Creating Bounding boxes for contours

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I used this code to draw "Bounding boxes". Now I want the largest rectangle while there are many rectangles. After finding the largest rectangle, I want to have the coordinates of this rectangle. By these coordinates, I want to be a part of the original image.The largest rectangle is my car.

#include "opencv2/imgcodecs.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/videoio.hpp"
#include <opencv2/highgui.hpp>
#include <opencv2/video.hpp>
#include <iostream>
#include <sstream> 
#include <fstream>
using namespace cv;
using namespace std;

Mat src; Mat src_gray;
int thresh = 0;
int max_thresh = 255;
RNG rng(12345);

/// Function header
void thresh_callback(int, void*);
/** @function thresh_callback */
void thresh_callback(int, void*)
{
    Mat threshold_output;
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;

    /// Detect edges using Threshold
    threshold(src_gray, threshold_output, thresh, 255, THRESH_BINARY);
    /// Find contours
    findContours(threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

    /// Approximate contours to polygons + get bounding rects and circles
    vector<vector<Point> > contours_poly(contours.size());
    vector<Rect> boundRect(contours.size());
    vector<Point2f>center(contours.size());
    vector<float>radius(contours.size());

    for (int i = 0; i < contours.size(); i++)
    {
        approxPolyDP(Mat(contours[i]), contours_poly[i], 3, true);
        boundRect[i] = boundingRect(Mat(contours_poly[i]));
        minEnclosingCircle((Mat)contours_poly[i], center[i], radius[i]);
    }


    /// Draw polygonal contour + bonding rects + circles
    Mat drawing = Mat::zeros(threshold_output.size(), CV_8UC3);
    for (int i = 0; i< contours.size(); i++)
    {
        Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
        drawContours(drawing, contours_poly, i, color, 1, 8, vector<Vec4i>(), 0, Point());
        rectangle(drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0);
    }

    /// Show in a window
    namedWindow("Contours", CV_WINDOW_AUTOSIZE);
    imshow("Contours", drawing);
}
/** @function main */
int main(int argc, char** argv)
{
    /// Load source image and convert it to gray
    src = imread("RGB2YCrCb_BC_SB.png");
    /// Convert image to gray and blur it
    cvtColor(src, src_gray, CV_BGR2GRAY);
    blur(src_gray, src_gray, Size(3, 3));
    /// Create Window
    char* source_window = "Source";
    namedWindow(source_window, CV_WINDOW_AUTOSIZE);
    imshow(source_window, src);


    thresh_callback(0, 0);

    waitKey(0);
    return(0);
}

I have worked hard and I have searched a lot.Please help if you can. See the picture.