ffmpeg transcoding.c problems about 'dimensions not set'

Question

I am trying to solve this problem in transcoding.c example about FFMPEG. but I don't know how to approach it...

I am getting this error:

[libx264 @ 00000000022c7120] dimensions not set
Cannot open video encoder for stream #0
Error occurred: Invalid argument

Here is my code to fix the transcoding.c example a little bit

static int open_output_file(const char *filename)
{
    AVStream *out_stream;
    AVStream *in_stream;
    AVCodecContext *dec_ctx, *enc_ctx;
    AVCodec *encoder;
    int ret;
    unsigned int i;

    ofmt_ctx = NULL;

    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);

    if (!ofmt_ctx) {
        av_log(NULL, AV_LOG_ERROR, "Could not create output context\n");
        return AVERROR_UNKNOWN;
    }

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        in_stream = ifmt_ctx->streams[i];
        out_stream = avformat_new_stream(ofmt_ctx, NULL);

        if (!out_stream) {
            av_log(NULL, AV_LOG_ERROR, "Failed allocating output stream\n");
            return AVERROR_UNKNOWN;
        }

        dec_ctx = avcodec_alloc_context3(in_stream->codec->codec);
        enc_ctx = avcodec_alloc_context3(out_stream->codec->codec);

        if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO || dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            /* in this example, we choose transcoding to same codec */
            encoder = avcodec_find_encoder(dec_ctx->codec_id);
            
            if (!encoder) {
                av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");
                return AVERROR_INVALIDDATA;
            }

            /* In this example, we transcode to same properties (picture size,
            * sample rate etc.). These properties can be changed for output
            * streams easily using filters */
            if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {

                enc_ctx->height = dec_ctx->height;
                enc_ctx->width = dec_ctx->width;
                enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;

                /* take first format from list of supported formats */
                enc_ctx->pix_fmt = AV_PIX_FMT_YUV420P;

                /* video time_base can be set to whatever is handy and supported by encoder */
                enc_ctx->time_base = dec_ctx->time_base;
            }

            else {
                enc_ctx->sample_rate = dec_ctx->sample_rate;
                enc_ctx->channel_layout = dec_ctx->channel_layout;
                enc_ctx->channels = av_get_channel_layout_nb_channels(enc_ctx->channel_layout);

                /* take first format from list of supported formats */
                enc_ctx->sample_fmt = encoder->sample_fmts[0];
                enc_ctx->time_base.num = 1;
                enc_ctx->time_base.den = enc_ctx->sample_rate;
            }

            /* Third parameter can be used to pass settings to encoder */
            ret = avcodec_open2(enc_ctx, encoder, NULL);
        
            if (ret < 0) {
                cout << "ret<0" << endl;
                av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", i);
                return ret;
            }
        }
        else if (dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {
            av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown type, cannot proceed\n", i);
            return AVERROR_INVALIDDATA;
        }
        else {
            /* if this stream must be remuxed */
            ret = avcodec_copy_context(ofmt_ctx->streams[i]->codec, ifmt_ctx->streams[i]->codec);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Copying stream context failed\n");
                return ret;
            }
        }
        if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
            enc_ctx->flags |= CODEC_FLAG_GLOBAL_HEADER;
    }

    av_dump_format(ofmt_ctx, 0, filename, 1);

    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Could not open output file '%s'", filename);
            return ret;
        }
    }

    /* init muxer, write output file header */
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Error occurred when opening output file\n");
        return ret;
    }
    return 0;
}

Why doesn't avcodec_open2 work in my code?