Explain the purpose and operation of std::condition_variable in C++.

std::condition_variable in C++ is used to organize synchronization between threads. It allows one or more threads to wait for a certain event to occur, while releasing the mutex so that other threads can modify the state that the first thread is waiting for.

Main points:

• A thread calls wait on the condition_variable, passing the mutex it holds.
• During waiting, the mutex is released, allowing other threads to access shared data.
• When the event occurs, another thread calls notify_one or notify_all to wake up the waiting threads.
• After waking up, the thread re-acquires the mutex and checks the condition.

Example usage:

#include <mutex>
#include <condition_variable>
#include <queue>

std::mutex mtx;
std::condition_variable cv;
std::queue<int> dataQueue;

void producer() {
    std::unique_lock<std::mutex> lock(mtx);
    dataQueue.push(42);
    cv.notify_one();
}

void consumer() {
    std::unique_lock<std::mutex> lock(mtx);
    cv.wait(lock, []{ return !dataQueue.empty(); });
    int data = dataQueue.front();
    dataQueue.pop();
    // process data
}

Thus, condition_variable helps efficiently organize waiting and notification between threads without active polling.