condition_variable使用方法

condition_variable是C++11中的一个同步原语，常和mutex搭配使用，用于阻塞一个线程或同时阻止多个线程，直到另一个线程修改共享变量并通知condition_variable。它可以防止多线程场景下，共享变量混乱。理解条件变量需要先理解三个概念：锁、wait等待和notify通知。

condition_variable的使用方法如下：
1.定义condition_variable对象和mutex对象

std::condition_variable cv;
std::mutex cv_m;

2.资源修改线程步骤

std::unique_lock<std::mutex> lk(cv_m);
// 修改共享变量
cv.notify_one(); // 通知等待的线程

3.资源等待线程步骤

std::unique_lock<std::mutex> lk(cv_m);
cv.wait(lk, []{return true/false;}); // 等待条件变量

其中，wait函数会自动释放锁，等待条件变量被通知后重新获取锁。

4.代码示例
4.1 无需notify场景

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mtx;
std::condition_variable cv;
bool ready = false;

void print_id(int id) {
    std::unique_lock<std::mutex> lck(mtx);
    while (!ready) cv.wait(lck);
    std::cout << "thread " << id << '\n';
}

void go() {
    std::unique_lock<std::mutex> lck(mtx);
    ready = true;
    cv.notify_all();
}

int main() {
    std::thread threads[10];
    for (int i = 0; i < 10; ++i)
        threads[i] = std::thread(print_id, i);

    std::cout << "10 threads ready to race...\n";
    go();

    for (auto& th : threads) th.join();

    return 0;
}

4.2 正常应用场景1

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mtx;
std::condition_variable cv;
bool data_ready = false;

void consumer() {
    std::unique_lock<std::mutex> lck(mtx);
    cv.wait(lck, []{return data_ready;});
    std::cout << "Consumer: data is ready\n";
}

void producer() {
    std::unique_lock<std::mutex> lck(mtx);
    data_ready = true;
    cv.notify_one();
}

int main() {
    std::thread t1(consumer);
    std::thread t2(producer);

    t1.join();
    t2.join();

    return 0;
}

4.3 正常应用场景2

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mtx;
std::condition_variable cv;
bool data_ready = false;

void consumer() {
    std::unique_lock<std::mutex> lck(mtx);
    cv.wait(lck, []{return data_ready;});
    std::cout << "Consumer: data is ready\n";
    data_ready = false;
}

void producer() {
    std::unique_lock<std::mutex> lck(mtx);
    data_ready = true;
    cv.notify_one();
}

int main() {
    std::thread t1(consumer);
    std::thread t2(producer);

    t1.join();
    t2.join();

    return 0;
}