::async
                     , &std::accumulate<decltype(v.begin()), double>
                     , v.begin()
                     , v.end()
                     , 0.0);

or:

auto r0 = std::async(std::launch::async
       , static_cast<double(*)(decltype(v.begin()), decltype(v.end()), double)>(&std::accumulate)
       , v.begin()
       , v.end()
       , 0.0);

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