使用std :: async启动类成员函数的模板函数

Question

I'd like to write a template function, which can take object of any class/struct and call any of its member function in a thread. Following does not compile, I guess it can not figure out: std::result_of< F(Args...) >::type .

Any suggestion, help..?

class test_f {
public:
  int f(int m) {
    std::cout << " call f : " << m << std::endl;
    return 1;
  }
};

template<typename F, typename T, typename... Args>
std::future<typename std::result_of<F(Args...)>::type>
Async(F&& f, T&& t, Args&&... params) {
  return(std::async(std::launch::async, std::forward<F>(f),
      std::forward<T>(t), std::forward<Args>(params)...));
}

int main() {
  test_f tf ;
  auto a = Async(&test_f::f, &tf, 1) ;
}

Answer 1

If you can use C++14, just use auto :

template<typename F, typename T, typename... Args>
auto Async(F&& f, T&& t, Args&&... params) {
  return(std::async(std::launch::async, std::forward<F>(f),
      std::forward<T>(t), std::forward<Args>(params)...));
}

Otherwise you need something like this:

auto Async(F&& f, T&& t, Args&&... params) 
    -> std::future<decltype( 
         (t->*f) (std::forward<Args>(params)...) )>            
       )> { // .. same as before

Answer 2

You're just missing an argument in your result_of declaration. Your function F takes a T and Args... and you forgot the T :

template<typename F, typename T, typename... Args>
std::future<typename std::result_of<F(T, Args...)>::type>
//                                   ^^^^
Async(F&& f, T&& t, Args&&... params) {
  return(std::async(std::launch::async, std::forward<F>(f),
      std::forward<T>(t), std::forward<Args>(params)...));
}

Alternatively, the T is totally unnecessary since you're arbitrarily limiting yourself to 1+ argument functions. And it's easy to forget. So you can just drop it:

template<typename F, typename... Args>
std::future<typename std::result_of<F(Args...)>::type>
Async(F&& f, Args&&... params) {
  return(std::async(std::launch::async, std::forward<F>(f),
      std::forward<Args>(params)...));
}