C++條件變量通知不按預期方式工作

Question

我試圖在以前worker_thread有開始工作，但可能已結束或不結束時啓動新線程。我已經替換了開始並結束了延時工作。我的代碼是：

#include <iostream> 
#include <string> 
#include <mutex> 
#include <condition_variable> 
#include <future> 
#include <atomic> 
#include <chrono> 
#include <thread> 

std::mutex m; 
std::condition_variable cv; 
bool started = false; 

void worker_thread() 
{ 
    std::unique_lock<std::mutex> lk(m); 

    static std::atomic<int> count(1); 
    std::this_thread::sleep_for(std::chrono::milliseconds{(count % 5) * 100}); 
    std::cerr << "Start Worker thread: " << count << "\n"; 

    started = true; 
    lk.unlock(); 
    cv.notify_one(); 

    std::this_thread::sleep_for(std::chrono::milliseconds{3000}); 
    std::cerr << "Exit Worker thread: " << count << "\n"; 
    ++count; 
} 

int main() 
{ 
    while(1) { 
     std::async(std::launch::async, worker_thread); 
     std::unique_lock<std::mutex> lk(m); 
     cv.wait(lk, []{return started;}); 
     started = false; 
    } 
} 

輸出看起來像這樣：

Start Worker thread: 1 
Exit Worker thread: 1 
Start Worker thread: 2 
Exit Worker thread: 2 
Start Worker thread: 3 
Exit Worker thread: 3 
Start Worker thread: 4 
Exit Worker thread: 4 
Start Worker thread: 5 
Exit Worker thread: 5 

這不是我想要的行爲。我想要的是像（不完全）這樣的：

Start Worker thread: 1 
Start Worker thread: 2 
Start Worker thread: 3 
Start Worker thread: 4 
Exit Worker thread: 1 
Exit Worker thread: 3 
Exit Worker thread: 4 
Exit Worker thread: 2 
Start Worker thread: 5 
Exit Worker thread: 5 

目前的工作而以前的線程完成下一個線程纔開始。但是，我想在上一個線程中開始工作並且不等待它結束時纔開始下一個線程，只能等待開始。

Answer 1

std::async返回一個std::future函數執行的保存結果。就你而言，它是一個被直接摧毀的臨時對象。爲std::future文檔說：

these actions will not block for the shared state to become ready, except that it may block if all of the following are true：

✔共享狀態是通過調用創建到std ::異步

✔共享狀態還沒有準備好

✔這是共享狀態的最後一個參考

所有這些都是真實的，所以破壞o f future將阻塞，直到工作器功能將完成執行。

您可以創建分離線程來避免這個問題：

std::thread(worker_thread).detach();