How to edit a frame's content using ffmpeg and opencv?

I am going to edit the content of one frame from a mp4 file using OpenCV and ffmpeg 3.3. However, I encountered some problems such as the width and the height of video are zero, some functions are deprecated. I have changed the old function to updated function, but still cannot extract a correct frame. Please help.

Can anyone show an example of extracting a frame from a mp4 file using ffmpeg 3.3?

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
// FFmpeg
extern "C" {
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavutil/avutil.h>
#include <libavutil/pixdesc.h>
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>

}
#define CODEC_TYPE_VIDEO      AVMEDIA_TYPE_VIDEO

int main(int argc, char* argv[])
{   
    // initialize FFmpeg library
    av_register_all();
    //  av_log_set_level(AV_LOG_DEBUG);
    int ret;

    // open input file context
    AVFormatContext* inctx = nullptr;
    //ret = avformat_open_input(&inctx, infile, nullptr, nullptr);
    ret = avformat_open_input(&inctx, "C:\\car.mp4", nullptr, nullptr);
    // retrive input stream information


    ret = avformat_find_stream_info(inctx, nullptr);
    if (ret < 0) {
    std::cerr << "fail to avformat_find_stream_info: ret=" << ret;
    return 2;
    }

    // find primary video stream


    AVCodec* vcodec = nullptr;
    vcodec = avcodec_find_decoder(AV_CODEC_ID_MPEG4);
    if (!vcodec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    const int vstrm_idx = ret;
    AVStream* vstrm = inctx->streams[vstrm_idx];

    // open video decoder context
    AVCodecContext *c = NULL;
    c = avcodec_alloc_context3(vcodec);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    if (avcodec_open2(c, vcodec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    // print input video stream informataion
    // initialize sample scaler

    c->pix_fmt = AV_PIX_FMT_YUV420P;
    c->width = 1280;
    c->height = 720;
    if (vcodec->capabilities & CODEC_CAP_TRUNCATED)
        c->flags |= CODEC_FLAG_TRUNCATED;

    c->flags2 |= CODEC_FLAG2_FAST;
    int width = 1280;
    int height = 720;
    SwsContext* swsctx = sws_getCachedContext(nullptr, width,
        height, AV_PIX_FMT_YUV420P, width, height, AV_PIX_FMT_RGB32,
        SWS_FAST_BILINEAR, NULL, NULL, NULL);
}