right way to setup cv::VideoCaptureModes

edit

In the opencv 3.4.2 documentation here , there is a provision to change the camera modes, so did I implemented as below,

int main(int argc, char argv) {

Mat frame;

VideoCapture cap(0);

    if (!cap.isOpened()) 
    {
        cout << "Cannot open the video cam" << endl;
        return -1;
    }

while (1) {

    cap.set(CAP_MODE_GRAY);   // mode I want
    cap.set(CAP_PROP_FRAME_WIDTH,320);
    cap.set(CAP_PROP_FRAME_HEIGHT,240);


    bool bSuccess = cap.read(frame);

     if (!bSuccess)
    {
         cout << "Cannot read a frame from video stream" << endl;           break;
    }


  imshow("simple video",frame);

    if (waitKey(30) == 27) 
   {
        cout << "esc key is pressed by user" << endl;            break; 

   }
}

its giving the error as

5_graycapmode.cpp:40:30: error: no matching function for call to 'cv::VideoCapture::set(cv::VideoCaptureModes)' cap.set(CAP_MODE_GRAY); ^

In file included from /usr/local/include/opencv2/highgui.hpp:51:0, from /usr/local/include/opencv2/opencv.hpp:68, from 5_graycapmode.cpp:6: /usr/local/include/opencv2/videoio.hpp:778:26: note: candidate: virtual bool cv::VideoCapture::set(int, double) CV_WRAP virtual bool set(int propId, double value); ^~~ /usr/local/include/opencv2/videoio.hpp:778:26: note: candidate expects 2 arguments, 1 provided

Is it probably as my syntax is wrong??

( Its my new account so my replies are kept frozen for 48 hrs)