void wait( std::unique_lockstd::mutex& lock );
//Predicate是lambda表達(dá)式。
template< class Predicate >
void wait( std::unique_lockstd::mutex& lock, Predicate pred );
//以上二者都被notify_one())或notify_broadcast()喚醒，但是
//第二種方式是喚醒后也要滿足Predicate的條件。
//如果不滿足條件，繼續(xù)解鎖互斥量，然后讓線程處于阻塞或等待狀態(tài)。
//第二種等價(jià)于
while (!pred())
{
wait(lock);
}

3 condition_variable用法

condition_variable必定至少有兩方，一方是資源修改線程，一方是資源等待線程。就跟打籃球一樣，同時籃球只會在一個人手中，投籃后就釋放了籃球所有權(quán)，其他方就會搶奪籃球所有權(quán)。

3.1 資源修改線程步驟

獲取一個mutex使用 std::unique_lock< std::mutex >
保持鎖定狀態(tài)，修改共享變量
condition_variable對象執(zhí)行notify_one或者notify_all（notify_one/notify_all執(zhí)行前可以釋放鎖）

3.2 資源等待線程步驟

獲取一個mutex使用 std::unique_lock< std::mutex > unlock用于保護(hù)要修改的共享變量
檢查條件變量，

(1)條件變量滿足，線程繼續(xù)執(zhí)行

(2)條件變量不滿足，wait會釋放unlock鎖，并掛起線程。

當(dāng)notify通知條件變量、超時過期或發(fā)生虛假喚醒時，線程被喚醒，互斥鎖unlock被原子地重新獲取。然后，線程應(yīng)該檢查條件，如果喚醒是假的，則繼續(xù)等待

4 代碼示例

4.1 無需notify場景

當(dāng)wait第一次執(zhí)行是，條件已經(jīng)滿足，則程序不會阻塞（即無需notify），會直接向下執(zhí)行。(僅為說明3.2 中第2點(diǎn)（1）的情況)

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    std::cout << "3、worker_thread子線程開始執(zhí)行"  << endl;
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    std::cout << "4、worker_thread子線程獲取到鎖,條件滿足無需notify，不阻塞向下執(zhí)行"  << endl;
    cv.wait(lk, []{return ready;});
 
    // after the wait, we own the lock.
    data += " after processing";
    // Send data back to main()
    processed = true;
    std::cout << "5、Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    std::cout << "6、worker_thread子線程交出執(zhí)行權(quán)限，主線程執(zhí)行"  << endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(2000));
    
    cv.notify_one();
    std::cout << "9、worker_thread調(diào)用 notify_one"  << endl;
}
int main()
{
    std::thread worker(worker_thread);
    std::cout << "1、主線程開始執(zhí)行"  << std::endl;
    data = "Example data";
    // send data to the worker thread
    {
        //std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        std::lock_guard<std::mutex> lk(m);
        ready = true;
    }
    std::cout << "2、鎖已經(jīng)釋放了，主線程休眠，子線程執(zhí)行"  << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    //cv.notify_one();
    {
        std::cout << "7、主線程data：" << data << endl;
        std::unique_lock<std::mutex> lk(m);
        std::cout << "8、主線程條件滿足無需notify" << endl;
        cv.wait(lk, []{return processed;});
    }
    
    worker.join();
     std::cout << "10、主線程結(jié)束" << endl;
}

執(zhí)行結(jié)果：

4.2 正常應(yīng)用場景1

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    std::cout << "3、worker_thread子線程開始執(zhí)行"  << endl;
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    std::cout << "4、worker_thread子線程獲取到鎖,條件不滿足，釋放lk鎖，子線程阻塞"  << endl;
    cv.wait(lk, []{return ready;});
    std::cout << "8、worker_thread子線程獲取到鎖,子線程繼續(xù)執(zhí)行"  << endl;
    // after the wait, we own the lock.
    data += " after processing";
    // Send data back to main()
    processed = true;
    std::cout << "9、Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    std::this_thread::sleep_for(std::chrono::milliseconds(5000));
    std::cout << "10、worker_thread調(diào)用 notify_one通知主線程執(zhí)行"  << endl;
    cv.notify_one();
}
int main()
{
    std::thread worker(worker_thread);
    std::cout << "1、主線程開始執(zhí)行"  << std::endl;
    data = "Example data";
    // send data to the worker thread
    {
        std::cout << "2、主線程休眠，子線程進(jìn)入執(zhí)行"  << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        std::cout << "5、主線程結(jié)束休眠，主線程獲取lk鎖，進(jìn)入執(zhí)行"  << std::endl;
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        
    }
    std::cout << "6、主線程釋放lk,調(diào)用notify通知子線程"  << std::endl;
    cv.notify_one();
    {
        std::cout << "7、由于主線程的執(zhí)行時鐘周期未結(jié)束，繼續(xù)執(zhí)行主線程獲取lk, wait檢查條件不滿足,釋放鎖" << endl;
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
   
    worker.join();
     std::cout << "11、主線程結(jié)束" << endl;
}

執(zhí)行結(jié)果：

這里notify執(zhí)行后不一定立即執(zhí)行子線程，如果cpu執(zhí)行時鐘周期未結(jié)束，則主線程會繼續(xù)執(zhí)行. 所以7,8，9,10順序可能變化參見4.3

同時4.1也會因?yàn)閏pu時鐘周期，執(zhí)行順序有所變動。

4.3 正常應(yīng)用場景2

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    std::cout << "3、worker_thread子線程開始執(zhí)行"  << endl;
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    std::cout << "4、worker_thread子線程獲取到鎖,條件不滿足，釋放lk鎖，子線程阻塞"  << endl;
    cv.wait(lk, []{return ready;});
    std::cout << "8、worker_thread子線程獲取到鎖,子線程繼續(xù)執(zhí)行"  << endl;
    // after the wait, we own the lock.
    data += " after processing";
    // Send data back to main()
    processed = true;
    std::cout << "9、Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    std::cout << "10、worker_thread調(diào)用 notify_one通知主線程執(zhí)行"  << endl;
    cv.notify_one();
}
int main()
{
    std::thread worker(worker_thread);
    std::cout << "1、主線程開始執(zhí)行"  << std::endl;
    data = "Example data";
    // send data to the worker thread
    {
        std::cout << "2、主線程休眠，子線程進(jìn)入執(zhí)行"  << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        std::cout << "5、主線程結(jié)束休眠，主線程獲取lk鎖，進(jìn)入執(zhí)行"  << std::endl;
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        
    }
    std::cout << "6、主線程釋放lk,調(diào)用notify通知子線程"  << std::endl;
    cv.notify_one();
    {
        for(int i = 0; i< 10000000; i++)
        {
            int j = i;
        }
        std::cout << "7、由于主線程的執(zhí)行時鐘周期未結(jié)束，繼續(xù)執(zhí)行主線程獲取lk, wait檢查條件不滿足,釋放鎖" << endl;
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
    
    worker.join();
    std::cout << "11、主線程結(jié)束" << endl;
}

執(zhí)行結(jié)果：