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i searched google "draw a line c++" and found this

the adaptation to your case is below

vector<Point> linePoints(int x0, int y0, int x1, int y1)
{
    vector<Point> pointsOfLine;

    int dx = abs(x1-x0), sx = x0<x1 ? 1 : -1;
    int dy = abs(y1-y0), sy = y0<y1 ? 1 : -1;
    int err = (dx>dy ? dx : -dy)/2, e2;

    for(;;)
    {
        pointsOfLine.push_back(Point(x0,y0));
        if (x0==x1 && y0==y1) break;
        e2 = err;
        if (e2 >-dx)
        {
            err -= dy;
            x0 += sx;
        }
        if (e2 < dy)
        {
            err += dx;
            y0 += sy;
        }
    }
    return pointsOfLine;
}

to test it you can try the code below

//modified from https://github.com/Itseez/opencv/blob/master/apps/annotation/opencv_annotation.cpp
// move the mouse on the window and try left click and right click
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>


using namespace std;
using namespace cv;

// Function prototypes
void on_mouse(int, int, int, int, void*);

vector<Point> linePoints(int x0, int y0, int x1, int y1)
{
    vector<Point> pointsOfLine;

    int dx = abs(x1-x0), sx = x0<x1 ? 1 : -1;
    int dy = abs(y1-y0), sy = y0<y1 ? 1 : -1;
    int err = (dx>dy ? dx : -dy)/2, e2;

    for(;;)
    {
        pointsOfLine.push_back(Point(x0,y0));
        if (x0==x1 && y0==y1) break;
        e2 = err;
        if (e2 >-dx)
        {
            err -= dy;
            x0 += sx;
        }
        if (e2 < dy)
        {
            err += dx;
            y0 += sy;
        }
    }
    return pointsOfLine;
}

// Public parameters
Mat image(600, 800, CV_8UC3, Scalar(220, 220, 220));
vector<Point> points;
int roi_x0 = 0, roi_y0 = 0, roi_x1 = 0, roi_y1 = 0;
bool start_draw = false;

// Window name for visualisation purposes
const string window_name = "OpenCV Mouse Event Demo";

// FUNCTION : Mouse response for selecting objects in images
// If left button is clicked, start drawing a rectangle and circle as long as mouse moves
//If right button is clicked, look what will happen !
// Stop drawing once a new left click is detected by the on_mouse function
void on_mouse(int event, int x, int y, int, void *)
{
    // Action when left button is clicked
    if (event == EVENT_LBUTTONDOWN)
    {


        if (!start_draw)
        {
            roi_x0 = x;
            roi_y0 = y;
            start_draw = true;
        }
        else
        {
            roi_x1 = x;
            roi_y1 = y;
            start_draw = false;
            vector<vector<Point> > contours;
            contours.push_back(linePoints(roi_x0,roi_y0,roi_x1,roi_y1));
            drawContours( image, contours, 0, Scalar(0, 0, 0), FILLED, 8 );
             imshow(window_name, image);
        }
    }
    // Action when mouse is moving
    if ((event == EVENT_MOUSEMOVE) && start_draw)
    {
        // Redraw bounding box and rectangle
        Mat current_view;
        image.copyTo(current_view);

        line(current_view, Point(roi_x0, roi_y0), Point(x, y), Scalar(0, 0, 255));
        imshow(window_name, current_view);
    }
}


int main(int argc, const char** argv)
{
    // Init window interface and couple mouse actions
    namedWindow(window_name, WINDOW_AUTOSIZE);
    setMouseCallback(window_name, on_mouse);

    imshow(window_name, image);

    int key_pressed = 0;

    do
    {
        // Keys for processing
        // Based on the universal ASCII code of the keystroke: http://www.asciitable.com/
        //      c = 99          add rectangle to current image
        //      <ESC> = 27      exit program
        key_pressed = 0xFF & waitKey(0);
        if (key_pressed==99)
        {
            image=Scalar(220&rand(), 220, 220);
            imshow(window_name, image);
        }
    }
    // Continue as long as the <ESC> key has not been pressed
    while (key_pressed != 27);

    // Close down the window
    destroyWindow(window_name);
    return 0;
}