为什么 "std::async" 没有按预期工作？

Question

#include <thread>
#include <functional>
#include <utility>

using namespace std;

template<typename Callable, typename... Args>
void Async(Callable&& fn, Args&&... args)
{
    auto fn_wrapper = [](Callable&& fn, Args&&... args)
    {
        invoke(forward<Callable>(fn), forward<Args>(args)...);
    };

    // ok
    fn_wrapper(forward<Callable>(fn), forward<Args>(args)...);

    // ok
    async(forward<Callable>(fn), forward<Args>(args)...);

    // error : no matching function for call to 'async'
    async(fn_wrapper, forward<Callable>(fn), forward<Args>(args)...);
}

void f(int, int) {}

int main()
{
    Async(f, 1, 2);
    this_thread::sleep_for(1min);
}

如下代码即可：

  fn_wrapper(forward<Callable>(fn), forward<Args>(args)...);

下面的代码也可以：

  async(forward<Callable>(fn), forward<Args>(args)...);

但是，下面的代码不没问题：

  // error : no matching function for call to 'async'
  async(fn_wrapper, forward<Callable>(fn), forward<Args>(args)...);

最后一个案例编译失败，出现以下错误：

main.cpp:27:5: fatal error: no matching function for call to 'async'
    async(fn_wrapper,
    ^~~~~
main.cpp:36:5: note: in instantiation of function template specialization 'Async<void (&)(int, int), int, int>' requested here
    Async(f, 1, 2);
    ^

  [...]

/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../include/c++/6.3.0/future:1739:5: note: candidate template ignored: substitution failure [with _Fn = (lambda at main.cpp:12:9) &, _Args = <void (&)(int, int), int, int>]: no type named 'type' in 'std::result_of<(lambda at main.cpp:12:9) (void (*)(int, int), int, int)>'
    async(_Fn&& __fn, _Args&&... __args)

为什么最后一个案例不行？

Answer 1

完美转发 (&&) 仅适用于模板参数上下文。但是您的 lambda 不是模板。所以 (Callable&& fn, Args&&... args) 只是将 && 添加到每个参数上，实际上意味着 "pass by rvalue reference"，这不会正确地 perfect-forward。

但更重要的是，std::async调用仿函数为INVOKE(DECAY(std::forward<F>(f)), DECAY(std::forward<Args>(args))...)，所以推导的仿函数的参数类型实际上是decayed Callable和Args.

改用这个包装器：

    auto fn_wrapper = [](auto&& fn, auto&&... args)
    {
        invoke(forward<decltype(fn)>(fn), forward<decltype(args)>(args)...);
    };

为什么 "std::async" 没有按预期工作？

Why does "std::async" not work as expected?

c++

lambda

std

c++17