Perfect Forwarding Variadic Template to Standard Thread

问题

I'm trying to make a form of std::thread that puts a wrapper around the code executed in the thread. Unfortunately I can't get it to compile due likely to my poor understanding of rvalues and the Function templated type I'm trying to pass. Here's my code:

#include <vector>
#include <thread>
#include <utility>

void Simple2(int a, int b) {}

template <typename Function, typename... Args>
void Wrapper(Function&& f, Args&&... a) {
  f(std::forward<Args>(a)...);
}

class Pool {
 public:
  template <typename Function, typename... Args>
  void Binder(Function&& f, Args&&... a) {
    std::thread t(Wrapper<Function, Args...>,
                  std::forward<Function>(f), std::forward<Args>(a)...);
  }
};

int main() {
  Wrapper(Simple2, 3, 4);       // Works

  Pool pool;
  pool.Binder(Simple2, 3, 4);   // Doesn't compile
}

The Clang3.0 output that seems important here is:

/usr/include/c++/4.6/functional:1286:9: error: non-const lvalue reference to type 'void (int, int)' cannot bind to a value of unrelated type 'void (*)(int, int)'

and

note: in instantiation of function template specialization 'std::thread::thread<void (void (&)(int, int), int &&, int &&), void (&)(int, int), int, int>' requested here

Which I think is hinting at a mismatch between Wrapper<Function, Args...> and the rvalues f, a... being given to std::thread.

Curiously this compiles in GCC4.9 and newer Clang if I change the std::forward<Function>(f) to std::ref(f).

回答1:

This is one of those rare cases where the difference between passing a function and passing a function pointer makes a difference. If you do:

pool.Binder(&Simple2, 3, 4);

it should work. Or alternatively you can have Binder decay its argument to a function pointer:

class Pool {
 public:
  template <typename Function, typename... Args>
  void Binder(Function&& f, Args&&... a) {
    std::thread t(Wrapper<typename std::decay<Function>::type, Args...>,
                  std::forward<Function>(f), std::forward<Args>(a)...);
  }
};

which in C++14 simplifies to:

class Pool {
 public:
  template <typename Function, typename... Args>
  void Binder(Function&& f, Args&&... a) {
    std::thread t(Wrapper<std::decay_t<Function>, Args...>,
                  std::forward<Function>(f), std::forward<Args>(a)...);
  }
};

来源：https://stackoverflow.com/questions/27029048/perfect-forwarding-variadic-template-to-standard-thread

标签

c++

c++11

rvalue-reference

stdthread

perfect-forwarding