How to run opencv code with usb cam?

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Hello I have .bag file which includes my test video and My opencv code is finished and it ishere :

   #include <iostream>
#include <stdio.h>
#include <iostream>
#include <fstream>
#include <string>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/nonfree/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/calib3d/calib3d.hpp"

#include "opencv2/nonfree/nonfree.hpp"
#include "opencv2/legacy/legacy.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <time.h> 


    using namespace cv;
    using namespace std;

char key = 'a';
int framecount = 0;

SurfFeatureDetector detector( 400);
//SurfFeatureDetector detector( 500,1,2,true,false);
SurfDescriptorExtractor extractor;
FlannBasedMatcher matcher;

Mat frame, des_object, image;
Mat des_image, img_matches, H;

std::vector<KeyPoint> kp_object;
std::vector<Point2f> obj_corners(4);
std::vector<KeyPoint> kp_image;
std::vector<vector<DMatch > > matches;
std::vector<DMatch > good_matches;
std::vector<Point2f> obj;
std::vector<Point2f> scene;
std::vector<Point2f> scene_corners(4);

int main()
{             
                //reference image
    Mat object = imread( "z.png", CV_LOAD_IMAGE_GRAYSCALE );

    if( !object.data )
    {
        std::cout<< "Error reading object " << std::endl;
        return -1;
    }


 detectors and descriptors of reference image
    detector.detect( object, kp_object );
    extractor.compute( object, kp_object, des_object );     

                //create video capture object
    VideoCapture cap(-1);
   // VideoCapture cap2()

    //Get the corners from the object
    obj_corners[0] = cvPoint(0,0);
obj_corners[1] = cvPoint( object.cols, 0 );
obj_corners[2] = cvPoint( object.cols, object.rows );
obj_corners[3] = cvPoint( 0, object.rows );

            //wile loop for real time detection
while (key != 27)
{
                            //capture one frame from video and store it into image object name 'frame'
    cap >> frame;

    if (framecount < 5)
    {
        framecount++;
        continue;
    }      


 resize(frame,frame,Size(),0.5,0.5);
                                //converting captured frame into gray scale
        cvtColor(frame, image, CV_RGB2GRAY);

                                //extract detectors and

descriptors of captured frame detector.detect( image, kp_image ); extractor.compute( image, kp_image, des_image );

                            //find matching descriptors of reference and captured image
                            matcher.knnMatch(des_object, des_image, matches, 2);

                            //finding matching keypoints with Euclidean distance 0.6 times the distance of next keypoint
                            //used to find right matches
                            for(int i = 0; i < min(des_image.rows-1,(int) matches.size()); i++)
                            {
                                            if((matches[i][0].distance < 0.6*(matches[i][1].distance)) && ((int) matches[i].size()<=2 && (int) matches[i].size()>0))
                                            {
                                                            good_matches.push_back(matches[i][0]);
                                            }
                            }                             

                            //Draw only "good" matches
                            drawMatches( object, kp_object, frame, kp_image, good_matches, img_matches,
                                            Scalar::all(-1), Scalar::all(-1), vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );

                            //3 good matches are enough to describe an object as a right match.
                            if (good_matches.size() >= 3)
    {                                     

        for( int i = 0; i < good_matches.size(); i++ )
        {
            //Get the keypoints from the good matches
            obj.push_back( kp_object[ good_matches[i].queryIdx ].pt );
            scene.push_back( kp_image[ good_matches[i].trainIdx ].pt );
        }
                                            try
                                            {
                                                            H = findHomography( obj, scene, CV_RANSAC );
                                            }
                                            catch(Exception e){}

        perspectiveTransform( obj_corners, scene_corners, H);

        //Draw lines between the corners (the mapped object in the scene image )
        line( img_matches, scene_corners[0] + Point2f( object.cols, 0), scene_corners[1] + Point2f( object.cols, 0), Scalar(0, 255, 0), 4 );
        line( img_matches, scene_corners[1] + Point2f( object.cols, 0), scene_corners[2] + Point2f( object.cols, 0), Scalar( 0, 255, 0), 4 );
        line( img_matches, scene_corners[2] + Point2f( object.cols, 0), scene_corners[3] + Point2f( object.cols, 0), Scalar( 0, 255, 0 ...