#2. Display feed from your webcam

#include
#include
#include
using namespace std;
using namespace cv;

int main()
{
 Mat frame;
//create a Mat object
 VideoCapture cap(0);
//creates a VideoCapture object that links to the default device(number 0) on your computer
 
        namedWindow("Video");
 while(char(waitKey(1))!='q'){

  cap >> frame;
//extracts a frame from the input device 'cap' to 'frame'
  imshow("Video",frame);
 }
 return 0;
}

Tweak:
You can also play a video from a file.
create a VideoCapture object as:

VideoCapture cap("video.mp4");