C++: What is std::condition_variable?

1#include <mutex>
2#include <condition_variable>
3#include <queue>
4
5std::queue<int> tasks;
6std::mutex mtx;
7std::condition_variable cv;
8
9// Producer
10void producer() {
11    for (int i = 0; i < 10; i++) {
12        {
13            std::lock_guard<std::mutex> lock(mtx);
14            tasks.push(i);
15        }
16        cv.notify_one();
17    }
18}
19
20// Consumer
21void consumer() {
22    while (true) {
23        std::unique_lock<std::mutex> lock(mtx);
24        cv.wait(lock, [] { return !tasks.empty(); });
25        int task = tasks.front();
26        tasks.pop();
27        lock.unlock();
28        // Process task
29    }
30}