场景需求

解决方案

通过锁+条件变量的方式来保证线程会按照特定的顺序执行。后续其实可以考虑如何封装一下，只需要传递需要线程数（threads_count）和所处理事件（task）,然后自动异步执行。

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <vector>
const int NUM_THREADS = 3;  // 3 thread

std::mutex mtx;
std::condition_variable cv;
int current_number = 0;


struct picture
{
    int val;
    picture(int v) : val(v) {}
    /* data */
};
void processPic(int thread_id, std::vector<picture> pictures) {
    
    for (;;) {
        std::unique_lock<std::mutex> lock(mtx);
        cv.wait(lock, [&]() { return current_number % NUM_THREADS == thread_id || current_number > pictures.size(); });

        if (current_number >= pictures.size()) {
            break;  
        }

        std::cout << pictures[thread_id].val << std::endl;
        ++current_number;
        cv.notify_all();
    }
}

int main() {
    std::thread threads[NUM_THREADS];
    std::vector<picture> pictures{};
    for (int i = 0; i < NUM_THREADS; ++i) {
        threads[i] = std::thread(processPic, i, pictures);
    }

    for (int i = 0; i < NUM_THREADS; ++i) {
        threads[i].join();
    }

    return 0;
}