当我使用异步任务（std::async Function 模板）时，我应该尊重指令的顺序吗？

Question

I was working on a multi-threaded program when I crossed a case in which when I ask some asynchronous implementations like the following:

#include <iostream>
#include <thread>
#include <chrono>
#include <future>

using namespace std::chrono;

int increment(int i){
     i++;
     std::this_thread::sleep_for(seconds(5));

     return i;
}

int main()
{

    int x(0),y(0);

    std::future<int> result = std::async(std::launch::async,
                                         [](int i)mutable throw()->
                                         int{
                                              i++;
                                              std::this_thread::sleep_for(seconds(5));
                                              return i;
                                          }, y);
    x=increment(x);
    y = result.get();
    return 0;

}

Focusing on the two instructions x=increment(x) and y = result.get() should they be in this order, or is there another explanation? Because:

Case 1:

x=increment(x);
y = result.get();

Time Execution: 5 seconds (As expected !)

Case 2:

y = result.get();
x=increment(x);

Time Execution: 10 seconds

Is there any logical explanations to this?

Answer 1

y = result.get();
x=increment(x);

In this case, the main thread will be blocked until the created thread finishes its work because get block the thread where it's working until it finishes its work

x=increment(x);
y = result.get();

Here is no blocking because y = result.get(); comes at the end and this way is the way threads should work.

See https://en.cppreference.com/w/cpp/thread/future/get