#include<iostream>
#include<opencv2/opencv.hpp>
using namespace cv;
using namespace std;
int main()
{
    VideoCapture capture;
    Mat frame;
    const string source = "/home/gear/big_disk_c/wangjd/shipintest/789.mp4";
    // frame= capture.open("789.mp4");
    frame= capture.open(source);
    if(!capture.isOpened())
    {
        printf("can not open ...\n");
        return -1;
    }
    printf("1213131\n");
    // namedWindow("output", CV_WINDOW_AUTOSIZE);

    while (capture.read(frame))
    {
        imshow("output", frame);
        waitKey(10);
    }
    capture.release();
    return 0;
}

Reason: Path. I have a permission problem with this server, and it will report an error if I write the full path. And the executable file I generated and the video file are not in the same directory, …/789.mp4 will not be read, only the two put together to read. Later I found out that
The constructor of capture.open() passed in a const string, so I instantiated a const string source first, then I could write the full path, and then passed it in.

const string source = "/home/gear/big_disk_c/wangjd/shipintest/789.mp4";
frame= capture.open(source);