什么可能导致此睡眠排序实现中的未定义行为？

Question

要了解C++中的线程,我做了这个睡眠排序实现.大多数时候,它工作正常.但是,也许每15个左右运行一次,我的睡眠排序函数返回的向量将包含一些垃圾值.有谁知道是什么原因引起的？

这是我输出的屏幕截图:

这是我的代码:

#include <stdio.h>
#include <thread>
#include <chrono>
#include <vector>

std::vector<unsigned int> sleepSort(std::vector<unsigned int> toSort){
    //vector to hold created threads
    std::vector<std::thread> threadList;
    //vector to hold sorted integers
    std::vector<unsigned int> sorted;

    //create a thread for each integer, n, in "toSort" vector
    //each thread sleeps for n seconds then adds n to "sorted" vector
    for(int i = 0; i < toSort.size(); i++){
        threadList.push_back(
            std::thread(
              [](int duration, std::vector<unsigned int>& v){
                std::this_thread::sleep_for((std::chrono::seconds)duration);
                v.push_back(duration);
                }, toSort.at(i), std::ref(sorted)
              )
            );
    }

    //wait for each thread to finish before returning sorted
    for(auto& thread : threadList){
        thread.join();
    }
    return sorted;
}

int main(int argc, char **argv)
{
    std::vector<unsigned int> v {5, 14, 6, 12, 17, 3, 15, 4, 10, 1, 
                                 2, 5, 7, 8, 9, 13, 11, 11, 11, 16
                        };

    printf("Unsorted:\n");
    for(int i = 0; i < v.size(); i++)
        printf("%d\n", v.at(i));

    printf("Sorting...\n");

    v = sleepSort(v);

    printf("Sorted:\n");
    for(int i = 0; i < v.size(); i++)
        printf("%d\n", v.at(i));

    system("PAUSE");
    return 0;
}

Answer 1

没有什么能阻止两个线程push_back同时或重叠时间调用.您需要互斥锁或其他形式的同步.