template <typename F>
auto foo (F && f)
 {
   // ...

   auto x = std::forward<F>(f)(/* some arguments */);

   // ...
 }

template <typename F>
auto foo (F && f)
 {
   using A = typename decltype(std::function{std::forward<F>(f)})::result_type;

   A val { std::forward<F>(f)() };

   return val;
 }

template <typename F>
auto bar (F && f)
 {
   using A = typename decltype(std::function{std::forward<F>(f)})::result_type;

   std::vector<A>  vect;

   for ( auto i { 0u }; i < 10u ; ++i )
      vect.push_back(std::forward<F>(f)());

   return vect;
 }